Cyclic Structure of Fructose

• As a ketohexose, fructose forms a 5-membered ring when the hydroxyl on C-5 reacts with the carbonly on C-2

CH2OH

C

C

C

C

CH2OH

O

HO H

OHH

OHH

D-Fructose

CH2OH

OH

OH

OH

CH2OHO

CH2OH

OH

OH

CH2OH

OHO

-D-Fructose -D-Fructose

Oxidation of Monosaccharides• Recall from Ch.15 that Benedict’s reagent (CuSO4) can oxidize

aldehydes with adjacent hydroxyl groups• The blue Cu2+ ions in the Benedict’s reagent are reduced to form

a brick-red precipitate, Cu2O• Normally, ketones are not oxidized, however ketones with an

adjacent hydroxyl group can rearrange to the aldehyde during reaction with Benedict’s reagent

• So, both aldoses and ketoses, in open chain form, can be oxidized by Benedict’s reagent to form carboxylic acids

• Sugars that can be thus oxidized are called reducing sugars

C

C

C

C

C

CH2OH

HO

OHH

HO H

OHH

OHH

C

C

C

C

C

CH2OH

OHO

OHH

HO H

OHH

OHH

+ Cu2+

D-Glucose D-Gluconic acid

+ Cu2O(s)

Reduction of Monosaccharides

• Reduction of the carbonyl group of a monosaccharide (in open-chain form) produces a sugar alcohol, or alditol

• D-Glucose is reduced to D-glucitol (also called D-sorbitol) using hydrogenation (H2 and a metal catalyst)

Formation of glycosides• Recall that an alcohol can react with a hemiacetal to form

an acetal (a di-ether)• When an alcohol reacts with a cyclic hemiacetal of a

monosaccharide the cyclic acetal product is called a glycoside

• The new ether bond is called a glycosidic bond• Monosaccharides are linked together by glycosidic bonds

to form disaccharides and polysaccharides• Alkyl glycosides can not undergo mutarotation, and so are

not reducing sugars

glycosidic bond

O

OH

OH

CH2OH

O

OH

CH3

HOCH3+

O

OH

OH

CH2OH

OH

OH

-D-Glucose Methanol Methyl--D-glucoside

Disaccharides• A disaccharide is formed when a hydroxyl group on one

monosaccharide reacts with the anomeric carbon of another monosaccharide to form a glycosidic bond

• Each disaccharide has a specific glycosidic linkage (depending on which hydroxyl reacts with which anomer)

• The three most common disaccharides are maltose, lactose and sucrose

• When hydrolyzed using acid or an enzyme, the following monosaccharides are produced:

Maltose + H2O D-Glucose + D-Glucose

Acid

orenzyme

Lactose + H2O D-Glucose + D-Galactose

Acid

orenzyme

Sucrose + H2O D-Glucose + D-Fructose

Acid

orenzyme

Maltose• Maltose (malt sugar or corn sugar) consists of two glucose

molecules linked by an -1,4-glycosidic bond• It comes from partial hydrolysis of starch by the enzyme

amylase, which is in saliva and also in grains (like barley)

• Maltose can be fermented by yeast to produce ethanol• Maltose is also used in cereals, candies and malted milk• Because one of the glucose molecules is a hemiacetal, it can undergo mutorotation, and so maltose is a reducing sugar

Lactose• Lactose (milk sugar) consists of one glucose molecule and

one galactose molecule linked by a -1,4 glycosidic bond• It comes from milk products (about 4-5% of cow’s milk)

• Because the glucose is a hemiacetal, it can undergo mutorotation, and so lactose is a reducing sugar

Hydrolysis of Lactose• Some people don’t produce enough lactase, the enzyme

that hydrolyzes lactose, and so can’t digest lactose• Many adults become lactose intolerant, and develop

abdominal cramps, nausea and diarrhea• Lactase can be added to milk products (or taken as a

supplement) to combat this problem

Sucrose• Sucrose (table sugar) consists of one glucose molecule and

one fructose molecule linked by an ,-1,2-glycosidic bond• Sucrose is the most abundant disaccharide and is

commercially produced from sugar cane and sugar beets• Because the glycosidic bond in sucrose involves both

anomeric carbons, neither monosaccharide can undergo mutorotation, and so sucrose is not a reducing sugar

CH2OH

OHCH2OH

OH

OHO

O

OH OH

OH

OH

CH2OH

alpha-D-Glucose

beta-D-Fructose

1

2

O

OH

OH

OH

CH2OH

1

CH2OH

CH2OH

OH

OHO

2

O

+alpha,beta-1,2-glycosidic bond

Sucrose

Hydrolysis of Sucrose

• Sucrose is hydrolyzed by the enzyme sucrase, which is secreted in the small intestine

• The glucose and fructose can then be absorbed into the bloodstream (disaccharides are too large to be absorbed)

Fermentation• A fermentation is defined as an energy-yielding

metabolic pathway with no net change in the oxidation state of products as compared to substrates

• Yeast can ferment glucose, fructose, maltose and sucrose• Ultimately, glucose is converted to pyruvate through

glycolosis, and the pyruvate is then converted to CO2 and ethanol by a two-step enzymatic process

• The net reaction is:

C6H12O6 2C2H5OH + 2CO2

O

O

Opyruvate

decarboxylase

H+ CO2

H

O

OHalcohol

dehydrogenase

NADH + H+ NAD+

Pyruvate Acetaldehyde Ethanol

Polysaccharides

• A polysaccharide is a polymer consisting of hundreds to thousands of monosaccharides joined together by glycosidic linkages

• Three biologically important polysaccharides are starch, glycogen and cellulose

- all three are polymers of D-glucose, but they differ in the type of glycosidic bond and/or the amount of branching

• Starch and glycogen are used for storage of carbohydrates

- starch is found in plants and glycogen in animals

- the polymers take up less room than would the individual glucose molecules, so are more efficient for storage

• Cellulose is a structural material used in formation of cell walls in plants

Plant Starch (Amylose and Amylopectin)• Starch contains a mixture of amylose and amylopectin• Amylose is an unbranched polymer (forms -helix) of D-

glucose molecules linked by -1,4-glycosidic bonds• Amylopectin is like amylose, but has extensive branching,

with the branches using -1,6-glycosidic bonds

Glycogen and Cellulose• Glycogen (animal starch) is like amylopectin, except it’s even

more highly branched- animals store glycogen in the liver (about a one-day supply in humans) and use it to maintain fairly constant blood sugar levels between meals

• Cellulose is an unbranched polymer of D-glucose molecules linked by -1,4-glycosidic bonds- cellulose forms -sheets of parallel strands held together by hydrogen bonding- we don’t have the enzyme to break down cellulose- some animals have microorganisms that do have the enzyme

Iodine Test for Starch

• The presence of starch can easily be identified using iodine (I2)

• Rows of iodine atoms form in the core of the -helix of amylose, forming a dark blue complex

• Because amylopectin, glycogen and cellulose do not form -helices, they do not complex well with iodine, so do not show the blue color (they show a purple or brown color)

• Monosaccharides do not interact with the iodine, so no color is produced