24 sugar

7/28/2019 24 Sugar

1/51

Richard F. Daley and Sally J. Daleywww.ochem4free.com

Organic

ChemistryChapter 24

Carbohydrates

24.1 Classification of Carbohydrates 1250

24.2 Monosaccharides 125324.3 Cyclic Forms of Monosaccharides 1256

Sidebar - The Sweet Taste 126024.4 Reactions of Monosaccharides 126324.5 Oxidation and Reduction Reactions 126524.6 Changing the Chain Length 126824.7 Fischer Proof of Glucose Structure 127124.8 Glycolysis - I 127524.9 Glycolysis - II 1280

Sidebar - Arsenic Poisoning 128524.10 Glycoside Formation 1286

24.11 Disaccharides 129124.12 Polysaccharides 1291Key Ideas from Chapter 24 1294

7/28/2019 24 Sugar

2/51

Organic Chemistry - Ch 24 1247 Daley & Daley

Copyright 1996-2005 by Richard F. Daley & Sally J. DaleyAll Rights Reserved.

No part of this publication may be reproduced, stored in a retrieval system, or

transmitted in any form or by any means, electronic, mechanical, photocopying,

recording, or otherwise, without the prior written permission of the copyright

holder.

www.ochem4free.com 5 July 2005

7/28/2019 24 Sugar

3/51


Chapter 24

Carbohydrates

Chapter Outline

24.1 Classification of CarbohydratesAn examination of the various categories of mono- andpolysaccharides

24.2 MonosaccharidesThe structures of the monosaccharides

24.3 Cyclic Forms of MonosaccharidesThe hemiacetal forms of monosaccharides

24.4 Reactions of MonosaccharidesMonosaccharides react much like alcohols, aldehydes,and ketones

24.5 Oxidation and Reduction ReactionsOxidizing and reducing the terminal carbons of

monosaccharides24.6 Changing the Chain LengthReactions that modify the length of the chain in amonosaccharide

24.7 Fischer Proof of Glucose StructureEmil Fischers proof of the relative stereochemistry ofthe stereogenic centers in glucose

24.8 Glycolysis - IA study of the mechanism for glycolysisconversion ofglucose to pyruvate

24.9 Glycolysis - IIContinuing the study of the mechanism of glycolysis

24.10 Glycoside Formation

Glycosides and glycosidic bonds24.11 Disaccharides

The structures of disaccharides24.12 Polysaccharides

The structures of polysaccharides


7/28/2019 24 Sugar

4/51


Objectives

Know the various types of carbohydrates

Be able to draw the Fischer and Haworth projections ofmonosaccharides

Recognize that the reactions of monosaccharides and simplerorganic molecules follow the same pathways

Understand the logic of the Fischer proof for the stereochemistry ofglucose

See the similarity of enzyme-catalyzed reaction pathways and thereaction pathways studied in earlier portions of this book

Understand the structure of glycosides Know the structural characteristics of di- and polysaccharides

By weight, carbohydrates occur more frequently innature than any other class of carbon-containingcompounds. Carbohydrates transport and store energy in bothanimals and plants. They also provide the structural framework forplants. Nearly all plants and animals synthesize carbohydrates. Manycarbohydrates are polymers of glucose. Glucose is converted to carbondioxide and water to provide energy for animals. Plants gain energyby converting carbon dioxide and water to glucose. Animals storeexcess chemical energy as glycogen, and plants store excess energy asstarch. Plants also convert glucose to cellulose, which provides thestructural framework for plants.

People depend on carbohydrates for a number of theiractivities. They consume carbohydrates and convert them to energy.They use cellulose in the form of wood for heat, housing, andfurnishings. People convert wood to paper for newspapers, magazines,and this book. They make clothing from the cellulose fibers of cottonand from the cellulose derivatives rayon and cellulose acetate.


7/28/2019 24 Sugar

5/51


At first glance, carbohydrate chemistry may seem complicated,but the principles of carbohydrate chemistry are the same as thechemical principles you have studied to this point. The molecules form

using the same structural chemistry as do other types of compounds.Carbohydrates contain ketone, aldehyde, and alcohol functionalgroups. Thus, their chemistry is similar to the chemistry of thosegroups.

24.1 Classification of Carbohydrates

Early chemists used the term carbohydrate to describe aclass of compounds that have the general molecular formulaCm(H2O)n. This formula suggests that carbon atoms somehow bond towater molecules to form "hydrates of carbon." Early chemists thought

that carbohydrates were simply hydrated carbons because heatingcarbohydrates produced water and carbon. However, this view isinaccurate. Although most of the carbohydrate examples presented inthis chapter have the empirical formula CH2O, their structures aremore complex than suggested by the term "hydrates of carbon."

The literal definition of

carbohydrate is the

hydrate of carbon.

Carbohydrates have

the molecular formula

Cm(H2O)n.

Biochemists often use the term sugar synonymously withcarbohydrate. Monosaccharides, or "simple sugars," are the simplestof the carbohydrate units because they cannot be hydrolyzed to othersimpler carbohydrates. Disaccharides, trisaccharides, andpolysaccharides are dimers, trimers, and polymers ofmonosaccharides. Most disaccharides and polysaccharides can behydrolyzed readily to monosaccharides. Biochemists classifymonosaccharides as either polyhydroxyaldehydes orpolyhydroxyketones. Biochemists call a polyhydroxyaldehydemonosaccharide an aldose and a polyhydroxyketone a ketose.

Monosaccharides are

the simplest

carbohydrate units.

Disaccharides,

trisaccharides, and

polysaccharides are

dimers, trimers, andpolymers of

monosaccharides.

An aldose is a

polyhydroxy aldehyde,

and a ketose is a

polyhydroxy ketone.

Both are

carbohydrates.

The two most familiar monosaccharides are glucose andfructose. Figure 24.1 shows the structures of these two compounds.The left structure of each type is a three-dimensional representation.The right structure is a Fischer projection of the same molecule.Glucose is an example of an aldose and fructose is an example of aketose.


7/28/2019 24 Sugar

6/51


CH2OH

CHO

OHH

HO

H OH

H

HO

H

Fructose

Glucose

OHC

OH

H

H

H

O

CH2OH

CH2OH

C

C

C

HO

OH

OH

H

H

H

O

CH2OH

CH2OH

C

HO

CHO

OH

OH

OH

HO

H

H

H

H

CH2OH

C

C

C

C

CHO

OH

OH

OH

HO

H

H

H

H

CH2OH

CH2OH

CH2OH

HO

H OH

H

HOH

O

Figure 24.1. Structures of glucose and fructose. The left structure of each groupshows the all-eclipsed structure on which the other two are based. The centerstructure shows the bonds with wedges and dashes. Most of the time, however,

biochemists draw the structures as Fischer projections, as shown on the right.Biochemists use Fischer projections to draw monosaccharides

because Fischer projections are easy to draw and clearly show thestereochemical relationships involved. Emil Fischer designed theFischer projection to draw the chemical structures in his proof of thestructure of glucose. For this proof, he received the Nobel Prize in1902. As shown in Figure 24.1, the vertical lines in a Fischerprojection are either in or behind the plane of the paper. Horizontallines project out from the plane of the paper. For reference purposes,when drawing carbohydrate structures of an aldose, always place thealdehyde carbon on the top. When drawing a ketose, place the ketone

closer to the top of the drawing.

See Section 11.5, page

000, for more details of

the Fischer projection.


7/28/2019 24 Sugar

7/51


Monosaccharides readily form cyclic hemiacetals when thesugar forms a five- or six-membered ring. The hemiacetal of glucose isparticularly stable because the groups on the ring are equatorial. In

the illustration of the acetal of glucose, the structure on the right is aHaworth projection. The Haworth projection is named for Sir W. N.Haworth from the University of Birmingham in England. He won theNobel Prize in 1937 for his work in carbohydrate chemistry. TheHaworth projection is equivalent to the cyclic structure on the left.

Although biology and biochemistry books widely use the Haworthprojection, chemists generally prefer to use the chair conformationbecause the chair conformation is closer to the shape of the moleculethan the flat Haworth projection. In this book we will use bothrespresentations.

The Haworth

projection is a

standardized method,

similar to the Fischer

projection, that is used

to represent the cyclic

hemiacetal forms of

monosaccharides.

Haworth projections

can also be linked to

show di, tri, and

polysaccharides.

O

H

H

H

H

H

CH2OH

OH

OH

HO

HO

A hemiacetal form ofD-glucose

Haworth projectionChair

O OH

H

H

H

H

OH

OH

CH2OH

HOH

A disaccharide is a sugar that hydrolyzes to twomonosaccharide molecules per disaccharide molecule. For example,

sucrose, "sugar" or "table sugar," readily hydrolyzes to one molecule ofglucose and one molecule of fructose.

H3OSucrose + FructoseGlucose

Both monosaccharides and disaccharides are readily soluble in waterand most have a characteristic sweet flavor.

