3 glycolysis+kreb's pdf

Phosphorylation of Glucose

Glucose + ATP Glucose–6–phosphate + ADPGlucokinase

Glucose + ATP Glucose–6–phosphate + ADPHexokinase

• There are TWO enzymes

Trapping of Glucose by phosphorylation

Glucokinase enzyme

Glucose + ATP Glucose-6-phosphate + ADPGlucokinase

Comparison between Hexokinase & Glucokinas

GlucokinaseHexokinaseFactorNo

Glucose onlyAll HexosesSubstrate1

Liver onlyAll tissuesDistribution2

Not inhibited by

Glucose-6-phosphate

Inhibited by

Glucose-6-phosphate

Product

inhibition3

High Km

(Low affinity)

Low Km

(High affinity)

Km for

glucose4

ActivatedNot affectedEffect of

Insulin5

ActivatedNot affectedEffect of

Carbohydrate6

InhibitedNot affectedEffect of

Starvation7

(Sigmoidal Curve)

(Hyberbolic Curve)

(Substrate concentration)

Comparison between Glucokinase & Hexokinase

Tissue-specific distribution of the two

enzymes ensures that:

At low blood glucose concentrations, liver

is prevented from utilizing glucose until the

nutrient requirements of other tissues are

satisfied

(Lactate Fermentation)(Ethanol + CO2)

36

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Stages of Glycolysis

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Stages of Glycolysis

Stages of Glycolysis

Stages of

Glycolysis

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Substrate-Level

Phosphorylation

Rx 7 and 10

Substrate-Level Phosphorylation

PFK is the regulatory (key) enzyme in glycolysis!

• The second irreversible reaction of glycolysis

• Large negative ∆G, means PFK is highly regulated

• PFK is regulated by:

– Citrate is an allosteric inhibitor

– ATP also inhibits PFK, while AMP activates PFK

– Fructose-2,6-bisphosphate is allosteric activator

– PFK increases activity when energy status is low

– PFK decreases activity when energy status is high

1

2

3

5 4

6

89

Metabolic Significance of Glycolysis

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1

2

3

• The three irreversible enzymes (Glucokinase,

Phosphofructokinase, Pyruvate kinase) are

under the control of Insulin

• Insulin induces the synthesis of:

Regulation of Glycolysis

1- After carbohydrate meal:

Blood glucose level Stimulates insulin

secretion Increases synthesis of

glucokinase, phosphofructokinase & pyruvate kinase

Enhances glycolysis

2- During fasting:

Blood glucose level Inhibits insulin

secretion & stimulates glucocorticoid secretion

Increases the synthesis of the four enzymes that

reverse glycolysis (Stimulate gluconeogenesis)

3- Pasteur effect:

Increased oxygen inhibits glycolysis, since

increased citrate and ATP or increased ATP/ADP

ratio which inhibit phosphofructokinase (the rate

limiting enzyme of glycolysis)

Decreased ATP/ADP ratio or increased ADP, AMP

& Pi activates phosphofructokinase

4. Glycolysis inhibited by iodoacetate, fluroacetate &

arsenite, since they inhibit Kreb’s cycle

Fate of Pyruvate

Pyruvate Decarboxylase

& Alcohol DHLactate DHPyruvate DH

In CytoplasmIn Mitochondria

2 Pyruvate

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We

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Lactate Dehydrogenase

For regeneration of NAD

• Lactate DH in Heart & Muscles:

Heart (H4) Muscle (M4)

H HHH

M MMM

H MMM

H HMH

H HMM

LDH5LDH1

LDH2 LDH3 LDH4

Different forms of Lactate Dehydrogenase

• Lactate DH in different tissues:

H3M H2M2 HM3

LDH

CPK

CatabolismAnabolism

LDH5 & LDH4LDH1 & LDH2

< TD>

TPP

12

Cyto

so

l

Acetaldehyde

Lactate & Ethanol Fermentation

Decarboxylase

Alcohol

Dehydrogenase

Rapoport-Luebering Cycle in RBCs

(R-L Cycle)

1,3- Diphosphoglycerate 2,3- Diphosphoglycerate

(2,3-DPG)

3- Phosphoglycerate

Mutase

Phoshatase

3-Phoshoglycerate

kinaseADP

ATP

Pi

• To meet this deficiency in ATP

synthesis, glycolysis rate in

RBCs increases.

7

Complete glycolysis

1

Rapoport-Luebering Cycle in RBCs

(R-L Cycle) 1

Pyruvate Kinase Deficiency in RBCs

1.The genetic deficiency of pyruvate kinase in

RBCs leads to hemolytic anemia.

2.This is due to inhibited (reduced rate of)

glycolysis and lowered level of ATP synthesis.

3.So the rate of synthesis of ATP is inadequate to

meet the energy needs of the cell to maintain

the structural integrity of erythrocytes.

3

1.Malate shuttle (Dicarboxylic Acid Shuttle).

2.Glycerol phosphate shuttle.

