carbohydrate ism gluconeogenesis,glycogenolysis

8/3/2019 Carbohydrate ism Gluconeogenesis,Glycogenolysis

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Carbohydrate Metabolism

Dr. Milind Dudhane

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Glycolysis.

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After being absorbed from the intestinal tract

the monosaccharides are carried by the portal

circulation directly to the liver.

In liver most of the D-Glucose is

phosphorylated to Glucose -6- phosphate which

can not diffuse back out of the cell as plasma

membrane is impermeable to the glucose -6-

phosphate.

Remaning glucose passes into systemic blood

supply.Other dietary monosaccharides D-fructose & D-

galactose are phosphorylated & may be

converted to glucose in the liver.

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Glucose -6- phosphate is an intermediate in severalmetabolic pathways that uses glucose in liver depe

Glycogen nding upon supply & demand.

Glucose-6-phosphateGlycogenesis

TG

Glycogen Pentose phosphate, NADPH

Pentose phosphatePathway

Pyruvate

Acetyl CoA

CO2 + H2O

Cholesterol

Fatty acid

Glycolysis

Blood Sugar

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Glycolysis (Embden Meyerhofpathway)Definition : Glycolysis is the sequence that

converts glucose into pyruvate in presence ofoxygen(aerobic) or lactate in absence of oxygen

(anarerobic) with the production of ATP

Location : Cytosol of cell

Reactions of Glycolysis:

The breakdown of glucose to two moles of

pyruvate is brought about by sequential action of

10 enzymes which can be devided into twophases.

First phase or energy requiring phase or preparative phase

First phase or energy requiring phase

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Phases ofGlycolytic pathwayGlucose

ATP

ATP

Fructose1,6 Bisphosphate

2- triose phosphate

2 ATP

2 ATP

2 NADH 6 ATP

2 Pyruvate

First phase:Energy

requiring phase

Second phase:Energy

releasing phase

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First phase or energy requiring phase orpreparative phase

1. Glucose is phosphorylated to Glu-6-phosphateby enzyme Hexokinase &ATP is required as

phosphate donar.[ Hexokinase occur in different

isoenzyme forms type I to IV]

Brain And Kidney Type I

Skeletal Muscles Type II

Adipose tissues Type I & II

Liver All types from I to

IV

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2. Conversion of glucose-6-phosphate to

fructose-6-phosphate by enzymephosphohexose isomerase is freely reversible

reaction.

3. fructose-6-phosphate is phophorylated to

fructose-1-6-bisphosphate byphosphofructokinase-I.

phosphofructokinase-Iis both allosteric and

inducible enzyme which is rate limiting &

regulatory enzyme of glycolysis.

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4. Fructose-1-6-bisphosphate is cleaved by

enzyme aldolase to two triose phosphates,glyceraldehyde-3-phosphate & dihydroxy

acetone phosphate(DHAP).

Several tissue specific isoenzymes of aldolase

exists.Aldolase A occurs in most tissues &aldolase B occurs in liver & kidney.

5. DHAP is isomerized to glyceraldehyde-3-

phosphate by the enzyme phosphotriose

isomerase,so that for every molecule of Glucoseentering glycolysis,2 moles of glyceraldehyde-3-

phosphate are formed.

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Second phase or energy releasing phase reactions1. Oxidation ofglyceraldehyde-3-phosphate to

1,3 bisphosphoglycerate by glyceraldehyde-3-

phosphate dH, an NAD dependent, reversible

reaction. NADH+

+ H+

are reoxidized by ETC togenerate 3 ATP molecules.

2. 1,3-bisphosphoglycerate to 3-

phosphoglycerate catalyzed by

phosphoglycerate kinase. This is the first step inglycolysis that generates ATP, by substrate level

phosphorylation(SLP).

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Since two molecules of triose phosphate are

formed per molecule of glucose 2 ATPs are

generated at this stage.Arsenate can uncouple

oxidation & phosphorylation at this step.

3. Phosphoglycerate mutase catalyze the

reversible reaction 3-phosphoglycerate to 2-phosphoglycerate.

4. Enolase converts 2-phosphoglycerate to

phosphoenol pyruvate. Enolase in inhibited by

fluoride.

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5. Conversion ofphosphoenol pyruvate to

pyruvate is brought about by Pyruvate

kinase(an allosteric enzyme)ATP is generated

by substrate level phosphorylation. (2 ATPs per

molecule of glucose).

Under aerobic condition pyruvate is takenup into mitochondria and after conversion to

acetyl Co A is oxidized to CO2 by TCA.

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Glucose Glucose-6- P

Fructose-6-

P P

P

Hexokinase

ATP Mg2+ ADP Phospho hexose isomerase

Fructose-1-6-bisphosphate

Dihydroxy acetoneGlyceraldehyde-3-

1,3-bisphosphoglycerate

3-Phosphoglycerate

Phospho fructokinaseATP

Mg2+

ADP

Aldolase

Phospho triose isomeraseGlyceraldehyde 3-

phosphate dH

NAD

NADH

Phospho glycerate kinase

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3-Phosphoglycerate

2 -Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Phosp

ho glycerate Mutase

EnolaseH2O

Pyruvate kinaseADP

Mg2+

ATP

Acetyl CoAPDH

Anaerobic

Oxidation

Lactate15


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Conversion of Pyruvate to Acetyl CoA

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Conversion of Pyruvate to Acetyl CoA occurs in

mitochondria & hence pyruvate must betranspoted into mitochondria by special pyruvate

transport.