Polysaccharides are carbohydrates that hydrolyze to a numberof monosaccharide molecules per polysaccharide. Two commonpolysaccharides are starch and cellulose. Starch is more water-soluble

than cellulose. A starch readily adds more monosaccharide units tostore excess energy or removes monosaccharide units to provideenergy to the organism. Starch easily hydrolyzes in the laboratory orin plant or animal systems. Cellulose is a larger molecule and is notwater-soluble. It does not easily hydrolyze, except by specializedorganisms or high temperature acid-catalyzed hydrolysis.


7/28/2019 24 Sugar

8/51


H3OStarch ~300 Glucose

H3OCellulose ~3000 Glucose

Of all the monosaccharides, glucose is by far the best known.Through the process of photosynthesis, plants form glucose in greatquantities from carbon dioxide and water. Environmental chemistsestimate that photosynthesis produces approximately 1011 to 1012tons of glucose per year. This amount of glucose represents anenormous resource of stored energy. Plants use the energy formed byphotosynthesis directly. Animals use that energy by consuming theplants.

Exercise 24.1

Draw the wedge-and-dash and the Fischer projections of theenantiomers of glucose and fructose.

24.2 Monosaccharides

Monosaccharides usually contain from three to seven carbons.The general name for each chain length is triose (3 carbons), tetrose (4carbons), pentose (5 carbons), hexose (6 carbons), and heptose (7carbons). Biochemists often combine these names with the aldose orketose designations. Thus, glucose, an aldose with six carbons, is analdohexose, and fructose, a ketose with six carbons, is a ketohexose.

Notice that all the monosaccharide illustrations used thus farin this chapter show the OH group on the chiral carbon furthestfrom the carbonyl group to the right side of the structure. Chemists inthe late 19th century made the observation that nearly all naturallyoccurring monosaccharides degrade to the dextrorotatory (+)enantiomer of glyceraldehyde, the only aldotriose. Only a very fewdegrade to the levorotatory () enantiomer of glyceraldehyde.

(S-(-)-glyceraldehyde)(R-(+)-glyceraldehyde)

HO H

CHO

CH2OH

H OH

CHO

CH2OH

D-Glyceraldehyde L-Glyceraldehyde

CHO

CH2OHHOH

CHO

CH2OHHHO

Because degradation shortens the chain from the carbonyl end of thechain, the single chiral center in glyceraldehyde corresponds to the


7/28/2019 24 Sugar

9/51


chiral carbon at the end of the carbon chain away from the carbonylgroup.

D-(+)-Glucose D-(-)-Arabinose D-(-)-Erythrose

H OH

CH2OH

OHH

HHO

OHH

CHO

H OH

CH2OH

OHH

CHO

HHO

H OH

CH2OH

OHH

CHO

D-(+)-Glyceraldehyde

H OH

CHO

CH2OH

The fact that the degradation leads to either (+) or ()

glyceraldehyde allows you to use the specification of thestereochemistry of the glyceraldehyde in the name of themonosaccharide. If the stereochemistry is the same as the (+)-glyceraldehyde, then prefix the name by a D. If the stereochemistry isthe same as ()-glyceraldehyde, then use the L prefix. The D,L systemof carbohydrate nomenclature is of limited use because it specifies theconformation of only one stereogenic center. It gives no informationabout any other stereogenic centers that may be present in themolecule.

D And L Notation vs Optical Activity

Note that the D and L notations refer only to the stereochemistry of the chiral carbonfurthest from the carbonyl group of a monosaccharide. This notation has nothing to dowith the direction that a particular sugar rotates a plane of polarized light. A sugarwith the D prefix may rotate a plane of polarized light either to the right or to the left.If you want to know the direction that a particular sugar rotates polarized light, youwill need to determine it experimentally.

A pair of structures that differ in the configuration of only onecarbon atom are called epimers.For example, mannose and glucoseare identical except for the configurations at C2. Generally, whenreferring to a pair of epimers, biochemists specify the differing carbon

atom with the carbon number. For example, glucose and mannose areC2 epimers, and glucose and galactose are C4 epimers.

Epimers are identical

structures except for

the configuration of a

single stereogenic

center.


7/28/2019 24 Sugar

10/51


D-GalactoseD-Mannose D-Glucose

C4 Epimers

C2 EpimersCHO

H

H

H

OH

OH

HO

HO

H

CH2OH

CHO

OH

OH

OH

HO

H

H

H

H

CH2OH

CHO

H

OH

OH

HO

HO H

H

H

CH2OH

Exercise 24.2

Draw Fischer projections ofD-allose, the C3 epimer ofD-glucose, and

ofL-altrose, the enantiomer of the C2 epimer ofD-allose.

Figure 24.2 shows all of the D-aldotrioses, D-aldotetroses, D-aldopentoses, and D-aldohexoses. The arrangement of these sugars inthis table allows you to easily recall their structures. Below eachstructure are two other structures. These two lower structures are C2epimers and have one more carbon atom than the structure above.

D-TaloseD-GalactoseD-IdoseD-GuloseD-MannoseD-GlucoseD-AltroseD-Allose

D-LyxoseD-XyloseD-ArabinoseD-Ribose

H OH

CH2OH

HHO

CHO

H OH

CH2OH

OHH

CHO

H OH

CH2OH

HHO

CHO

OHH

H OH

CH2OH

OHH

CHO

OHH

H OH

CH2OH

OHH

CHO

HHO

H OH

CH2OH

HHO

CHO

HHO

H O

CH2OH

H

H

HHO

CHO

HO

HO

HH OH

CH2OH

H

H

OHH

CHO

HO

HO

H OH

CH2OH

H

OH

HHO

CHO

HO

H

H OH

CH2OH

OH

OH

OHH

CHO

H

H

H OH

CH2OH

H

OH

OHH

CHO

HO

H

H OH

CH2OH

OH

OH

HHO

CHO

H

H

H OH

CH2OH

OH

H

OHH

CHO

H

HO

H OH

CH2OH

OH

H

HHO

CHO

H

HO

D-ThreoseD-Erythrose

D-Glyceraldehyde

H OH

CHO

CH2OH


7/28/2019 24 Sugar

11/51


Figure 24.2. Configuration of the D aldotrioses, aldotetroses, aldopentoses, andaldohexoses. The arrangement of the structures shows the structural relationshipsbetween the various structures.

Exercise 24.3

Figure 24.2 shows only D-sugars. Draw the Fischer projections of L-xylose, L-mannose, and L-idose.

24.3 Cyclic forms of Monosaccharides

An alcohol reacts with a carbonyl group producing ahemiacetal. Two moles of alcohol produce an acetal.Acetals are found in

Section 7.5, page 000.

C

O

ROHROHORC

OR

ORC

OH

Hemiacetal Acetal

When both the alcohol and the carbonyl group are in the samecompound, intramolecular nucleophilic addition can take place to forma cyclic hemiacetal. Five- and six-membered cyclic hemiacetals formparticularly easily.

HHO

O

O OH

Many monosaccharides exist in equilibrium between the open chainform and the cyclic hemiacetal form.

In aqueous solution, glucose exists almost entirely in the six-membered pyranose ring. The ring forms by a nucleophilic additionof the hydroxyl oxygen at C5 to the aldehyde at C1. In aqueoussolution, fructose exists about 20% as a five-membered furanose ring

formed by the addition of the hydroxyl group oxygen at C5 to the C2ketone. Biochemists derive the names pyranose and furanose from thenames of the simple cyclic ethers pyran and furan.

A pyranose ring is

named for the cyclic

six-membered ether

pyran.

Furanose is named for

the cyclic five-

membered ether furan.


7/28/2019 24 Sugar

12/51


OO

Pyran Furan

Biochemists usually represent the pyranose and furanose ringsas the Haworth projection rather than the Fischer projection.

Although the Haworth projection shows the relationship between thesubstituents on the ring, it is misleading because, in reality, the ringsare not flat.

An easy method to convert from the Fischer projection of ahemiacetal to a Haworth projection starts with the Fischer projectionof the acyclic, or open chain, molecule that forms the hemiacetal. For

reference, lay the Fischer projection on its side with the carbonylgroup to the right and toward the rear. Hydroxyl groups that are onthe right in a Fischer projection are down in the Haworth projection.Conversely, hydroxyl groups that are on the left in a Fischer projectionare up on the Haworth projection. The CH2OH group is always up inD sugars and always down in L sugars. Arrange the ring by placing theoxygen in the ring at the upper right. Figure 24.3 shows theconversion of the Fischer projection of D-glucose to the Haworthprojection of the pyranose form ofD-glucose.

O

OHH

H

H

OHOH

CH2OH

HOH

H

+C1C4

C5

-H

H

Haworth

Fischer

D-Glucose

D-Glucose

H

H

H

OH

OH

CH2OH

HOH

OH

O

H

C4-C5 bond

Rotate

H

H

H

OH

OH

OH

CH2OH

OH

H

O

H

Sideways

Turn

CHO

OH

OH

OH

HO

H

H

H

H

CH2OH

O OH

HH

H

H

OHOH

CH2OH

HOH

C4

C5

C4

C5

C5

C4

C1

C1C1

C1C4

C5

Figure 24.3. Converting the Fischer projection of D-glucose to the Haworthprojection of the hemiacetal.


