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The Citric Acid Cycle

Syllabus

• Pyruvate Dehydrogenase Complex

• Overview of the Citric Acid Cycle

• The Reactions of the Citric Acid Cycle

• Regulation of the Citric Acid Cycle

• Amphibolic character of the CAC

• Anaplerotic Sequences of the CAC

• Glyoxylate Cycle

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Pyruvate Dehydrogenase

Complex

• Pyruvate oxidation: a major entry route for carbon into the

citric acid cycle

• the three enzymes involved are assembled into a highly

organized multienzyme assembly

– pyruvate decarboxylase (dehydrogenase) (E1)

– dihydrolipoamide transacetylase (E2)

– dihydrolipoamide dehydrogenase (E3)

1

2

3

4

5

6

3

4

Citrate synthase

G°´ = -32,2 kJ/mol G = negative

acetyl-CoA oxaloacetate citrate

CH3

O

SC CoA

C

CH2

COO

COO

O-

-

CoA SH+ + + + H+

C

CH2

COO

COO

CH2

CO

O

-

-

-

HO

O

H2

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Aconitase

G°´ = +6,3 kJ/mol G 0

citrate cis-aconitate (2R,3S)-isocitrate

C

CH

COO

COO

CH2

COO-

-

-

C

C

COO

COO

CH2

COO

OH

H

H

-

-

-

H O2

H O2

H O2

H O2

C

C

COO

COO

CH2

COO

H

H

OH

-

-

-

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Isocitrate dehydrogenase

G°´ = -20,9 kJ/mol G = negative

(2R,3S)-isocitrate oxalosuccinate a-ketoglutarate

COO

CH2

C

C

CO O

HOH

CHO

O

-

-

-COO

CH2

C

C

CO O

O

CHO

O

-

-

-

Mn2+

COO

CH2

C

C

CO O

O

H

-

-

Mn2+

-

NAD+ + +

H+

H NADH CO2

COO

CH2

C

C

CO O

O

H H

-

-

G°´ = -33,5 kJ/mol G = negative

a-ketoglutarate succinyl-CoA

NAD+ CO

2+ + CoA-SH + + NADH

COO

CH2

C

C

CO O

O

H H

-

-COO

CH2

C

C

S

O

H H

CoA

-

a-ketoglutarate dehydrogenase

a-ketoglutarate decarboxylase

dihydrolipoamide succinyltransferase

dihydrolipoamide dehydrogenase

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Succinyl-CoA-synthetase

G°´ = -33,5 kJ/mol G = negative

succinyl-CoA succinylphosphate

COO

CH2

C

C

S

O

H H

CoA

-

+ O P O

OH

O-

- + CoASH

COO

CH2

C

C O

H H

O PO3

-

2-

H+

COO

CH2

C

C O

H H

O PO3

-

2-

+ +

COO

CH2

C

C O

H H

O

-

-

enzyme His. enzyme 3-phosphohistidine

+GDP

+ GTPenzyme His.succinylphosphate succinate

Succinate dehydrogenase

G°´ = 0 kJ/mol G 0

succinate fumarate

COO

C

C

COO

H H

H H

-

-

+ E FADC

COOC H

H COO

+ E FADH2

-

-

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Fumarase

G°´ = -3,8 kJ/mol G 0

(S)-malate fumarate

+ H O2

C

COOC H

H COO

-

-

C

COOC

H COOH

OH

H

-

-

Malate dehydrogenase

G°´ = +29,7 kJ/mol G 0

(S)-malate oxaloacetate

C

COOC

H COOH

O

H

-

-

H+ + + H

+NADHNAD

+ C

COOC

H COOH

O-

-

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Regulation of the Citric Acid Cycle

Three key enzymes of the Citric Acid Cycle

operate under physiologic conditions far

from equilibrium (G is negative)

• citrate synthase

• isocitrate dehydrogenase

• a-ketoglutarate dehydrogenase

Control of the citric acid cycle by three simple ways

• availability of substrates (Ac-CoA, oxaloacetate)

• inhibition by product (NADH)

• competitive inhibition by other intermediates of

the cycle

Regulation of the Citric Acid Cycle

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Regulation of the CAC

Amphibolic character of CAC

• biosynthesis of glucose (gluconeogenesis)

– in cytosol - uses malate from mitochondria

• biosynthesis of lipids

– in cytosol - Ac-CoA by spliting of citrate (ATP-citrate lyase)

• biosynthesis of amino acids

– a-ketoglutarate for glutamate synthesis

– a-ketoglutarate and oxaloacetate - transamination

with Ala

• synthesis of porphyrin

– Cy+Mit - uses succinyl-CoA as a building material

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Anaplerotic sequences of CAC

• Oxidative carboxylation - pyruvate carboxylase

• Reductive carboxylation - malate dehydrogenase

+ + +H+

NADP+ + H2OC O

CH3

COO-

CO3H-

NADPH +C O

COO

CH2

COO

H H

-

-

+ + ATP + ADP + Pi +C O

CH3

COO-

CO3 -

HC O

COO

CH2

COO-

-

H+

• Oxidation of FA with odd C - production of succinyl-CoA

• Degradation of isoleucine, valine, methionine - succinyl-CoA

• Degradation of phenylalanine, tyrosine, aspartate - fumarate

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Glyoxylate cycle

• in plants different fate of Ac-CoA - oxaloacetate

• reactions are localised into two cell organels

– mitochondrion

– glyoxysome

• conversion of Ac-CoA into glyoxylate

• glyoxylate and Ac-CoA form malate in

glyoxysome - the formation of glucose

Glyoxylate cycle

• 2 enzymes differ from citrate acid cycle

C

C

COO

COO

CH2

COO

H

H

OH

-

-

-

CH2

CH2

COO

COO-

-

+O C COO

H

-

– isocitrate lyase

succinate glyoxylate

O C COO

H

-+ CH

3C

O

S CoA + H O2

COO

C

CH2

COO

HOH

-

-

+ CoA SH + H+

– malate synthase

malate

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