Within the mitochondria pyruvate is oxidatively

decarboxylated to acetyl Coa by multienzymecomplex,consisting .

Enzymes : Pyruvate decarboxylate

Dihydrolipoate transacetylase &

Dihydrolipoate dH

Coenzymes: TPP, Lipoate, CoASH,FAD & NAD


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Oxidative decarboxylation ofPyruvate to by Pyruvate dH complex

..

O

II

CH3 C COO

Pyruvate

O

II

CH3 C SCoA

Acetyl CoA

NAD+ NADH + H+

PDH CO2

TPP, Lipoate, CoA-SH,FAD

Significance : Conversion of Pyruvate to Acetyl CoA is a

central step linking glycolitic pathway with TCA.

Acetyl CoA is also an important intermediate of lipid

metabolism, Cholesterol biosynthesis and acetylation

reactions.


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Pyruvate

-OH ethylthiamine pyrpphosphate

Thiamine pyrpphosphate

CO2

Oxidizedlipoate

Pyruvate

Decarboxylase

DihydrolipoateTransacetylase

Pyruvate

decarboxylase

Reducedlipoate

S-Acetyl lipoate

FADH2

FAD

DihydrolipoatedH

NADH + H+

NAD

3ATP

ETCCoASH CH3 C SCoA

II

OAcetyl CoA

Oxidative decarboxylation ofPyruvate to by Pyruvate dH complex


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Metabolic fates of Acetyl CoA

Xenobiotic metabolism

N-Acetylglutamate

in urea biosynthesis

Acetyl Choline Acetylation of aminosugar

Glycoprotein synthesis

Ketone bodies N-Acetylneuraminic acidganglioside synthesis

Acetyl CoA

Catabolism

TCA

cycleATPCholesterolSynthesis

CO2+ H2O

Fatty acidSynthesis


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Anaerobic Glycolysis :

If anaerobic condition prevail the reoxidation of

NADH(formed in glyceraldehyde-3-phosphatestep) by transfer of reducing equivalents through

the respiratory chain to oxygen is prevented. But

are reoxidized by conversion of pyruvate to lactate

By LDH.

Tissues that function under anaerobic

condition produce lactate,e.g. skeletal muscle,

smoth muscle & erythrocytes.

In erythrocytes even under aerobic conditions,

glycolysis terminates in lactate because of

absence of Mitochondria.

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The liver, kidney & heart usually take up

lactate & oxidize it under hypoxic condition. This

allows other active muscle cells to utilizeglucose.

During vigorous exercise the rate of

formation of pyruvate exceeds its utilization.Moreover rate of formation of NADH is greater

than its utilization. This accumulated NADH is to

oxidized to NAD in order to supply enough NAD

for glycolysis in skeletal muscle & erythrocytes.

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Significance ofAnaerobic Glycolysis :

The reoxidation of NADH via lactate

formation allows glycolysis to proceed in the

absence of oxygen by regenerating sufficient

NAD for another cycle of reaction catalyzed by

glyceraldehyde-3-phosphate dHAnaerobic glycolysis is an emergency source

of ATP.

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Energetics of Glycolysis:

The overall reaction of aerobic glycolysis alone

using either free glucose, fructose or galactose

yielding pyruvate generates:

2 molecules of NADH

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molecules of ATP at SLP But2 molecules of ATP per mole of hexose are

consumed.

The NET gain of2 moles of each NADH & ATP .

The NADH thus formed must be transportedto mitochondria so that it can be utilized there.

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Mitochondrial membrane is impermeable to

both NADH & NAD & hence the shuttle systems

namely.

Glycerol phosphate & Malate aspartate

shuttle which yield 2 &3 moles of ATP

respectively per mole of NADH.Further oxidation of Pyruvate to CO2 & H2O

yields 38 ATPs in citric acid cycle.

In anaerobicglycolysis on the other

hand only 2 moles of ATP& NONADHare produced.

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Consumption ofATPin Glycolysis

Reaction Reaction Catalyzed by No. of ATP

consumed/mole Glucose

Gluc to Gluc-6-

phosphate

Fruct-6-phosphateto Fruct-1,6-

bisphosphate

Hexokinase,

Glucokinase

Phosphofructokinase

-1

-1

Total = -2

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Production ofATP in Glycolysis

Reaction Reaction

catalyzed by

Mechanism

of ATP

Production

No.of ATP

formed/Gl

u molecule

Glyc-3-P to 1,3

bisphosphoglycerat

e

Glycaldehyde-

3-phsphate

dH

ETC

+ 6

1,3 bisphospho-

glycerate to

3-hosphoglycerate

Phosphoglycer

ate kinase

SLP

+2

Phosphoenol

pyruvate topyruvate

Pyruvate

kinase

SLP +2

Totalproduction

10

So the NET production is 10 2 = 8 ATP26


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Rapoport Lubering Cycle OR

Bisphosphoglycerate Shunt :

Mature erythrocytes contain no mitochondria.