7/28/2019 24 Sugar

13/51


Figure 24.4 shows the conversion of the Fischer projection of D-riboseto the Haworth projection.

HaworthD-Ribose

OH

OH

OHOH

HOH2C

H H

H

C1

C3

C4

FischerD-Ribose

H

-H

OH

OHOH

H H

H

OH

HOH2C

C3-C4 bond

Rotate

CH2OH

OHOH

HO

H H

H H

Osideways

TurnOHH

OHH

CHO

OHH

CH2OH

OOH

H

OHOH

HOH2C

H H

H

C4C4

C3 C3

C3

C4

C1

C1C1

C1

C3

C4+

Figure 24.4. Converting the Fischer projection of D-ribose to the Haworth projectionof the hemiacetal.

To name the hemiacetal form of the monosaccharides, replacethe se ending from the name of the open chain form of the sugar with

the pyranose or furanose ending. For example, the cyclic form ofglucose is called glucopyranose and the cyclic form of ribose is calledribofuranose.

Exercise 24.4

Draw the Haworth projection of the pyranose form ofD-gulose.

With the formation of the pyranose or furanose ring, thereaction produces a new stereogenic center at the site that was thecarbonyl carbon. Because the carbonyl group is planar and can react

from either side, the reaction produces a pair of diastereomers. Thesediastereomers are called anomers and the hemiacetal carbon atom iscalled an anomeric center. Glucose forms a 36:64 mixture of the twoanomers designated as and anomers. With both the D and Lisomers, the anomer has the OH group at C1 and the CH2OHgroup on C5 trans, but in the anomer both groups are cis.

Anomers are the

diastereomers

produced when a

monosaccharide forms

a hemiacetal.

The former carbonyl

carbon is called the

anomeric center.


7/28/2019 24 Sugar

14/51


-D-glucopyranose-D-glucopyranose

Anomers of the cyclic forms ofD-glucose

O

H

H

H

OH

OH

CH2OH

H

OH

H

OH

O OH

H

H

H

H

OH

OH

CH2OH

H

OH

Both anomers of glucose can be isolated and purified. Asexpected from diastereomers, they have different physical properties.Pure -D-glucopyranose has a melting point of 146oC and a specificrotation, []D, of +112.2o, whereas pure -D-glucopyranose has a

melting point of 152oC and an []D of +18.7o. The pure anomercrystallizes from water at room temperature. The pure anomercrystallizes when the water is allowed to evaporate above 97oC.

For more on

diastereomers see

Section 11.5, page 532.

If you dissolve a sample of either anomer ofD-glucose in water,the optical rotations change and ultimately converge to a constantvalue of +52.6o. This phenomenon, known as mutarotation, is due tothe slow conversion of either anomer to the 36:64 equilibrium mixtureof the two anomers. Mutarotation occurs via a reversible ring openingof either anomer to the open chain form followed by a ring closure.

Mutarotation is the

conversion, in water, of

one anomer to the other

anomer.

C2C2 C1 C1

Rotate C1-C2

Bond C2C2 C1C1

OH

OH

HOHO

H

H

H

CH2OH

H O

H

D-Glucose

OH

OH

HOHO

H

H

H

H

CH2OH

H

O

O

OH

HO

HO

OH

HH

H

H CH2OH

HO

OHOH

HO

HO

H

H

H

H

CH2OHH

-D-Glucopyranose-D-Glucopyranose Exercise 24.5


7/28/2019 24 Sugar

15/51


Draw the Haworth projection of the two anomers of the furanose formof fructose.

Exercise 24.6

Explain why there is approximately twice as much of the -D-glucopyranose at equilibrium in water as there is of the -D-glucopyranose form.

Exercise 24.7

All sugars with the hemiacetal structure exhibit mutarotation. Forexample, -D-galactopyranose has an []D of +150.7o, and -D-galactopyranose has an []D of +52.8o. If either anomer is dissolved in

water and allowed to reach equilibrium, the []D of the resultingsolution is +80.2o. Draw the pyranose forms of both anomers andcalculate the percentages of each in solution.

[Sidebar]

The Sweet Taste

Sugar is sweet, but many artificial sweeteners are sweeter.Sodium cyclamate is 30 times sweeter than sucrose, aspartame is 160

times sweeter, sodium saccharin is 500 times sweeter, and 5-nitro-2-propoxyaniline is 4100 times sweeter than sucrose. Looking at thestructures of these sweeteners, the relationship between these variousstructures and their sweetness is hard to see.

O

OO OH

OH

OH

HOHO

CH2OH

CH2OHCH2OH

Sucrose

Sodium cyclamate

NHSO3 Na


7/28/2019 24 Sugar

16/51

7/28/2019 24 Sugar

17/51


O

O O

H H

H

AB

CH2

HO

OH

OH

The tripartite model applied to -D-glucose

This model also fits well with the saccharin molecule. The Bcomponent is an oxygen of the SO2 group, the NH group is the

AH component, and the benzene ring is the component.

SO2N

H

O

H

AB

The tripartite model applied to saccharin

The theory about the relationship of structure to sweetnessexplains the sweet flavor of many sweeteners, but no search for a

sweetener based on these criteria has really turned up a newsweetener. The discovery of many sweeteners was by accident. In fact,many of these discoveries are the result of the researcher notpracticing good hygiene in the lab. For example, the investigator whodiscovered saccharin did so while eating his lunch in the labwithoutwashing his hands. Likewise, the investigator who discoveredcyclamate was smoking in the labagain without washing his hands.

Aspartame and Acesulfame-K were discovered when the investigatorslicked their fingers to pick up a piece of weighing paper. A student,who was not very proficient in English, discovered sucralose when hemisunderstood a request to test the compound with a request to taste

the compound.If chemists today used the low-tech approach of tasting newcompounds in an effort to find new artificial sweeteners, they wouldno doubt uncover many sweeteners. Tasting new chemical compoundswas standard practice in the nineteenth century. Every new compoundwas tasted as a part of its characterization. Unfortunately, if tastingnew compounds were part of a chemist's job description today,chemical companies would risk huge lawsuits. Many more of the


7/28/2019 24 Sugar

18/51


compounds that chemists work with now are potentially more harmfulthan the compounds that early chemists worked with.

Biochemists will continue to discover new sweeteners mostly by

accident. Of course, research based on accidents is a contradiction interms. True research on sweetness and sweeteners is impossible untilsomeone isolates the primate sweet taste receptor. Once biochemistsfind this receptor, they can conduct research on hundreds orthousands of compounds by finding those compounds that bind best tothe receptor.

24.4 Reactions of Monosaccharides

Monosaccharides are polyfunctional compounds that undergoany of the reactions that are typical of each of the functional groups

present on the compound. Thus, monosaccharides exhibit reactivitytypical of alcohols and ketones or aldehydes. However, the reagentsthat chemists normally use with monofunctional compounds often giveundesirable side reactions with the sugars containing these functionalgroups. These undesirable side reactions occur because the presence ofmultiple functional groups tends to make them much more reactivethan their monofunctional counterparts. Although many of thereactions in this section are similar to those reactions that you alreadyknow, they are adapted for carbohydrate compounds.

A strong base causes two isomerization reactions incarbohydrates. Both reactions result from the formation of the enolateion of the carbonyl group. In the first reaction, the product is theformation of the C2 epimer of the aldose. The example below showsthe epimerization ofD-glucose to D-mannose.

C

OHH

HHO

OHH

OHH

CH2OH

OH

HO

C

OH

HHO

OHH

OHH

CH2OH

OH

C

OH

HHO

OHH

OHH

CH2OH

H O

C

HHO

HHO

OHH

OHH

CH2OH

OH

H OH

D-Glucose D-Mannose

The second reaction is the movement of the carbonyl groupbetween C1 and C2. This reaction proceeds via a similar mechanism tothe reaction above. In general, this reaction, as well as theepimerization reaction above, are equilibrium reactions. Thus, thereaction favors the most stable form of the sugar. Epimerization and


7/28/2019 24 Sugar

19/51


carbonyl group movements are competitive reactions. The conversionofD-glucose to D-fructose is the most common example of this reaction.

OH

C

HHO

OHH

OHH

CH2OH

H OH

O H

H OH

CH2OH

C

HHO

OHH

OHH

CH2OH

O

C

OH

HHO

OHH

OHH

CH2OH

H O

HO

C

OHH

HHO

OHH

OHH

CH2OH

OH

H OH

D-Fructose

D-Glucose

Exercise 24.8

Fructose epimerizes at C3. Write a mechanism for this reaction.

Biochemists often prepare the ester and ether derivatives ofmonosaccharides because these derivatives are easier to work withthan are the sugars. Monosaccharides contain so many hydroxylgroups that they are soluble in water but are soluble in only a few

organic solvents. An attempt to recrystallize monosaccharides fromwater often forms a syrupy material. The strong hydrogen bonds thatform with water make removing all the water difficult. The ester andether derivatives of monosaccharides are similar to most other organiccompounds, so they purify more easily.

