Chapter 4 Carbohydrates Metabolism

1 OverviewCarbohydrates in general are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.

Biosignificance of CarbohydratesThe major source of carbon atoms and energy for living organisms.Supplying a huge array of metabolic intermediates for biosynthetic reactions. The structural elements in cell coat or connective tissues.

Glucose transporters (GLUT)GLUT1~5 GLUT1: RBC GLUT4: adipose tissue, muscle

The metabolism of glucoseglycolysisaerobic oxidationpentose phosphate pathwayglycogen synthesis and catabolismgluconeogenesis

glycogenGlycogenesisGlycogenolysisPentose phosphate pathwayRibose, NADPHGlycolysislactateH2O+CO2aerobic oxidationDigestion absorptionstarchLactate, amino acids, glycerolglucoseGluconeo-genesis

2 Glycolysis

GlycolysisThe anaerobic catabolic pathway by which a molecule of glucose is broken down into two molecules of lactate. glucose 2lactic acid (lack of O2)All of the enzymes of glycolysis locate in cytosol.

1. The procedure of glycolysisGpyruvatelactic acidglycolytic pathway

1) Glycolytic pathway : G pyruvate including 10 reactions.

G phosphorylated into glucose 6-phosphate Phosphorylated G cannot get out of cell Hexokinase , HK (4 isoenzymes) , glucokinase, GK in liver ;Irreversible .

hexokinase glucokinaseoccurrence in all tissues only in liverKm value 0.1mmol/L 10mmol/LSubstrate G, fructose, glucose mannoseRegulation G-6-P InsulinComparison of hexokinase and glucokinase

(2) G-6-P fructose 6-phosphate

(3) F-6-P fructose 1,6-bisphosphateThe second phosphorylation phosphofructokinase-1, PFK-1

(4) F-1,6-BP 2 Triose phosphatesReversible

(5) Triose phosphate isomerizationG2 molecule glyceraldehyde-3-phosphate, consume 2 ATP .

(6) Glyceraldehyde 3-phosphate glycerate 1,3-bisphosphate

(7) 1,3-BPG glycerate 3-phosphateSubstrate level phosphorylation

(8) Glycerate 3-phosphate glycerate 2-phosphate

(9) Glycerate 2-phosphate phosphoenol pyruvate

(10) PEP pyruvateSecond substrate level phosphorylationirreversible

2) Pyruvate lactate

Summary of Glycolysis

Total reaction: C6H12O6 + 2ADP + 2Pi 2CH3CHOHCOOH + 2ATP + 2H2OFormation of ATP: The net yield is 2 ~P or 2 molecules of ATP per glucose.

2. Regulation of GlycolysisThree key enzymes catalyze irreversible reactions : Hexokinase, Phosphofructokinase & Pyruvate Kinase.

1) PFK-1The reaction catalyzed by PFK-1 is usually the rate-limiting step of the Glycolysis pathway. This enzyme is regulated by covalent modification, allosteric regulation.

bifunctional enzyme

2) Pyruvate kinaseAllosteric regulation: F-1,6-BP acts as allosteric activator ATP and Ala in liver act as allosteric inhibitors;

Covalent modification: phosphorylated by Glucagon through cAMP and PKA and inhibited.

3) Hexokinase and glucokinaseThis enzyme is regulated by covalent modification, allosteric regulation and isoenzyme() regulation.Inhibited by its product G-6-P.Insulin induces synthesis of glucokinase.

3. Significance of glycolysis 1) Glycolysis is the emergency energy-yielding pathway. 2) Glycolysis is the main way to produce ATP in some tissues, even though the oxygen supply is sufficient, such as red blood cells, retina, testis, skin, medulla of kidney. In glycolysis, 1mol G produces 2mol lactic acid and 2mol ATP.

3 Aerobic Oxidation of Glucose

The process of complete oxidation of glucose to CO2 and water with liberation of energy as the form of ATP is named aerobic oxidation. The main pathway of G oxidation.