So they are totally dependent upon glycolysis for

ATP production. Erythrocytes metabolizes

excessive amount of glucose in the glycolytic

pathway to maintain the structural integrity of the

erythrocyte membrane,resulting in generation of

more ATPs.Those are in excess & can not be

used by erythrocytes.This may inhibit glycolysisby inhibiting phosphofructokinase-I

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In Rapoport Lubering cycle ATP production

by SLP from 1,3 BPG is bypassed in RBCs.

1,3 BPG is converted to 2,3 BPG by an enzyme

bisphosphoglycerate mutase.

2,3 BPG is converted to 3-phosphoglycerate by

2,3 bisphosphoglycerate phosphatase with lossof energy in the form of heat. Due to lackof

mitochondria in RBCs glycolysis occur

anaerobically producing lactate.

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Rapoport Lubering cycle

.

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Glucose

Glyceraldehyde-3-phosphate

1,3-bisphosphoglycerate

3-phosphoglycerate

Pyruvate

NAD

Pi

NADH+ H+

Glyceraldehyde-3-phosphatedH

ADP

ATP 2,3-bisphosphoglycerate

Bisphosphoglyceratemutase

2,3BisphosphoglyceratephosphatasePi


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Significance of Rapoport Lubering cycle :

It prevents accumulation of ATP not needed by

the erythrocyte

It supplies 2,3-BPG in oxygen transport which is

required for Hb function.2,3-BPG regulatesthe

binding& release of oxygen from Hb. 2,3 BPG accounts for 16% of noncarbonate

buffer value.

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Krebs Cycle, TCA Cycle.

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. TCA CYCLE

(CITRIC ACIDOR KREB CYCLE OR COMMON

METABOLIC PATHWAY)

The TCA cycle is a series of cyclic reaction that

catalyse the oxidation of acetyl Co A to CO2

and water liberating reducing equivalents.


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.SITE :

The TCA cycle occurs inside the mitochondria asthe enzymes required for the cycle are located in

the mitochondria matrix, either free or attached

to the inner surface of inner mitochondria

membrane.The enzyme of TCA cycle are located in

mitochondria matrix, in close proximately to the

electron transport chain.


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.TCA CYCLE - THE CENTRAL METABOLIC

PATHWAY

The citric acid cycle is the final common

oxidative pathway for carbohydrates, fats and

amino acid. The cycle not only supplies energy

but also provides many intermediates requiredfor the synthesis of amino acids glucose etc.

Krebs cycle is the most important central

pathway.


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. TCA Cycle is an open cycle

Krebs cycle is the a cyclic process. It should notbe viewed as closed circle many compounds

enter the cycle and leaves. TCA cycle is

comparable to a heavy traffic circle in a national

highway with many connecting roads.Reactions of TCA cycle

Oxidative decarboxylation of pyruvate to

acetyl COA by pyruvate dehydrogenase complex

is discussed above. The TCA cycle connectingglycolysis.


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.Steps

The various steps of the cycle are as follows Step-1

Formation of citrate

Acetyl CoA condense with oxaloacetate to

from citrate. It is catalyzed by condensing

enzyme citrate synthase. the condensationproduct citryl CoA is hydrolysed to yield citrate

and CoA.

citrate synthase

Acetyl CoA + Oxaloacetate CitrateH2O CoASH


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. Step -2

Citrate is Isomerized to Isocitrate

Citrate is converted to isocitrate by the enzymeaconitase. the enzyme contain iron in in the from ofiron sulfur protein.

The conversion takes place in

2stage

First, there occur dehydration of citrate to cisaconitate.

Next, Cis aconitate undergoes rehydration to formisocitrate.

H2O H

2O

Citrate Cis-aconitase Isocitrate


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. STEP-3

Formation of - ketoglutarate

Isocitrate is converted to - ketoglutarate in 2

stage by the enzyme isocitrate dehydrogenase.

First, There occurs dehydrogenation of isocitrate

to from oxalosuccinate. NAD acts as onhydrogen acceptor. Next , Oxalosuccinate

undergoes decarboxylation to form

-ketoglutarate.

Isocotrate + NAD Oxalosuccinate +

NADH + H+

Oxalosuccinate - ketogutarateCO2


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. Step -4

Conversion of - ketoglutarate to succinyl CoA

-Ketoglutarate undergoes oxidative decarboxylation

to produce succinyl CoA. The enzyme is

- ketoglutarate dehydrogenase complex and the

reaction is irreversible.

the enzyme requires NAD ,FAD ,thiamine

pyrophosphate, lipoate and co-enzyme A as

cofactors similar to PDH complex

- ketoglutarate + CoA + NAD Succinyl CoA+NADH + H+ CO2

- ketoglutarate dHcomplex


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. Step 5

Formation of succinate

Succinyl Co A is converted to succinate by the

enzyme succinate thiokinase requires & GDP

converted GTP.

The enzyme nucleoside diphosphate kinase

converts GTP in to ATP.