Biochemists normally carry out an esterification by treatingthe carbohydrate with either an acid chloride or an acid anhydride inthe presence of a mild base. All the hydroxyl groups react, including


7/28/2019 24 Sugar

20/51

7/28/2019 24 Sugar

21/51


example, a reduction ofD-glucose by sodium borohydride results in theformation of glucitol.

(also called sorbitol)

GlucitolD-Glucose

NaBH4

CH2OH

OHH

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

OHH

OHH

CH2OH

Additionally, D-fructose can be catalytically reduced to glucitol andmannitol.

Ni

H2+

Mannitol

CH2OH

HHO

HHO

OHH

OHH

CH2OH

GlucitolD-Fructose

CH2OH

OHH

HHO

OHH

OHH

CH2OH

CH2OH

C O

HHO

OHH

OHH

CH2OH

In water, bromine readily reacts with aldoses but not ketoses,oxidizing the aldoses to aldonic acids. Thus, this reaction provides aconvenient test to differentiate between aldoses and ketoses. Analdose is present when the orange bromine solution rapidly loses itscolor. An example of this reaction is the formation of D-mannonic acidfrom D-mannose with bromine in water.

An aldonic acid is an

oxidized aldose that

results when the

aldehyde is oxidized to

a carboxylic acid.


7/28/2019 24 Sugar

22/51


CHO

H

HHO

OHH

OHH

CH2OH

HO

D-Mannose

COOH

HHO

HHO

OHH

OHH

CH2OH

D-Mannonic acid

Br2

H2O

In water, nitric acid is a much stronger oxidizing agent thanbromine. Nitric acid oxidizes both the aldehyde and the CH2OH ofan aldose to carboxylic acids. These dicarboxylic acids are called

aldaric acids. For example, D-mannose forms mannaric acid.An aldaric acid is the

product of a reaction of

an aldose oxidized to a

dicarboxylic acid.

HNO3

D-Mannaric acid

COOH

HHO

HHO

OHH

OHH

COOH

D-Mannose

CHO

H

HHO

OHH

OHH

CH2OH

HO

The Tollens test is the process that was used years ago to

form most mirrors. The Tollens test uses a solution of silver nitrate inammonium hydroxide. The formula for the Tollens reagent is

Ag(NH3)2 c- OH. This solution oxidizes an aldehyde to a carboxylicacid and forms silver metal. If the glass surface is very clean, thesilver forms a beautiful though somewhat fragile mirror on the glass.If the glass is not clean, the silver forms a black precipitate. Thesugars that reduce the Tollens reagent are called reducing sugars.Reducing sugars include all the monosaccharides, both aldoses as wellas ketoses with a CH2OH next to the carbonyl group. In addition,

any hemiacetal structure can also act as a reducing sugar.

In a Tollens test, silver

ions are oxidized to

metallic silver by all

monosaccharides.

A reducing sugar is

any saccharide with

either an aldehyde or

ketone functional

group.


7/28/2019 24 Sugar

23/51


D-Talonic acid

COOH

HHO

HHO

HHO

OHH

CH2OH

D-Talose

CHO

H

HHO

HHO

OHH

CH2OH

HO

Ag(NH3)2 OH+ Ag

Benedicts reagent readily oxidizes the aldehyde functionalgroup of reducing sugars. This reagent is a very mild oxidizing agentand has long been used as a test to screen for levels of reducing sugars

in blood. An unusual concentration of glucose in the blood is anindication of diabetes.

Benedicts reagent is a

solution of copper (II)

sulfate with sodium

citrate in aqueous base.

Exercise 24.10

The product of the reaction of D-glucose with nitric acid produces anoptically active product, whereas the reaction with D-galactoseproduces an optically inactive product. Explain.

24.6 Changing the Chain Length

Chemists devoted much of their early work on carbohydrates tothe understanding of the stereochemical relationships among themonosaccharides. They found that when they changed the chainlength of a particular monosaccharide, they could easily see thesestereochemical relationships. For example, when they degraded twodifferent aldohexoses to the same aldopentose, they knew that thealdohexoses were C2 epimers. Section 24.7 examines this conceptfurther.

The most common method for shortening an aldose chain is theRuff degradation. The Ruff degradation can be used to degrade avariety of natural sugars to D-glyceraldehyde. Otto Ruff, a Germanchemist, developed the Ruff degradation in about 1900.

Ruff degradation is a

method for shortening

the chain of a

monosaccharide. The Ruff degradation is a two step process. The first step is tooxidize the aldose to the corresponding aldonic acid. Second, treat thealdonic acid with hydrogen peroxide and iron(III) sulfate. The aldonicacid loses the carboxylic acid group as CO2, and the second carbonbecomes the aldehyde in the new shorter chain. The remainingcarbons do not change stereochemically.


7/28/2019 24 Sugar

24/51


D-ArabinoseD-Gluconic acidD-Glucose

H2O2

Fe2(SO4)3H2O

Br2 HHO

OHH

OHH

CH2OH

CHO

COOH

OHH

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

OHH

OHH

CH2OH

The Kiliani-Fischer synthesis is effectively the reverse of theRuff degradation. Heinrich Kiliani, of the University of Freiburg inGermany, published a paper that described his method for converting

an aldose to a cyanohydrin. Emil Fischer, realizing the importance ofKiliani's method, developed it further. Fischers method took thenitrile of a cyanohydrin and converted the nitrile into an aldehydegroup.

The Kiliani-Fischer

synthesis is a method

that lengthens the

chain of an aldose.

See Section 7.4, page

000, for more about

cyanohydrins.

The result of the Kiliani-Fischer synthesis is the formation oftwo new aldoses. These two aldoses are C2 epimers of an aldose onecarbon longer than the starting aldose. A new stereogenic center isgenerated with the formation of the cyanohydrin.

HCN

Epimeric cyanohydrins

HHO

HHO

OHH

OHH

CH2OH

CN

OHH

HHO

OHH

OHH

CH2OH

CN

D-Arabinose

HHO

OHH

OHH

CH2OH

CHO

+

The two cyanohydrins can be separated and converted to thecorresponding aldoses. A variety of methods are used for thisconversion. One commonly used method is the hydrolysis of the nitrileto a carboxylic acid. This aldonic acid rapidly forms a lactone.


7/28/2019 24 Sugar

25/51


OHH

HHO

OHH

OHH

CH2OH

CN

1) Ba(OH)2

2) H3O

OHH

HHO

OHH

OHH

CH2OH

COOH

O

OOH

HOHO

CH2OH

H

H

H

H

The cyanohydrin The aldonic acidThe lactone

The lactone can be reduced using a sodium amalgam in water at a pHof 3-5. The reduction forms both the and pyranoses.

OHH

HHO

OHH

OHH

CH2OH

CHO

O

OOH

HOHO

CH2OH

H

H

H

H

Na(Hg), H2O

pH 3 - 5

O

OH

HOHO

CH2OH

H

H

H

H

OH

H

Both and formed

D-Glucose

Hydrolysis and reduction of the epimeric cyanohydrin produces D-mannose.

O

O

HOHO

CH2OH

H

H

H

HO

H

HHO

OHH

OHH

CH2OH

COOH

HO H

2) H3O

1) Ba(OH)2HHO

OHH

OHH

CH2OH

CN

HO H


7/28/2019 24 Sugar

26/51


D-Mannose

Both and formed

O

HOHO

CH2OH

H

H

H

OH

H

H

HO

pH 3 - 5

Na(Hg), H2OO

O

HOHO

CH2OH

H

H

H

H

HO HHO

OHH

OHH

CH2OH

CHO

HHO

Exercise 24.11

What two aldohexoses would give D-xylose on Ruff degradation? Whattwo aldohexoses result from a Kiliani-Fischer synthesis that uses D-

ribose as a substrate?

24.7 Fischer Proof of Glucose Structure

Emil Fischer published his proof of the structure of glucose in1891. In 1902, he received the Nobel Prize in chemistry for his work.When you consider the state of chemistry at that time, Fischeraccomplished an amazing feat with his proof. Fischer was challengedby the complexity of the structure of glucose, by the fact that many ofthe modern theories of chemistry were new, and by laboratory toolsthat were limited according to today's standards. The theory of the

tetrahedral carbon, an important concept relative to his work, hadonly been published in the late 1870s. He conducted his laboratorywork much differently than chemists do today. He had no modernmeans, such as chromatography, to purify his products, so his workwas laborious and time consuming. His research also required that hepurify the product from each reaction step and conduct numerouschemical and physical tests on it, as he had no IR, MS, or NMR to helpwith identification.