1. Process of aerobic oxidation

1) Oxidative decarboxylation of Pyruvate to Acetyl CoAirreversible;in mitochodria.

Pyruvate dehydrogenase complex: E1 pyruvate dehydrogenaseEs E2 dihydrolipoyl transacetylase E3 dihydrolipoyl dehydrogenase thiamine pyrophosphate, TPP (VB1) HSCoA (pantothenic acid) cofactors lipoic Acid NAD+ (Vpp) FAD (VB2)

HSCoANAD+Pyruvate dehydrogenase complex:

The structure of pyruvate dehydrogenase complex

HSCoA

CO2 CoASHNAD+NADH+H+

2) Tricarboxylic acid cycle, TCAC The cycle comprises the combination of a molecule of acetyl-CoA with oxaloacetate, resulting in the formation of a six-carbon tricarboxylic acid, citrate. There follows a series of reactions in the course of which two molecules of CO2 are released and oxaloacetate is regenerated.Also called citrate cycle or Krebs cycle.

(1) Process of reactions

Citrate cycle

Summary of Krebs Cycle Reducing equivalents

The net reaction of the TCAC:acetylCoA+3NAD++FAD+GDP+Pi+2H2O 2CO2+3NADH+3H++FADH2+GTP+ HSCoA Irreversible and aerobic reaction The enzymes are located in the mitochondrial matrix.

Anaplerotic reaction of oxaloacetate

(2) Bio-significance of TCAC Acts as the final common pathway for the oxidation of carbohydrates, lipids, and proteins. Serves as the crossroad for the interconversion among carbohydrates, lipids, and non-essential amino acids, and as a source of biosynthetic intermediates.

Krebs Cycle is at the hinge of metabolism.

2. ATP produced in the aerobic oxidation acetyl CoA TCAC : 3 (NADH+H+) + FADH2 + 1GTP 12 ATP.pyruvate acetyl CoA: NADH+H+ 3 ATP1 G 2 pyruvate : 2(NADH+H+) 6 or 8ATP 1mol G 36 or 38mol ATP 123 2 6 8 36 38

3. The regulation of aerobic oxidationThe Key Enzymes of aerobic oxidation The Key Enzymes of glycolysis Pyruvate Dehydrogenase Complex Citrate synthase Isocitrate dehydrogenase (rate-limiting ) -Ketoglutarate dehydrogenase

(1) Pyruvate dehydrogenase complex

(2) Citrate synthaseAllosteric activator: ADPAllosteric inhibitor: NADH, succinyl CoA, citrate, ATP(3) Isocitrate dehydrogenaseAllosteric activator: ADP, Ca2+Allosteric inhibitor: ATP(4) -Ketoglutarate dehydrogenaseSimilar with Pyruvate dehydrogenase complex

Oxidative phosphorylationTCAC

ATP/ADP inhibit TCAC, Oxidative phosphorylation ATP/ADPpromote TCAC Oxidative phosphorylation

4. Pasteur EffectUnder aerobic conditions, glycolysis is inhibited and this inhibitory effect of oxygen on glycolysis is known as Pasteur effect.The key point is NADH NADH mitochondria Pyr TCAC CO2H2O Pyr cant produce to lactate.

4 Pentose Phosphate Pathway

1. The procedure of pentose phosphate pathway/shunt

In cytosol

1) Oxidative Phase

2) Non-Oxidative PhaseRibose 5-pXylulose 5-pXylulose 5-pFructose 6-pGlyceraldehyde 3-pFructose 6-p Transketolase: requires TPP Transaldolase Glycolysis

The net reation:3G-6-P + 6NADP+ 2F-6-P + GAP + 6NADPH + H+ + 3CO22. Regulation of pentose phosphate pathwayGlucose-6-phosphate Dehydrogenase is the rate-limiting enzyme. NADPH/NADP+, inhibit; NADPH/NADP+, activate.