Succinyl CoA + GDP Succinate + GTP + CoA

Succinyl thiokinaseGTP+ ADP ATP + GDP


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. Step -6

Conversion of succinate to formula :-

Succinate undergoes dehydrogenation to from

fumarate The enzyme is concerned is succinate

dehydrogenase which is found to inner surface

of inner mitochondrial membrane unlike otherenzymes of the cycle.

Succinate + FAD Fumarate + FADH2

The step is inhibited competitively by malonate

or oxaloacetate.


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. Step -7

Formation of Malate

A molecule of water is added to fumarate to form

malate. The enzyme required is fumarase which

is specific informing :- Isomer of malate.

FumaraseFumarate + H2O L-Malate


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. Step - 8

Conversion of malate to oxaloacetate

Malate undergoes dehydrogenation to form

oxaloacetate

the enzyme is malate dehydrogenase NAD as

the hydrogen acceptor.Malate dH

L-Malate + NAD Oxaloacetate + NADH


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. ENERGETICS OF GLUCOSE OXIDATION

When a molecule of glucose undergoes,glycolysis 2 molecules of pyruvate or located areproduced pyruvate is oxidatively decarboxylatedto acetyl Co A. Which enters the citric acid

cycle and gets completely oxidized to andglycolysis and citric acid cycle.

C6H12O6 + 6O2 +38 ATP +38 Pi 6 CO2 +6H2O +38 ATP


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. The enzyme of glucose metabolism responsible

for generating ATP.

When a molecule of glucose is burnt in a calorie

meter. In the living system energy is trapped

leading to the synthesis of 38 ATP which isequivalent to 1159 KJ.

The 48% of the energy in glucose combustion is

actually captured for ATP generation.


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. Regulation ofTCA

The cellular demands of ATP are crucial incontrolling the rate of citric acid cycle. The

regulation is brought about either by enzymes or

the levels of ADP.

Their enzymes Citrate synthase. In inhibited by ATP , NADH,

acyl CoA and succinyl CoA

Isocitrate dehydrogenase Is activated by ADP

and inhibited by ATP and NADH. and

- ketoglutarate dehydrogenase. Is inhibited by

succinyl CoA and NADH.


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. Amphibolic Nature of the TCA

The citric acid cycle is the final commonoxidative pathway in the living cell. The cycle

provides various intermediates for the synthesis

of many compounds needed by the body. Krebs

cycle is the both catabolic anabolic nature henceregarded as amphibolic.


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. The important synthetic reaction connected

with TCA cycle are given :-

1. Oxaloacetate and - Ketoglutarate,

respectively serves as precursors for the

synthesis of aspartate, glutamate which is turn

are required for the synthesis of other nonessential amino acid, Purines & Pyrimidines.

2. Succinyl CoA is used for the synthesis of

porphyrins andheme.

3. Mitochondrial citrate is transported to thecytosol were it is cleaved to provide acetyl CoA

for the biosynthesis of fatty acids.


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.Anaplersis orAnaplerotic Reaction

The synthesis reaction described abovedeplete the intermediates of citric acid cycle.

The reaction concerned to replenish or fill upthe intermediates of citric acid cycle are calledanaplerotic reaction or anaplerosis.

The important synthetic pathways that draw theintermediates of TCA cycle and the anapleroticreaction are described.

1. Pyruvate carboxylase catalyses the

conversion of pyruvate to oxaloacetate. This isan ATP dependent carboxylation reaction.


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.1. Pyruvate carboxylase catalyses the

conversion of pyruvate to oxaloacetate. This is

an ATP dependent carboxylation reaction.

2. Pyruvate is converted to malate by NADP+

dependent malate dehydrogenase.

3. Transamination is process where in an aminoacid transfers its amino groups to a keto acid

and itself gets converted to a keto acid.

4. - Ketoglutarate can also be synthesized form

glutamate by glutamate dehydrogenase action.The formation of - ketoglutarate and

oxaloacetate occurs by this mechanism.


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Pentose phosphate pathway

HMPShunt Pathway.

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This is aerobic pathway for the oxidation of glucose

in the liver , adipose tissue in addition to the

Embdonmeyorhop pathway for glycolysis.It isanabolic in nature , since it is concerned with the

biosynthesis of NAPDH and pentose's .

HMP shunt -a unique multifunctional pathway:

The enzyme of this pathway are present in

extra mitochondrial portion of the cell . This pathway

is active in adipose tissue , adrenal cortex, liver ,

thyroid , erythrocytes.