Fischer's work consisted of a series of observations from whichhe made various predictions concerning glucose. His first observationwas that (+)-glucose is an aldohexose. An aldohexose has fourstereogenic centers, so he knew the compound had 16 possiblestereoisomers. These stereoisomers make up eight pairs ofenantiomers. After doing a number of chemical tests and reactionsusing (+)-glucose, he learned that successive Ruff degradations on (+)-glucose produced (+)-glyceraldehyde. At this point, he predicted that(+)-glyceraldehyde has the OH group on the right when drawn as aFischer projection with C1 at the top. With this assumption, he couldeliminate eight of the possible structures for (+)-glucose. Thus, heexcluded the eight enantiomers with the OH group on the left in the


7/28/2019 24 Sugar

27/51


Fischer projection from further study. Although he knew that hisprediction had only a 50/50 chance of being correct, he also knew thateven if he was wrong, his logic was correct. In the early 1950s, using

X-ray crystallography, other chemists showed that he had made thecorrect choiceindeed (+)-glyceraldehyde has the OH group on theright in a Fischer projection.

Fischer's second observation was that the Kiliani-Fischersynthesis with ()-arabinose formed a mixture of (+)-glucose and (+)-mannose. This observation meant that glucose and mannose were C2epimers. The fact that the Ruff degradation on either (+)-glucose or(+)-mannose produces ()-arabinose confirmed his observation.

His third observation was that nitric acid oxidizes ()-arabinoseinto an optically active aldaric acid. Of the four aldopentoses only twowould give optically active aldaric acids. Thus, ()-arabinose is eitherstructure B or D in Figure 24.5. Because the aldaric acids from A andC (Figure 24.5) have a plane of symmetry, they are not opticallyactive.

CHO

OHH

OHH

OHH

CH2OH

HNO3

COOH

OHH

OHH

OHH

COOH

Plane ofsymmetry

A

B

COOH

HHO

OHH

OHH

COOH

HNO3

CHO

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

OHH

CH2OH

HNO3

COOH

OHH

HHO

OHH

COOH

Plane ofsymmetry

C


7/28/2019 24 Sugar

28/51


CHO

HHO

HHO

OHH

CH2OH

HNO3

COOH

HHO

HHO

OHH

COOH

D

Figure 24.5. The possible aldopentoses from Fischers structural work on glucose.Each aldopentose also shows the structure of the aldaric acid produced by reactionwith nitric acid.

Fischer's fourth observation was that a Ruff degradation of ()-

arabinose produces ()-erythrose. Reaction of ()-erythrose with nitricacid produces optically inactive meso-tartaric acid. For ()-erythrose tobe optically inactive, its structure must have both of the OH groupson the same side. Fischer's observation showed that ()-arabinose isstructure B in Figure 24.5.

CHO

HHO

OHH

OHH

CH2OH

Plane ofsymmetry

(-)-Arabinose

CHO

OHH

OHH

CH2OH

(-)-Erythrose

COOH

OHH

OHH

COOH

HNO3Ruff

Degradation

meso-Tartaric acid

At this point Fischer proposed two structures (X and Y), C2epimers, for (+)-glucose and (+)-mannose. When subjected to Ruffdegradation, structures X and Y yield ()-arabinose.

CHO

HHO

OHH

OHH

CH2OH

(-)-Arabinose

RuffDegradation

HHO

OHH

OHH

CH2OH

CHO

HHO

HHO

OHH

OHH

CH2OH

CHO

OHH

OR

X Y


7/28/2019 24 Sugar

29/51


When reacted with nitric acid, these two structures, X and Y, bothgive optically active aldaric acids.

Next, Fischer developed a method that converts an aldehyde

group of the aldose to the CH2OH group and converts the CH2OHgroup to the aldehyde. The net result is to switch the two ends of themonosaccharide. With the ends switched, (+)-glucose produces adifferent sugar that degrades to ()-glyceraldehyde, and (+)-mannoseproduces (+)-mannose.

L-Gulose

180o

Rotate

CH2OH

CHO

H

H

H

H

HO

HO

HO

OH

Switch ends

CH2OH

CHO

OH

OH

OH

HO

H

H

H

H

X

HHO

OHH

OHH

CH2OH

CHO

OHH

D-Mannose

HHO

OHH

OHH

CH2OH

CHO

HHO

180o

RotateSwitch ends

CH2OH

CHO

H

OH

OH

HO

HO

H

H

H

Y

HHO

OHH

OHH

CH2OH

CHO

HHO

At that point, Fischer concluded that X is (+)-glucose and Y is (+)-mannose.

Using this method, Fischer determined the structures of 12 ofthe 16 aldohexoses. His work represents a remarkable achievementwhen you consider the chemical knowledge and laboratory toolsavailable to him to work with.

Exercise 24.12

An aldose, G, is optically active but, when treated with HNO3, givesan optically inactive aldaric acid. Ruff degradation of G gives H, whichalso gives an optically inactive aldaric acid with HNO3. Ruffdegradation of H produces D-glyceraldehyde. What are the structuresand names of G, H, and the aldaric acids?


7/28/2019 24 Sugar

30/51


24.8 Glycolysis - I

In the summer of 1896, a German chemist, M. Hahn, wasattempting to isolate some proteins from yeast. He ground the yeast ina mortar with fine sand and diatomaceous earth and filtered thismixture through cheesecloth, but the extracts were difficult topreserve. Hahn's colleague, Hans Buchner, remembering that fruitpreserves were made by adding sugar, suggested this as a method ofpreserving the extracts. When Hahn added the sucrose to the solution,bubbles formed in the solution. Buchner had demonstrated that a lifeprocess (fermentation) occurred outside a living cell. He hypothesizedthat this fermentation arose from the activity of an enzyme that hecalled zymase. Today biochemists call this process, which actually

involves 10 enzymes, glycolysis.Glycolysis is thebiochemical process

that converts glucose to

pyruvate and a small

amount of energy.

In the process of glycolysis, glucose, and a number of othermonosaccharides, form pyruvate and adenosine triphosphate (ATP).The organic portion connected to the phosphates in ATP is sometimesabbreviated asAdo.

Adenosine 5'-triphosphate (ATP)

N

NN

N

NH2

HH

H

OHOH

CH2O

H

OP

O

O

O

P

O

O

O

P

O

O

O

One molecule of glucose forms two molecules of pyruvate and twomolecules of ATP. ATP is an important molecule in biological systemsbecause it can be used as a source of energy in various reactions.

The net Go for glycolysis is 17.5 kcal/mole of glucose. Thus,glycolysis is a very exothermic process. The process of glycolysis

produces pyruvate, which is needed for the citric acid cycle. The citricacid cycle produces most of the energy for the organism. This sectionand the next examine glycolysis from the viewpoint of an organicchemist with the emphasis on the fact that most of the steps arefamiliar reactions.

The first step in the glycolysis process is a reaction that forms aphosphate ester at C6 of glucose. The phosphate ester forms via anucleophilic substitution reaction involving the C6 hydroxyl group of


7/28/2019 24 Sugar

31/51


glucose and the terminal phosphorus of ATP. The reaction requiresthe presence of a magnesium ion for the transfer to occur. Cells usethe enzyme hexokinase to catalyze the reaction.

Mg2

AdoO

OO O

O

PO

O O

O P PO

-Glucopyranose 6-phosphate-Glucopyranose

O OH

H

H

HO

OH

OHH

H

H

CH2OPO32

O OH

H

H

HO

OH

OHH

H

CH2OH

H

Next, -D-glucopyranose 6-phosphate isomerizes to -D-fructofuranose 6-phosphate. The enzyme glucose 6-phosphateisomerase catalyzes this reaction. Note that the mechanism dependson having an amine and carboxyl group fixed into the active site of theenzyme. Through a series of proton transfers, the enzyme catalyzesthe opening of the glucopyranose ring, the formation of the endiolintermediate, and the closing of the ring to form the fructofuranose.The curved lines in the mechanism represent those segments of theenzyme surrounding the active site.


7/28/2019 24 Sugar

32/51


H

H

CH2OPO32

HOH

O

HO

H

H

OH

OH

D-Fructose 6-phosphate

O

H

H

HO

O

OHH

CH2OH

CH2OPO32

H

OO

H

H2N

Enzyme

OO

H2NH

Enzyme

Endiol intermediate

D-Glucose 6-phosphate

Enzyme

H2N

OO

H

OH O

H

H

HO

OH

OHH H

H

CH2OPO32

OO

H2N

HEnzyme

-D-Glucopyranose 6-phosphate

O O

H

H

HO

OH

OHH

H

H

CH2OPO32

H

O

H

OH

OH

CH2OH

OH

H

H

2 O3POCH2

-D-Fructofuranose 6-phosphate

The third step in glycolysis is the transfer of another phosphategroup from ATP to -D-fructofuranose 6-phosphate to form -D-fructofuranose 1,6-bisphosphate. The phosphofructokinase-1 enzymecatalyzes this step of the reaction.