3. Significance of pentose Phosphate pathway1) To supply ribose 5-phosphate for bio-synthesis of nucleic acid;2) To supply NADPH as H-donor in metabolism; NADPH is very important reducing power for the synthesis of fatty acids and cholesterol, and amino acids, etc.

NADPH is the coenzyme of glutathione reductase to keep the normal level of reduced glutathione;

So, NADPH, glutathione and glutathione reductase together will preserved the integrity of RBC membrane.

Deficiency of glucose 6-phosphate dehydrogenase results in hemolytic anemia.

NADPH serves as the coenzyme of mixed function oxidases (mono-oxygenases). In liver this enzyme participates in biotransformation.

5 Glycogen synthesis and catabolism

Glycogen is a polymer of glucose residues linked by (14) glycosidic bonds, mainly (16) glycosidic bonds, at branch points.

The process of glycogenesis occurs in cytosol of liver and skeletal muscle mainly. 1. Glycogen synthesis (Glycogenesis)

UDPG: G active pattern, G active donor.In glycogen anabolism, 1 G consumes 2~P. Glycogen synthase: key E.

UDPG

Branching enzyme

Phosphorylase: key E;The end products: 85% of G-1-P and 15% of free G;There is no activity of glucose 6-phosphatase (G-6-Pase) in skeletal muscle.2. Glycogen catabolism (glycogenolysis)

Debranching enzyme: glucan transferase -1,6-glucosidase

Nonreducing ends(16) linkageGlycogen phosphorylase(16) glucosidase activity of debranching enzymeGlucoseTransferase activity of debranching enzyme

3. Regulation of glycogenesis and glycogenolysis1) Allosteric regulationIn liver: G phosphorylase glycogenolysisIn muscle:

2) Covalent modification

GlucagonepinephrineAdenylyl cyclasecAMPG proteinreceptorPKAglycogenolysisPhosphorylase Glycogen synthaseglycogenesisBlood sugar

quiz1NADH-p/oA0 B1 C2 D3

2( )ACoQ BCytb CCoA DNAD+

3A B C DCO2

4 A BFAD CNADP+ DNAD+

5A1,3 BC D6EA

6ATPA1112 B1314 C1516 D1718

7 A BH+C DATP

8TCAA BC D

6 Gluconeogenesis

Concept: The process of transformation of non-carbohydrates to glucose or glycogen Materials: lactate, glycerol, pyruvate and glucogenic amino acidSite: mainly liver, kidney.

Daily consumption of glucosebrain 120g Red blood cell 40gResting muscle 40gTotal 200gDaily available glucose: 150g (stored in liver as glycogen)only last for 12h

Gluconeogenesis Pathway essentially a reversal of glycolysis but three energy barriers obstructing a simple reversal of glycolysis.

Gluconeogenesis and glycolysis

1,

3-

NAD+

2 3-

NADH H

1.3-

ADP

2

3-

2

H20 ADP ATP

2- 2

G= -0.6kcal/mol

()

G= -0.4kcal/mol

()

1

G= +0.3kcal/mol

()

ATP

G= +0.2kcal/mol G=-0.8kcal/mol

()

2

2

G= -4.0kcal/mol ()

carboxylation of Pyruvate

pyruvate carboxylaseBiotin as coenzyme

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biotin

N subject to carboxylation

lysine residue

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_968414373.cdx

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carboxybiotin

oxaloacetate

pyruvate

biotin

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_1061636507.cdx

_1061636588.cdx

_1061636655.cdx

_1061636210.cdx

_1061635428.cdx

_1061635895.cdx

_1061634596.cdx

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_968414373.cdx

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pyruvate oxaloacetate PEP

Pyruvate Carboxylase PEP Carboxykinase

_1001272491.cdx

16-23

ATP

6- 1-

6-

ATP

16

3-

G= -7.5kcal/mol

()