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Glu-6-P Glu-6-P Glu-6-P

6-Phosphogluconolactone 6-Phosphogluconolactone 6-Phosphogluconolactone

6-Phosphogluconate 6-Phosphogluconate 6-Phosphogluconate

Ribulose-5-P Ribulose-5-P Ribulose-5-P

NADP++ H2O

Mg2+ or Ca2+

NADP++ H2O NADP++ H2O

NADP++ H+ NADP++ H+ NADP++ H+

H2O

Mg2+ or Ca2+

NADP+

NADP + H+

NADP+

NADP + H+

NADP+

NADP + H+

CO2 CO2CO2

Glu-6-P

dH

6-Phospho

gluconolactone

hydratase

6-Phospho

gluconate

dH

P

H

A

S

E

I

Rib lose 5 P Rib lose 5 P Rib lose 5 P


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Ribulose-5-P Ribulose-5-P Ribulose-5-P

Ribulose-5-P

3-epimeraseKetoisomerase

Ribulose-5-P

3-epimeraseXylulose-5-P Xylulose-5-PRibose-5-P

Transketolase TPP

Glyceraldehyde-3-P Sedoheptulose-7-P

Transaldolase

Fructose-6-P Erythrose-4-P

Fructose-6-P Glyceraldehyde-3-P

Glucose-6-P Glucose-6-P 1/2 Glucose-6-P

P

H

A

S

E

II


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The pathway starts with glucose 6-phosphate

.As such no ATP is directly utilize or produced

in HMP pathway.

It is unique and multifunctional , since their

are several interconvertible substance

produced which proceed in different direction

in the metabolic reaction.

REACTIONS :

Reactions of HMP shunt is divided in to 2 phases:- Oxidative phase

Non-oxidative phase

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1. Oxidative phase : Glucose 6-phosphate

dehydrogenase (G6PD)is an NADP dependent

enzyme that converts glucose 6-phosphate to6-phospogluconate.the latter is then hydrolysed

by the gluconoactone hydrolase to

2. 6-phospogluconate.The next reaction involving

the synthesis of NADPH is catalysed by 6-

phosphogluconate which then undergoes

decaboxylation to give ribulose 5-phosphate.

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2.Non-oxidative phase :The non-oxidative

reaction are concerned with theinterconvertion of 3,4,5,&7carbon

monosaccharide . Ribulose 5-phosphate is

acted upon by an epimer to produce xylulose

5-phospate while ribose 5-phosphate

ketoisomerase converts ribulose5-phosphate

to ribose 5-phosphate.

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Significance of HMP shunt

HMP shunt is unique in generating twoimportant products Pentose & NADPH needed

for the biosynthetic reactions & other

function:

A)Importance ofPentose

In the HMP shunt hexos are converted into

pentose, the most important being ribose 5-

phosphate.The pentose or its derivative areuseful for the synthesis of nucleic acid & many

nucleotides such as ATP ,NAD+, FAD,& CoA.

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B) Importance ofNADPH

1) NADPH is required for the reductivebiosynthesis of fatty acid & steroids, hence

HMP shunt is more active in the tissue

concerned with lipogenesis . Example adipose

tissue, liver.

2)NADPH is used in the synthesis of certain

ammino acid involving the enzyme glutamate

dehydrogenase.

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3)Microsomal cytochrome P450 system(in

liver)brings about the detoxification of drugs &

foreign compounds by hydroxylation reactionsinvolving NADPH.

4)Phagocytosis is the engulfment of foreign

particles including microorganism , carried out

by W.B.C.The process requires the supply of

NADPH.

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5)Special function ofNADPH in RBC: NADPH

produced in erythrocytes has special function to

perform. It maintain the concentration ofreduced glutathione, which is essentially

required to preserve the integrity of RBC

membrane.

6)NADPH is also necessary to keep the ferrous

iron (Fe2+) of hemoglobin in the reduced state

so that accumulation of methemoglobin (Fe3+)

is prevented .

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G6PD deficiency is more severe in RBCs. HMPshunt is the only means of providing NADPH in

erythrocytes .

Decreased activity of G6PD impairs the

synthesis of NADPH in RBCs .

This results in the accumulation of

methamoglobin & peroxides in erythrocytes

leading to hemolysis .

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Glucose 6- Phosphate dehydrogenase

deficiency:


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The G6PD deficiency develop Hemolytic Anemia

.

If they are admired with drugs such as

primaquine (antimalaria), sulfamethaxoazole

(antibiotic), produce hemolytic jaundice in

patients.

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This is a genetic disorder associated with HMPshunt An alteration in Transketolase activity

reduce affinity with thiamine pyrophosphate is

the biochemical lesion.

The symptoms include the mental disorder, lossof memory & partial paralysis.

The symptoms are manifested whose diet are

vitamin deficient .

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Wernicke Korsakoff Syndrome :


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URONIC Acid Pathway

This is an alternative oxidative pathway for glucoseand is known as glucuronic acid pathway.

1.uronic acid pathway is concerned with the

production of glucuronic acid (involved in

detoxification)/pentose & vitamin C.2.In uronic acid pathway the free sugar or sugar

acids are involved

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F i & I f UDP Gl


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Glucose 6-phosphate is first converted to

glucose1-phosphate UDP-glucose is then

synthesized by the enzyme UDP-Glucose

Pyrophosporylase.

UDP- glucuronate is the metabolically active

form of glucuronate which is utilized for

conjugation with many substance like bilirubin ,

steroid hormones & certain drugs

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Formation & Importance of UDP- Glucuronate


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Several insoluble compounds are converted to

soluble ones through conjugation & further the

drugs are detoxified. UDP- glucuronate is also require for the

synthesis of glycosaminoglycans &

proteoglycan.