-D-Fructofuranose 6-phosphate

O

H

OH

OH

CH2OH

OH

H

H

2

O3POCH2 O

H

OH

OH

OH

H

H

2 O3POCH2

CH2OPO32

ATP

Phosphofructokinase-1

-D-Fructofuranose 1,6-bisphosphate


7/28/2019 24 Sugar

33/51


The formation of -D-fructofuranose 1,6-bisphosphate is therate-determining step in glycolysis. Thus, phosphofructokinase-1 isthe critical enzyme in the regulation of glycolysis. Although the

regulation of glucose is different in different organisms, a significantregulator is the presence of-D-fructofuranose 2,6-bisphosphate. Thismolecule signals the presence of glucose in the system and increasesthe activity of the phosphofructokinase-1 enzyme. An absence of-D-fructofuranose 2,6-bisphosphate signals an absence of glucose anddecreases the activity of the enzyme.

-D-Fructofuranose 2,6-bisphosphate

O

H

OH

OH

H

H

2 O3POCH2

CH2OH

OPO32

In the next step in glycolysis, the enzyme aldolase splits D-fructose 1,6-bisphosphate (a hexose) into dihydroxyacetone phosphateand D-glyceraldehyde 3-phosphate (two trioses). As its name suggests,aldolase is an enzyme that catalyzes an aldol condensation reaction.In this step, the reaction of interest is the reverse aldol reaction.

HHO

OHH

C O

CH2OPO32

OHH

CH2OPO32

CH2OH

C O

CH2OPO32

CHO

OHH

CH2OPO32

+

D-Fructose 1,6-bisphosphate

Dihydroxyacetonephosphate

D-Glyceraldehyde3-phosphate

Aldolase

The mechanism for the reverse aldol condensation reaction

involves a series of four steps. In these steps the D-fructose 1,6-bisphosphate interacts with several different amino acid residues atthe active site of the enzyme. The first step is the nucleophilic additionto the ketone of the fructose by the -amine group of a lysine residueat the active site. This enzyme-fructose complex has the atoms alloriented properly to allow the reverse aldol condensation to occur. Thenext step is the actual reverse aldol producing D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The final two steps are a


7/28/2019 24 Sugar

34/51


proton transfer and hydrolysis of the enzyme-dihydroxyacetonecomplex.

H2O

+ Enzyme

HHO

C

CH2OPO32

N(CH2)4

H

HHO

C

CH2OPO32

N(CH2)4

HHO

OH

C

CH2OPO32

OHH

N(CH2)4

H

CH2OPO32

HHO

OHH

C O

CH2OPO32

OHH

CH2OPO32

CH2OH

C O

CH2OPO32

H OH

CHO

CH2OPO32

+

Lys

Lys

Cys

His

H2N

S

H

H2N(CH2)4

H3N

HN NH

Enzyme

Lys

Lys

Cys

His

H3N

S

H3N

HN NH

Enzyme

Lys

Lys

Cys

His

H3N

SH

H3N

HN NH

Enzyme

Lys

Lys

Cys

His

H3N

SH

H3N

HN N

Enzyme

The formation of D-glyceraldehyde 3-phosphate anddihydroxyacetone phosphate completes what biochemists often call thefirst stage of glycolysis. This first stage consumes two molecules of

ATP to produce two molecules of ADP. The second stage, which isdiscussed in Section 24.9, produces four molecules of ATP from ADP.The Go for the first stage of glycolysis is slightly exothermic at 1.3kcal/mole of glucose. The Go for the second stage is much moreexothermic at 16.2 kcal/mole of glucose.

Exercise 24.13


7/28/2019 24 Sugar

35/51


What is the effect on the rate of glycolysis as the concentration of D-glucopyranose 6-phosphate changes? How is the rate affected byconcentration changes of D-fructofuranose 1,6-bisphosphate? D-

Fructofuranose 2,6-bisphosphate?

24.9 Glycolysis - II

The second stage of glycolysis involves the transformation ofglyceraldehyde 3-phosphate and dihydroxyacetone phosphate topyruvate. What then happens to the pyruvate in the metabolic processdepends on the cell that generated the pyruvate and on theavailability of oxygen. Under anaerobic conditions the pyruvate isreduced to lactic acid. Under aerobic conditions the pyruvate entersthe citric acid cycle. Further detailed descriptions of pyruvate and the

citric acid cycle are beyond the scope of this book.

Anaerobic literally

means without air or

oxygen.

An aerobic process

requires oxygen. In the first step in the second stage of glycolysis is theconversion of dihydroxyacetone phosphate to D-glyceraldehyde 3-phosphate. Triose phosphate isomerase catalyzes the reaction. Themechanism of the reaction is similar to the mechanism of glucose 6-phosphate isomerase, as described in Section 24.8.


7/28/2019 24 Sugar

36/51


C

C O

CH2OPO32

OH

H

H

C

C OH

CH2OPO32

O

H

H

C

C OH

CH2OPO32

O

H

H + Enzyme

C

O

O

NH2

H

Enzyme

Lys

Glu

C

O

OH

NH2

Enzyme

Lys

Glu


Dihydroxyacetonephosphate

Both halves of the glucose molecule are now the same. Thismakes it possible to consider the rest of the glycolysis as happening onone molecule, even though each molecule of glucose actually producestwo molecules ofD-glyceraldehyde 3-phosphate.

The next step in glycolysis requires both an enzyme and acoenzyme to catalyze the reaction. The active site of the enzyme fitsboth the coenzyme and the substrate. The enzyme, glyceraldehyde 3-phosphate dehydrogenase, catalyzes the oxidation ofD-glyceraldehyde

3-phosphate to 1,3-bisphospho-D-glycerate.


7/28/2019 24 Sugar

37/51


CH2OPO32

OHH

O

CH2OPO32

OHH

CH2OPO32

O

HGlyceraldehyde

3-phosphate

dehydrogenase

D-Glyceraldehyde 3-phosphate 1,3-Bisphospho-D-glycerate

The mechanism of the oxidation of D-glyceraldehyde requiresthat the active site of the enzyme first bind to a molecule of NAD+.NAD+ is a coenzyme and is a biological oxidizing agent. After theNAD+ binds to the enzyme, the molecule of D-glyceraldehyde 3-phosphate enters the active site.

Coenzymes were

introduced in Section

23.10, page 000.

NADHNAD+

N

O

NH2

R

HH

H

N

O

NH2

R

The oxidation of D-glyceraldehyde means that NAD+ is reduced toNADH. In the process, NAD+ loses its aromaticity. As a result NADH

readily loses a Hc- to reform NAD+. This reaction is important becauseof the reactivity of the hydride ion in aqueous systems. Most of thehydride sources (e.g. LiAlH4) you have studied react violently withwater. There are only a limited number of reagents that can act as ahydride donor in aqueous solution.

The transformation of D-glyceraldehyde 3-phosphate to 1,3-bisphospho-D-glycerate is a key reaction in glycolysis. Simultaneously,an aldehyde is oxidized to a carboxylic acid and inorganic phosphate isincorporated. The product of this transformation is energy-rich. Thistransformation is essential to the formation of ATP from ADP.

Once the NAD+ coenzyme enters the active site of the enzyme,the next step is the nucleophilic addition of the SH group of acysteine to the carbonyl of the D-glyceraldehyde 3-phosphate. TheNAD+ removes an Hc- from the carbon bearing the sulfur. Inorganicphosphate (HPO42c- ) then attacks the carbonyl carbon to form the 1,3-bisphospho-D-glycerate.


7/28/2019 24 Sugar

38/51


+ Enzyme

C

OHH

O OPO32

CH2OPO32

P

O

O

O

O

S

H OH

C

O

CH2OPO32

P

O

OH

O

O

N NHH

CO

OHH

H

CH2OPO32

N

O

NH2

R

H

SH

Enzyme

His

Cys

N NH

N

O

NH2

R

H

S

H OH

OC

H H

CH2OPO32

Enzyme

His

Cys

N NHH

N

O

NH2

R

H H

Enzyme

His

CysC

O

OHH

S

CH2OPO32

N NHH

N

O

NH2

R

H H

Enzyme

His

Cys

NAD+


NADH

1,3-Bisphospho-D-glycerate

In the next step, the NADH dissociates from the enzyme. After

the dissociation of the NADH, another molecule of NAD+ binds to theenzyme, and the catalytic cycle begins again. Phosphoglycerate kinasetransfers the phosphate group introduced into the product to amolecule of ADP; thus, regenerating ATP and producing 3-phospho-D-glycerate.


7/28/2019 24 Sugar

39/51


kinase

Phosphoglycerate

C

OHH

CH2OPO32

O O

+ ATP+ ADP

C

OHH

CH2OPO32

O OPO32

1,3-Bisphospho-D-glycerate 3-Phospho-D-glycerate

Glycolysis has three more steps. In the first step, the enzymephosphoglycerate mutase transfers the phosphate group in 3-phospho-D-glycerate to C2 to form 2-phospho-D-glycerate.

3-Phospho-D-glycerate

mutasePhosphoglycerate

C

H

O O

OPO32

CH2OH

C

OHH

CH2OPO32

O O


In the next step, the enzyme enolase converts 2-phospho-D-glycerate to phosphoenolpyruvate. The active site of enolase contains amanganese(II) ion (Mn2), which acts as a Lewis acid accepting theelectron pair from the OH group on C3 of 2-phospho-D-glycerate.This interaction forms phosphoenolpyruvate.