ADP

G= -0.6kcal/mol

()

G= -5.0kcal/mol

()

ADP

G= -0.3kcal/mol

()

Two Glycolysis irreversible reactions are bypassed by simple hydrolysis reactions:

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fructose-1,6-bisphosphate fructose-6-phosphate

Fructose-1,6-bisphosphatase

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glucose-6-phosphate glucose

Glucose-6-phosphatase

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Summary of Gluconeogenesis Pathway:.

glyceraldehyde-3-phosphate

NAD+ + Pi

NADH + H+

1,3-bisphosphoglycerate

ADP

ATP

3-phosphoglycerate

Phosphoglycerate Mutase

2-phosphoglycerate

H2O

phosphoenolpyruvate

CO2 + GDP

GTP

oxaloacetate

Pi + ADP

HCO3 + ATP

pyruvate

Glyceraldehyde-3-phosphate

Dehydrogenase

Phosphoglycerate Kinase

Enolase

PEP Carboxykinase

Pyruvate Carboxylase

Gluconeogenesis

glucose

Gluconeogenesis

Pi

H2O

glucose-6-phosphate

Phosphoglucose Isomerase

fructose-6-phosphate

Pi

H2O

fructose-1,6-bisphosphate

Aldolase

glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate

Triosephosphate

Isomerase

(continued)

Glucose-6-phosphatase

Fructose-1,6-bisphosphatase

gluconeogenesis

Key enzymes of gluconeogenesis PEP carboxykinase Pyr carboxylase Fructose-bisphosphatase Glucose-6-phosphatase

What Materials can be Transformed to Glucose by Gluconeogenesis?1All those that can be transformed to pyruvate and any intermediate in TCAC, via oxaloacetate.

Glucogenic AA(mainly precursor when starving)

cellulosefermentationgluconeogenesisglucose

2glycerol

3lactate Cori

Lactic acid (Cori) cycleLactate, formed by the oxidation of glucose in skeletal muscle and by blood, is transported to the liver where it re-forms glucose, which again becomes available via the circulation for oxidation in the tissues. This process is known as the lactic acid cycle or Cori cycle.prevent acidosisreused lactate

Lactic acid cycle

Gluconeogenesis is energy consuming

1pyruvate ATP HCO3 oxaloacetate ADP Pi H

2 GTP PEP CO2 GDP

33P ATP 13 ADP Pi

2 pyruvates glucoseuse 6 ATP 1 glucose 2 pyruvate, net productioin 2 ATP

Net ATP consumption: 4ATP

So, whats the significance of lactic acid cycle?

3. Significance of gluconeogenesisReplenishment() of Glucose by Gluconeogenesis and Maintaining Normal Blood Sugar Level.Replenishment of Liver Glycogen.Regulation of Acid-base Balance.Beneficial for prey() and predatorIn survival competition

6 Blood Sugar and Its Regulation

1. The source and fate of blood sugar

Blood sugar level must be maintained within a limited range to ensure the supply of glucose to brain.

The blood glucose concentration is 3.896.11mmol/L normally.

2.Hormones Regulating blood sugar level1insulin decreasing (unique)2glucagonincreasing3glucocorticoid:increasing4adrenalineincreasing

3. Abnormal Blood Sugar LevelHyperglycemia: > 7.227.78 mmol/L The renalthreshold for glucose (: 8.8910.00mmol/L Hypoglycemia: < 3.333.89mmol/L

1st stage(cytosol)2nd stage(Mt.)3rd stage(Mt.)

Summary of carbohydrate metabolism

Exercise1,A B C 3- D

2A B C A D E

3A B C PEP D

4A B C D

5AMP

Pantothenic acid, pantoic acid,pyrophosphate,mercaptoethylamine,phosphopantotheine,Anaplerotic reaction, malic enzyme, Crossroad,Hinge,Glucagon,epinephrine,PKAAShunt,