Conversion of UDP- Glucuronate to L- Gluonate

UDP- glucuronate loses its moiety in a hydrolytic

reaction & releases D glucuronate which is

reduced to L

gluonate by an NADPHdependent reaction.

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Effect ofdrugson Uronic acid pathway

Administration of drugs (barbital ,

chlorobutanol) significantly increase the

uronic acid pathway to achive more synthesisof glucouronate from glucose.Certain drugs

(aminopyrine , antipyrine)were found to

enhance the synthesis of ascorbic acid in rats.

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Gluconeogenesis

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Definition :The formation of Glucose or

Glycogen from non-carbohydrate precursors is

called gluconeogenesis.

The major non-carbohydrate substrates for

gluconeogenesis are

Lactate, Glycerol, Glucogenic amino acids

Propionate & Intermediates of citric acid cycle

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Significance : Blood sugar level is maintained

via gluconeogenesis.

D-glucose is absolutely necessary for the

tissues such as brain, erythrocytes, kidney &

eyes.

Glu-6-phosphatase is absent in muscle.

Therefore, during exercise and starvation, the

large amount of lactate produced by glycolysis

& glycerol generated by lipolysis of TGs areused up by gluconeogenesis.

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Location :

In animals, gluconeogenesis takes place

mainly in the liver. (85 95 %). In the cortex of kidneys during periods of

fasting, starvation, or intense exercise about50 %

The pathway can begin in the mitochondria orcytoplasm, depending on the substrate beingused.

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Characteristics of Gluconeogenesis :

Gluconeogenesis involves Glycolysis, Citricacid cycle plus some special reactions.

Glycolysis & Gluconeogenesis share the

same pathway but in opposite direction. Seven

reactions of glycolysis are common. Howeverthree reactions are irreversible involving

following enzymes.

Pyruvate carboxylase (Mitochondrial)

Phosphoenolpyruvate carboxykinase

Fructose-1,6 bisphosphatase and

Glucose -6-phosphatase81

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Pi Glucose ATP

Glucose-6-phosphatase Glucokinase

Hexokinase

H2O Glucose-6-phosphate ADP

Pi Fructose-6-phosphate ATPFructose-1,6-bisphosphatase Phosphofructokinase

H2O Fructose-1,6-bisphosphate ADP

DHAP

Glyceraldehyde-3-phosphate

Contd

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.

Glyceraldehyde-3-phosphate DHAP


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Glyceraldehyde-3-phosphate DHAP

NAD NADH +H+

Glycerol-3-phosphate dH

NADH +H+ NAD1,3-Bisphosphoglycerate Glycerol-3-phosphate

ADP

Glycerolkinase

ATP

3-phosphoglycerate Glycerol

2-phosphoglycerate

PhosphoenolPyruvate83

..

Phosphoenolpyruvate


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Phosphoenolpyruvate

CO2 ADP CYTOSOL

GDP Phosphoenolpyruvate Pyruvate kinase

GTP carboxykinase ATP

Pyruvate Lactate

Oxaloacetate NADH+H+ NAD+

Pyruvate 1

NADH+H+ ATP,CO2 Pyruvate Carboxylase

MDH ADP+Pi

NAD NADH+H+ Oxaloacetate 2

NAD

Malate Malate -KetoGlutarate 3

MITO.

5 Fumarate SuccinylCoA 4

Propionate84

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The Nos. allotted in above Pathway are for

Gluconeogenic amino acids are

( Gluconeogenesis from amino acids)

1.Pyruvate : Alanine, Glycine, Serine Cysteine,Threonine & Tryptophan

2.Oxaloacetate : Aspartate & Arginine

3. Ketoglutarate : Arginine, Glutamate, Glutamine,Histidine, Prolein

4.Succinyl CoA: Isoleucine, Methionine, Valine

5.Fumarate : Phenylalanine, Tyrosine

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Reactions of gluconeogenesis :


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Reactions of gluconeogenesis :

Only those reactions which are not common

to glycolysis are :

Reaction 1 : Carboxylation ofpyruvate

Pyruvate is converted to oxaloacetate with the

help of CO2, ATP & pyruvate carboxylase.Biotin,

Mg++, Mn++ are required. There is an absoluterequirement of acetyl CoA.

This reaction occurs in mitochondrial matrix.

Oxaloacetate produced in mitochondria cannot

cross the membrane. It is first reduced to

malate, which then crosses mito. Membrane

where it is re-oxidized to Oxaloacetate .

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.

Reaction 2 : Conversion of Oxaloacetate to


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Reaction 2 : Conversion of Oxaloacetate to

phosphoenolpyruvate :

The enzyme required is phosphoenolpyruvatecarboxykinase, it requires Mn++ & GTP. CO2 &

GDP are formed. The reaction occurs in cytosol.

Reversal of reactions of glycolysis now occurs

until fructose 1-6 bisphosphate.Reaction 3 : Dephosphorylation of fructose 1-6 bisphophate :

Fructose 1-6 bisphosphatase, the major

regulatory enzyme then forms fructose-6-phosphate.

The enzyme is activated by citrate & inhibited

by AMP and fructose2-6 bisphosphate87

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Reaction 4 : Dephosphorylation ofglucose 6

phosphate:

This reaction gives free Glucose & inorganicphosphate.