Two Shuttle Pathways for

Oxidation of Cytoplasmic NADH

(Dicarboxylic

Acid Shuttle)

Cytoplasm

(3 ATP)

(From glycolysis)1.Malate Shuttle

electron respiratory chain

2- -Glycerol Phosphate Shuttle

Cytosol

2 ATP(2 ATP)

2- -Glycerol Phosphate Shuttle

2 ATP

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Fate of Pyruvate

2 pyruvate + 2 NAD+ + 2 CoA ----> 2 acetyl CoA + 2 NADH + 2 carbon dioxide

H

Oxaloacetate

Alanine

Alanine

Cyto

pla

sm

Mit

och

on

dri

a

12

5

3

46

Acetaldehyde

1. Decarboxylation: Removal of CO2 (Decarboxylase,

TPP as coenzyme).

2. Oxidation of the remaining two-carbon compound

and reduction of NAD+ (Dehydrogenase, CoASH,

FAD & NAD+).

3. Trans-acetylating function: Attachment of CoA with

a high energy thio-ester bond to form Acetyl CoA

(Transacetylase, Lipoic acid).

Pyruvate + NAD+ + CoASH Acetyl CoA + NADH + CO2

• Pyruvate Dehydrogenase:

• A multienzyme complex has 3 Functions:

Pyruvate Acetyl CoA

FAD CoA-SH

NADH + CO2

NAD+ TPP

Lipoic acid

Pyruvate DH

1 2

3

5

4

NADH & ATP

Pyruvate Acetyl CoA1- Product inhibition

2- Covalent modification

(Protein kinase)

(directly)

3- Insulin

Pyruvate DH

Regulation of Pyruvate DH

-

--

+

Pyruvate Acetyl CoA

Insulin Allosterically

Pyruvate DH

Carboxylation of Pyruvate

-

+

Pyruvate OxaloacetateBiotin

ATP + CO2 ADP + Pi

Pyruvate

Carboxylase

-+

OH

(( ) )CH3

-Alanine Pantoic acid

Sources & Fate of

Acetyl CoA

12

3

5

Ketone

Bodies

4

Cholesterol

Tricarboxylic Acid Cycle (TCA)

Citric Acid Cycle, Kreb’s Cycle

Third Stage of Metabolism

2 Carbons4 Carbons

6 Carbons

5 Carbons4 Carbons

4 Carbons 4 Carbons

4 Carbons

3 Carbons

Thiokinase

3 ATP

3 ATP

1 ATP

2 ATP

3 ATP

• It is the final pathway for oxidation (3rd stage) of

all foodstuffs to CO2 + H2O + Energy.

• It is important for the interconversion of

carbohydrates, fats & proteins.

• All reactions are reversible except: Citrate

synthase, Isocitrate DH & -Ketoglutarate DH.

• The rate limiting enzyme is Citrate synthase.

Comments & Biological

Significance of TCA Cycle

• Mitochondrial isocitrate DH is NAD+

linked, while cytoplasmic isocitrate DH is

NADP+ linked.

• TCA is the major source of succinyl Co A

which used for:

– Heme synthesis.

– Ketolysis.

– Detoxication reactions.

Comments & Biological

Significance of TCA Cycle

1. Insulin activates pyruvate DH & inhibits pyruvate

carboxylase, thus directing pyruvate towards complete

oxidation through kreb’s cycle.

2. Acetyl Co A inhibits pyruvate DH & activates pyruvate

carboxylase, thus directing pyruvate & glucose towards

formation of oxaloacetate to combine with excess Acetyl

Co A for the optimal activity of kreb’s cycle.

• During starvation (glucose supply is low & fat oxidation

provides excess Acetyl Co A), so oxaloacetate is

required.

Regulation of Kreb’s Cycle

3. So, Kreb’s cycle is inhibited by:

a) Starvation (No carbohydrates).

b) Diabetes mellitus (No insulin).

c) Anaerobic conditions (No oxygen).

4. It is inhibited in vitro by fluroacetate & iodoacetate

which form flurocitrate & iodocitrate that inhibits

aconitase.

5. Malonic acid is a competitive inhibitor of succinate

dehydrogenase.

6. Arsenite inhibits Kreb’s cycle.

Regulation of Kreb’s Cycle

Effect of Fluroacetate on TCA

• Flurocitrate inhibits aconitase

23

4

Metabolic Significance of Kreb’s Cycle

1

Sources of Oxaloacetate

1. Pyruvate, in mitochondria (Pyruvate

carboxylase).

2. Malate, in mitochondria, by malate DH.

3. Citric acid, in cytoplasm (ATP-Citrate lyase).

4. Aspartic acid, by transamination in both

cytoplasm & mitochondria.

aspartate -ketoglutarate oxaloacetate glutamate

Aminotransferase (Transaminase)

COO

CH2

CH2

C

COO

O

COO

CH2

HC

COO

NH3+

COO

CH2

CH2

HC

COO

NH3+

COO

CH2

C

COO

O + +

Fate of Oxaloacetate

1. Aspartic acid, by transamination.

2. Citric acid, by citrate synthase.

3. Malate, in mitochondria, by malate DH.

4. Phosphoenolpyruvate by reversal glycolysis

(Gluconeogenesis), in cytoplasm, by PEP

Carboxykinase.

1. Excretion through lungs (main fate).

2. Combined with ammonia to form urea.

3. Combined with ammonia to form pyrimidine.

4. Enters in the formation of C6 of purines.

5. Fixation into organic acids (Carboxylation):

1. Pyruvic acid + CO2 Oxaloacetic acid.

2. Acetyl Co A + CO2 Malonyl Co A.

3. Propionyl Co A + CO2 Methylmalonyl Co A.

Fate of CO2