CH2

OOC OPO32

NH2

C

H

O O

OPO32

CH2 OH

EnzymeMn2 Phosphoenolpyruvate

Phosphoenolpyruvate, an energy-rich compound, readilydonates its phosphate group to ADP to form ATP and pyruvate. Thisstep is the end of glycolysis. However, this step is not the end of theenergy production from glucose. Under aerobic conditions, thepyruvate enters the citric acid cycle to produce a number of additionalmolecules of ATP. Under anaerobic conditions, pyruvate forms lacticacid or ethanol and CO2 and regenerates essential NAD+, thusallowing further glycolysis to occur.


7/28/2019 24 Sugar

40/51


++ ATPADPkinase

Pyruvate

CH3

OOOC

CH2

OOC OPO32

Phosphoenolpyruvate Pyruvate

Exercise 24.14

Using a sample of glucose labeled at C1 with 14C and incubated withthe Hahn's zymase extract, where would you find the label inpyruvate?

[Sidebar]

Arsenic Poisoning

For centuries arsenic has had an aura of mystery and horror asan ultimate poison. Literature tells of many fictional murders inwhich unwitting victims had some arsenic placed in their food ordrink. Arsenic (As2O3) is a white solid with an appearance similar topowdered sugar or flour. When prepared in food, arsenic is somewhatwater-soluble and nearly tasteless.

White arsenic has been widely used in a variety of ways fromweed killer and sheep-dip to fruit spray and rat poison. In the 18th

and 19th centuries physicians used arsenic to cure a variety ofillnesses. Around the beginning of the 20th century, physicians used itas a treatment for Graves' disease (hyperthyroidism). The prevalentthought was if white arsenic was toxic to the people, lowerconcentrations might harm the disease-causing agent more than thepatient. Unfortunately, some patients received the wrong dosage.

The symptoms of a toxic dose of arsenic begin withgastrointestinal distress: diarrhea, vomiting, and abdominal pain.Death may occur within a few hours to a few days. However, chronicarsenic poisoning does not exhibit any gastrointestinal problems.Instead, the person experiences a gradual weight loss, a skin rash,and changes in skin pigmentation. In either case, arsenic affects theheart, kidneys, and other major organs. If death does not occur as adirect result of a toxic dose, it may occur because of the deteriorationof these systems.

Arsenic compounds made their way into chemical warfare aswell. Phenarsazine chloride was used in World War I as a war gas. Itwas dispersed in the air as minute particles or "smoke." It causes aprofuse watery nasal discharge; severe pain in the nose, sinuses, and


7/28/2019 24 Sugar

41/51


chest; nausea; vomiting; severe depression, and weakness. Profoundsensory disturbances occured later.

As

N

H

Cl

Phenarsazine chloride

The toxicity of arsenic is attributed to its chemical similarity tophosphorus. White arsenic is oxidized to the AsO

4

3c- (arsenate) ion,which is chemically very similar to the PO43c- (phosphate) ion, andcompetes with the phosphate ion in for D-glyceraldehyde 3-phosphatein the reaction that forms 1,3-bisphospho-D-glycerate. Instead, 1-arseno-3-phospho-D-glycerate forms, which readily hydrolyzes to 3-phospho-D-glycerate and an arsenate ion.

1-Arseno-3-phospho- D-glycerate

+ AsO43

H OH

CO O

CH2OPO32

H2OH OH

CO OAsO3

2

CH2OPO32


The formation of 3-phospho-d-glycerate from 1-arseno-3-phospho-d-glycerate allows glycolysis to proceed. However, the ATPthat forms in the conversion of 1,3-bisphospho-d-glycerate to 3-phospho-d-glycerate is lost. Thus, the presence of arsenate stops theproduction of ATP by glycolysis. Moreover, the supply of ATPdecreases because the earlier steps of glycolysis continue to use ATP.Cells that cannot regenerate ATP eventually die.

24.10 Glycoside Formation

An acetal forms when a hemiacetal reacts with an alcohol inthe presence of an acid catalyst.

Hemiacetal and acetal

formation was

discussed in Section

7.5, page 000.


7/28/2019 24 Sugar

42/51


C

OH

OR

COR

OR'R'OH

H

Hemiacetal Acetal

The cyclic forms of monosaccharides are hemiacetals. Treatment withan alcohol in the presence of an acid catalyst yields an acetal thatreplaces the anomeric hydroxyl group by an alkoxy group. Theproduct, called a glycoside, is an acetal. For example, the reaction of-D-glucopyranose with methanol and HCl forms methyl--D-glucopyranoside along with a small amount of methyl--D-glucopyranoside.

A glycoside, the cyclic

acetal form of a

monosaccharide,

converts the anomeric

OH group to an

ether, thus forming an

acetal.

+

Methyl -D-glucopyranoside

O

OH

H

CH2OH

HOHO

OCH3

CH3OH, HCl

Methyl -D-glucopyranoside-D-Glucopyranose

O

OH

OCH3

CH2OH

HOHO

H

O

OH

OH

CH2OH

HOHO

H

Biochemists name glycosides with a prefix of the alkyl groupfrom the alcohol and by changing the ose suffix of the specific sugarto oside. The glycoside of glucopyranose is glucopyranoside.Glycosides include aminals and thioacetals as well as acetals.Biochemists often term glycosides as O, N, or Sglycosidesdepending on which atom bonds to the anomeric carbon.

An aminal has a

nitrogen attached to

the anomeric carbon

instead of an oxygen of

a glycoside. A

thioacetal has sulfur.


7/28/2019 24 Sugar

43/51


Sinigrin, an S-glycoside, providesthe characteristic flavor of horseradishand mustard.

O

OH

HOHO

CH2OH

SNCH2CH CH2

H

NOSO3 K

Adenosine, an N-glycoside,the "A" part of ATP.

O

OHOH

HOCH2

H

H H

H

N

N

N

N

NH2

CH2

H

HOHO

OH

O

OCH

CN

O

H

CH2OH

HOHO

OH

O

Amygdalen, an O-glycoside, isfrom bitter almonds and the pitsof peaches and apricots.

O-Glycosidic bonds are important in the formation of oligomers

and polymers of monosaccharides. N-Glycosidic bonds are importantin the structure of ribonucleic acid (RNA) and deoxyribonucleic acid(DNA). Nucleotides contain an N-glycosidic link to the anomericcarbon. An example of the nucleotides is the nucleotide of -D-ribofuranose in RNA. In DNA, an H replaces the OH group on C2of the ribofuranose. RNA also has uracil instead of thymine in DNA.Thymine has a CH3 group on the ring instead of the H on uracil.The various nucleotides connect together by a phosphate diester groupto make either RNA or DNA. Table 24.1 lists the names of the variousbases and their corresponding ribonucleotides or deoxyribonucleotides.Figure 24.6 shows a segment of a hypothetical DNA molecule.

Chapter 25 covers

nucleotides in detail.

Base Structure NamesBase Nucleotide


7/28/2019 24 Sugar

44/51


Base Structure NamesBase Nucleotide

N

N

N

N

H

NH2

Adenine Adenosine

N

N

NH2

O

H

Cytosine Cytidine

N

N

N

N

O

H

NH2

Guanine Guanosine

N

N

H

O

H

O

CH3

Thymine Thymidine

N

N

H

O

H

O

Uracil Uridine

Table 24.1. The bases and their names as well as the names of the nucleotides inRNA and DNA.


7/28/2019 24 Sugar

45/51


H

H H

O

N

P

O O

O

O CH2

H

H H

O

N

P

O O

O

CH2

H

H H

O

N

P

OO

O

CH2

H H

HH

H H

O

O

O

N

O

CH2

O

OO

P

N

O

HH

H

NH

N

NNH

N

N

N

NH2

NH2

NH2

O

O

O

O

CH3

HH

Adenosine

Guanosine

Thymidine

Cytidine

Figure 24.6. A segment of a DNA molecule showing theN-glycosidic links with eachof the four nucleotides.

Exercise 24.15

Glycosides are relatively stable under both neutral and basicconditions, but they readily hydrolyze in acidic conditions. Usingmethyl -D-glucopyranoside as an example, write a mechanismillustrating the acid hydrolysis of glycosides.


7/28/2019 24 Sugar

46/51


24.11 Disaccharides

Disaccharides contain a glycosidic acetal bond that connectstwo monosaccharides. The most common link is between the anomericcarbon atom of one monosaccharide and C4 of the othermonosaccharide. This is called the 1,4' link. Another common link, the1,6' link, is between the anomeric carbon and C6. A link that formsbetween the two anomeric carbons is called the 1,1' or 1,2' links. Eachof the anomeric carbons has either the or the stereochemistry.Thus, the linkage of two anomeric carbons has six possiblestereochemical variants: ', ', , , ', or '. If the twomonosaccharides being linked arethe same, then there are only threepossibilities (, , or). Below are some examples of disaccharides.