It is catalyzed by glucose 6phosphatase which

ispresent only in liver, kidney& epithelial

cells of small intestine.

The overall summary of Gluconeogenesis for

conversion of pyruvate to glucose.

2 Pyruvate + 4 ATP +2 GTP + 2 NADH+ 2H+ + 6H2O --

Glucose + 2 NAD + 4 ADP + 2 GDP + 6 Pi + 6 H+

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Gluconeogenesis from Glycerol :

Glycerol liberated by fat hydrolysis in

adipose tissues. In liver & kidneys glycerol is

activated to Glycerol-3- P by the enzyme

Glycerokinase (Which present in liver & kidneys

and not in adipose tissues).Glycerol-3-phosphate is then converted to DHAP which is

the intermediate of glycolysis by the enzyme

Gly.-3-P dH.

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.

Gluconeogenesis from Propionate :


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Gluconeogenesis from Propionate :

Oxidation of odd chain fatty acid & breakdown of

methionine & isoleucine yields propionyl CoA.This is carboxylated to form methyl malonyl CoA

which requires Biotin & ATP by propionyl CoA

carboxylase. It further forms Succinyl CoA, the

intermediate of TCAPropyonylCoA

Methyl malonyl CoA

Succinyl CoA90

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Gluconeogenesis from Lactate :

Lactate produced during anaerobic oxidation of

glucose in muscles. Due to absence of enzymes

Glucose-6-phosphatase & Fructose 1,6

bisphosphatase,

Lactate can not be used in muscle &to betransported to liver for Gluconeogenesis.

LDH

Pyruvate + NADH + H+ Lactate + NAD

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.

Regulation of Gluconeogenesis :


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Regulation of Gluconeogenesis :

The hormone Glucagon & the availability of

substrate mainly regulates gluconeogenesis.Glucagon stimulates gluconeogenesis by two

mechanisms.

1. Active form of pyruvate kinase is converted to its

inactive form. Decreased pyruvate kinase resultsin reduced conversion ofphosphoenol pyruvate

to pyruvate & former is diverted for synthesis of

glucose.

2. Glucagon reduces the concentration offructose 2,6 bisphosphate. This compound

allosterically inhibit phosphofructokinase and

activates fructose 1,6 bisphosphatase.94


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.


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Availability of substrate :

Glucogenic amino acids have stimulating effect

on gluconeogenesis. In conditions like diabetes

mellitus amino acids are mobilized from muscle

protein for the purpose of gluconeogenesis.

During starvation acetyl CoA accumulates inliver due to lypolysis. Acetyl CoA allosterically

activate pyruvate carboxylase resulting in

enhanced glucose production.

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Glycogen Metabolism

Glycogenesis & Glycogenolyss

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Glycogenesis

Glycogen is storage form of glucose in animals. It is stored mostly in liver (6-8 %) &

muscle(1-2 %).

Due to more muscle mass, the quantity ofglycogen in muscle(250 g) is about three times

higher than that in the liver (75 g).

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Glycogenesis:

Glycogenesis is the formation of

G

lycogenfrom Glucose. Glycogen is synthesized

depending on the demand for glucose and ATP

(energy)

If both are present in relatively high amounts,

then the excess of insulin promotes the

glucose conversion into glycogen for storage in

liver and muscle cells (In fed state).

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In the synthesis of glycogen, one ATP is

required per glucose incorporated into thepolymeric branched structure of glycogen.

Glucose-6-phosphate is synthesized directly

from glucose or as the end product of

gluconeogenesis.

The synthesis ofGlycogen from glucose takes

place in cytosol and requires ATP, UTP &

glucose.

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Reactions of Glycogenesis:

Glucose is phosphorylated to Glu-6-phosphate

catalyzed by hexokinase in muscle &glucokinase in liver.

Glu-6-phosphate is converted to Glu-1-

phosphate byph

osph

oglucomutase. Glu-1-phosphate reacts with uridine tri

phosphate (UTP) to form uridine di phosphate

glucose(UDPGlc) by the enzyme UDP-glucose

phosphorylase.

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By the action ofglycogen synthase the C1 of

activated glucose ofUDP-Glu form glycosidic

bond with C4 of terminal residue of glycogen

primer to 1-4 glycosidic linkage.

When the chain has been lenthened to a

minimum of 11 residues the branchingenzyme amylo-1,4 to 1-6 transglucosidase

transfers a part of the 1,4 chain minimum

lenth of 6 glucose residue to neighbouringchainto form -1,6 linkage as a branching

point.

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.


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Shifting

New 1,6 bond

Glycogen Branching

Primer enzyme

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Glycogenolysis

.

105

The conversion of glycogen to glucose 6


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The conversion of glycogen to glucose -6-

phosphate and glucose, which occurs in Muscle

& liver respectively. Glycogenolysis is not reverse of glycogenesis

but is a separate pathway.

Reactions of Glycogenolysis

The enzyme glycogen phophorylase cleaves

-1,4 linkage yielding glu-1-phosphate &

residual Glycogen molecule. This continues til

about4

glucose residue remain on either side ofbranch point. The four unit chain is then

transferred to one branch by glucan transferase.

The debranchingenzyme splits -1,6 linkage106

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-1,4 bond

Glucagon Debranching

Phosphorylase Transferase enzyme

Pi Glu-1-phosphate

Free Glucose

107

.

N t l 1 h h t i t d t


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Next, glu-1-phosphate is converted to

glu-1-phosphate by thephosphoglucomutase.

In the liver but not in muscle, glu-6-phosphatase

removes phosphate to form glucose, which difuse

from cell into blood.

Regulation ofGlycogenesis & Glycogenolysis

Glycogen synthase-a, an active or

DEPHOSPHORYLATED form

Glycogen synthase-b, an inactive or

PHOSPHORYLATED form

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.

Regulation of Glycogenesis & Glycogenolysis


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g f y g y g y

GlycogenGlu-6-P

ATP

_ Insulin

_

+ _

GlycogenPhosphorylase cAMP Glycogen synthase

+

+ Glucagon, Epinephrine +

Ca++ ,AMP Glucose-1-phosphate Glu-6-phosphate

-

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.

.

110

I k l l l l i d d d


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In skeletal muscle glycogen is degraded toglucose 6-phosphate, which is then converted

into pyruvate and used in ATP productionduring glycolysis and the Krebs cycle .

However, pyruvate can also be converted, in

the liver, to glucose; thus muscle glycogen isindirectly a source of blood glucose.

111

.Reaction cascade for the controlofGlycogen Synthesis

Glucagon(Liver) ,Epinephrine(Muscle) ATP Insulin


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g ( ) p p ( )

Inactive adenylate Active adenylate Phosphodiesterase

cyclase cyclase c-AMP 5-AMP

Active c-AMP dependent Inactive c-AMP dependent

Protein kinase Protein kinase

ATP ADP

Insulin

Active Glycogen Inactive Glycogen

synthase a Glu-6-Phsphate synthase b

Pi H2O

Insulin Protein phosphatase - 1 Glycogen synthesis

+ + +

+

P

+

+

+

-

112

.Reaction cascade for the controlofGlycogen Breakdown

Glucagon(Liver) ,Epinephrine(Muscle) ATP Insulin


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Inactive adenylate Active adenylate Phosphodiesterase

cyclase cyclase c-AMP 5-AMP

Inactive c-AMP dependent Protein kinase

Active c-AMP dependent GlycogenPhosphorylase a

Protein kinase ADP activeATP

Ca2+

Inactive phosphorylase Active phosphorylase Glu-6-P

kinase kinase

Ca2+

Pi H2O Insulin

Protein phosphatase 1 GlycogenPhosphorylase b

ATP Inactive

+

P

-

+ +

+

+

+

Proteinphospha

tase

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Uronic acid pathway (Glucuronic acid cycle)


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Definition : A pathway in liver for conversion of

Glucose to Glucuronic acid & pentoses asreferred to as the uronic acid pathway.

Reactions of pathway:

Glucose is phosphorylated to Glu-1-phosphate

by phosphoglucomutase. with Glu-1-1P then reacts withUTP to form

Uridine diphosphate Glu (UDP-Glc) an active

nucleotide by UDP- glucose phosphorylase.

UDP-glucose is oxidized at carbon 6 to

glucuronate via UDP-glucuronate by UDP-

glucose dH ( NAD dependent)114

.

Glucuronate is reduced at carbon 1 to L-gulonate


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by NADPH dependent enzyme gulonate dH.

L-gulonate is precursor of ascorbate ( Vit C) in

those animals capable of synthesizing this vitamin.

In humans & other primates due to absence of

enzyme L-gulonolactone oxidase.

L-gulonate is oxidized and decarboxylated to thepentose, L-xylulose by the enzyme L-gulonate

decarboxylase.

L-xylulose is reduced to xylitol catalysed by

NADPH dependent L-xylulose dH.

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.

Xylitol is oxidised to D-xylitol by NAD dependent


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D-xylulose dH.

D-xylulose is phophorylated by ATP in thepresence ofXylulose kinase to yield xylulose-5-

phosphate, which is further metabolized in

pentose phosphate pathway & lead to formation of

glucose.

116

ron c ac pa way ucuron c ac cyc e

-D-Glucose-6-phosphatePentose phosphate pathway


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.Phosphoglucomutase

Glucose-1-phosphate

UTP

UDP

UDP-glu-pyrophosphorylase

Uridine diphosphate glucose(UDP-Glc)

Uridine diphosphate glucose(UDP-Glc)

UDP-glucose-dH 2NAD+

+ H2O

2NADH + 2H+

UDP

H2O

D-GlucuronateNADPH + H+

NADP

L-Gulonate

Gulonate-dH

D-Xylulose-5- phosphate

D-Xylulose

ADP

ATP

Mg+ +D-xylulose kinase

D-xylulose dH NADH + H+

NAD+

XylitolNADPH + H+

NADP

L-Xylulose

NAD+

NADH + H+

CO2

L-Gulonate-dH

Block inessentialpentosuria

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Thanks

carbohydrate ism gluconeogenesis,glycogenolysis

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