Disaccharides have two

monosaccharides

connected by a

glycosidic bond.

O

CH2OH

CH2OH

OH

HO

O

O

OHHO

HO

CH2OH

O

OH

HOHO

CH2OH

CH2OH

OHO

OH

O

OH

CH2

HO

OH

O

OH

O

CH2OH

HOHO

OH

O

HO

Gentiobiose(1,6' Glycosidic link)

Cellobiose(1,4' Glycosidic link)

Sucrose(Link between anomeric carbons)

24.12 Polysaccharides

Polysaccharides are polymers made up of tens, hundreds, oreven thousands of monosaccharide repeating units. One example of apolysaccharide is cellulose, which is the main constituent of plant cellwalls. Two common cellulose-containing plant products are wood andcotton. Wood contains 40-50% cellulose; cotton contains 90%. Celluloseis a polymer of -D-glucopyranose with 1,4' O-glycosidic links. Woodhas approximately 10,000 glucose units per chain; cotton has about15,000.

A polysaccharide is a

polymer made up of

monosaccharide

repeating units linked

by glycosidic links.


7/28/2019 24 Sugar

47/51


O

OHHO

CH2OH

CH2OH

OHO

OH

O

O

O

O

OHHOO

CH2OHCH2OH

O

HO OH

O

Segment of cellulose(A 1,4'-O-( -D-glucopyranoside) polymer)

The 1,4'-O-(-D-glucopyranoside) polymer structure of celluloseis very stable because all the substituents are equatorial. The cellulosefibers consist of individual cellulose molecules held together byhydrogen bonding in long parallel chains. These groups of chains arecalled fibrils. Fibrils are approximately 200 in diameter andaccount for the rigidity of plant materials. Another major componentof wood is lignin. Lignin is a complex phenolic polymer that crosslinksthe fibrils, thereby greatly increasing the strength of the wood. Thestructure of lignin is only partially understood.

Fibrils are groups of

polysaccharide chains

held together byhydrogen bonding.

Fibrils account for the general chemical resistance of wood andother fibrous plant materials. Most chemical reagents cannotpenetrate the surface of the fibril structures because the hydrogenbonding is so strong that it does not allow disruption by mostreagents. Alkylation and acylation of some of the surface hydroxylgroups may take place, but the overall structure of the fibrils usuallyremains intact.

Under more extreme reaction conditions, the fibrils do

dissociate, which allows a number of the hydroxyl groups to react. Forexample, if you react cellulose with acetic acid followed by the additionof acetic anhydride with sulfuric acid, you get cellulose triacetate.Typically, the number of glucose repeating units in cellulose acetate isconsiderably lower than with the original cellulose. The number canbe as low as 200 repeating units. Industrial chemists use celluloseacetate in plastic and fiber production to produce acetate, rayon, oracetate rayon.

Segment of cellulose triacetate

O

O

OAcAcO

O

CH2OAc

CH2OAc

O

AcO

OAc

O


7/28/2019 24 Sugar

48/51


One cellulase, the enzyme -glucosidase, is found in somespecies of bacteria and protozoa. -Glucosidase hydrolyzes the -glycoside linkage between the glucose units of cellulose. Cud-chewing

animals (ruminants) host colonies of these bacteria in their stomachsand intestines. When a cow eats hay, the bacteria convertapproximately 20-30% of the cellulose to digestible carbohydrates.None of the enzymes found in mammals hydrolyze cellulose.Therefore, mammals cannot utilize the glucose found in cellulose.

Cellulases are a group

of related enzymes that

hydrolyze cellulose.

Starches are somewhat shorter polymers of glucose. 1,4'-O--Glycosidic bonds connect the glucose units. Starch from plant sourcescan be separated into two fractions. The first fraction, which isinsoluble in cold water, is called amylose. The second fraction, whichis soluble in cold water, is called amylopectin. A typical sample ofstarch consists of approximately 25% amylose and 75% amylopectin.

Except for its -glycosidic linkage, amylose is a linear polymer similarto cellulose.

Starch is a polymer

made up of a few

hundred

monosaccharide units.

Amylose is a starch

that is insoluble in cold

water.

Amylopectin is a starch

soluble in cold water.

Segment of amylose(A 1,4'-O-( -D-glucopyranoside) polymer)

CH2OH

HOOH

O

O

O

CH2OH

HOOH

O

O

OHHO

CH2OH

O

O

O

OHHO

O

CH2OH

The structure of amylopectin is more complex than thestructure of amylose. Amylopectin has 1,6'--glycoside branches every25 glucose units in the main chain. Thus, amylopectin has a complexthree-dimensional structure and a more compact shape than amylose.


7/28/2019 24 Sugar

49/51


Segment of amylopectin(1,6' Branched 1,4'-O-( -D-glucopyranoside) polymer)

O

OH

O

CH2OH

HOOH

O

O

O

OHHO

CH2

O

O

O

OHHO

O

CH2OH

O

OH

O

OCH2OH

CH2OHHO

HO

1,6'- -Glycosidic link

Animals have enzymes that hydrolyze starches. -Glycosidasesbegin the hydrolysis of starches in the mouth and continues it in thesmall intestine. These enzymes digest only the glycoside links instarch, not the glycoside links in cellulose. Thus, animals can eatgrains for energy, but not wood.

Exercise 24.16

How do the structural differences between amylose and amylopectinaccount for their different water solubilities?

Key Ideas from Chapter 24

Carbohydrates are a class of compounds with the generalmolecular formula Cm(H2O)n.

Monosaccharides, or simple sugars, have two general forms: 1)an open chain linear polyhydroxy aldehyde or ketone and 2) acyclic hemiacetal of the open chain molecule. Most

monosaccharides prefer the cyclic hemiacetal form to the openchain form.

A polyhydroxy aldehyde is called an aldose. A polyhydroxyketone is called a ketose.


7/28/2019 24 Sugar

50/51


Biochemists usually represent the stereochemistry of an openchain monosaccharide by a Fischer projection and the cyclicforms by a Haworth projection.

There are two groups of monosaccharides. One group degradesto D-glyceraldehyde. The other group degrades to L-glyceraldehyde. Most naturally occurring monosaccharidesdegrade to D-glyceraldehyde.

The D and L designations refer only to the stereochemistry ofthe monosaccharides and how they compare to glyceraldehyde.Except for glyceraldehyde, the designations have no connectionwith the direction of the rotation of a plane of polarized light.

A pair of monosaccharides that are epimers differ in thestereochemistry of only one carbon atom.

The reactions of monosaccharides are typical of the functionalgroups they contain, so they react like aldehydes, ketones, andalcohols.

Bases cause an epimerization at C2 via an enolate ion.

Esters form by treatment of the sugar with an acid anhydrideor acyl halide.

Ethers form by a variation of the Williamson ether synthesisthat use methyl iodide and silver oxide as the reagents.

Sodium borohydride and catalytic hydrogenations reduce thecarbonyl groups of monosaccharides to alcohols, thus formingpolyhydroxy alkanes called alditols.

Bromine in water oxidizes the aldehyde group in an aldose toan acid, thus forming an aldonic acid. An aldonic acid is apolyhydroxy carboxylic acid.

Nitric acid oxidizes both ends of an aldose to form a

dicarboxylic acid called an aldaric acid.

The Ruff degradation shortens the chain length of amonosaccharide by one carbon without affecting thestereochemistry of the other carbons.


7/28/2019 24 Sugar

51/51


The Kiliani-Fischer synthesis lengthens the chain of an aldoseby one carbon, thus forming a pair of C2 epimers one carbonlonger than the original aldose.

The Fischer proof of the structure of glucose used a series ofreactions and logical processes to prove the relativestereochemistry of each of the stereogenic centers found inglucose.

Glycolysis consists of a series of enzyme-catalyzed reactionsthat convert glucose to pyruvate. Glycolysis starts the processto provide heat and chemical energy to biological systems.

The first steps of glycolysis involve the isomerization of glucoseand the transfer of two phosphate groups from two molecules of

ATP. The products of the reaction are fructofuranose 1,6-bisphosphate and two molecules of ADP.

The enzyme aldolase splits the fructofuranose 1,6-bisphosphateinto two components. One of these components,dihydroxyacetone phosphate, is isomerized into the other D-glyceraldehyde 3-phosphate.

Once one molecule ofD-glucose is converted to two molecules ofD-glyceraldehyde 3-phosphate, a molecule of ADP and anotherof D-glyceraldehyde 3-phosphate is converted to pyruvate and

two molecules of ATP.

A glycoside forms when an alcohol, amine, or thiol reacts withthe hemiacetal form of a monosaccharide.

Disaccharides have two monosaccharides linked by a glycosidicbond. These glycosidic bonds are generally between theanomeric carbon of one monosaccharide and the hydroxyl groupat the anomeric site, or of C4 or C6, of another.

Polysaccharides are polymers of monosaccharides (usuallyglucose) linked by glycosidic bonds. Cellulose and starches are

two examples of polysaccharides.

24 sugar

Documents