19

19-1

The Citric

Acid Cycle

Hans Krebs,

1900–1981

19

19-2

Learning Objectives 1. What Role Does the Citric Acid Cycle Play in Metabolism?

2. What Is the Overall Pathway of the Citric Acid Cycle?

3. How Is Pyruvate Converted to Acetyl-CoA?

4. What Are the Individual Reactions of the Citric Acid

Cycle?

5. What Are the Energetics of the Citric Acid Cycle, and How

Is It Controlled?

6. What Is the Glyoxylate Cycle?

7. What Role Does the Citric Acid Cycle Play in Catabolism?

8. What Role Does the Citric Acid Cycle Play in Anabolism?

19

19-3

The Citric Acid Cycle • Three processes play central role in aerobic

metabolism

• the citric acid cycle

• electron transport

• oxidative phosphorylation

• Metabolism consists of

• catabolism: the oxidative breakdown of nutrients

• anabolism: the reductive synthesis of biomolecules

• The citric acid cycle is amphibolic; that is, it

plays a role in both catabolism and anabolism

• It is the central metabolic pathway

19

19-4

Mitochondrion

19

19-5

The Citric Acid Cycle • TCA cycle= Krebs cycle= citric acid cycle

• In eukaryotes, cycle occurs in the mitochondrial matrix

mitochondrion

19

19-6

The Citric Acid Cycle Pyruvate

Acetyl -CoA

GDP GTP

F A D

F A D H 2

N A D +

N A D H

N A D +

N A D H

C O 2

N A D +

N A D H

C O 2

Citric acid cycle

(8 steps)

Coenzyme A

N A D +

N A D H CO2

19

19-7

19

19-8

Pyruvate to Acetyl-CoA • Oxidative decarboxylation reaction

• Occurs in the mitochondria

this reaction requires NAD+, FAD, Mg2+, thiamine

pyrophosphate, coenzyme A, and lipoic acid

• G°’ = -33.4 kJ•mol-1

CH3 CCOO- + NAD+CoA-SH +

CH3 C-SCo A + CO2

pyruvatedehydrogenase

complex

Pyruvate Coenzyme A

Acetyl -CoA

O

O

+ NADH

19

19-9

Structure of the pyruvate

dehydrogenase complex

E1, pyruvate dehydrogenase (yellow) (; E2, dihydrolipoyl

transacetylase;(green) and E3,dihydrolipoyl dehydrogenase

(red). The lipoyl domain of E2 (blue)

19

19-10

19

19-11

Beriberi-

A vitamin-deficiency disease first described in 1630

by Jacob Bonitus, a Dutch physician working in

Java:

"A certain very troublesome affliction, which

attacks men, is called by the inhabitants Beriberi

(which means sheep). I believe those, whom this

same disease attacks, with their knees shaking

and the legs raised up, walk like sheep. It is a

kind of paralysis, or rather tremor: for it

penetrates the motion and sensation of the hands

and feet indeed sometimes of the whole body."

19

19-12

The Citric Acid Cycle

• Step 1: Formation of citrate by condensation of

acetyl-CoA with oxaloacetate; G°’= -32.8 kJ•mol-1

citrate synthase (condensing E) is an allosteric

enzyme, inhibited by NADH, ATP, and succinyl-CoA

C H 3 C - S C o A

Acetyl-CoA

C - C O O -

C H 2 - C O O -

Oxaloacetate

+ C - C O O - H O

C H 2 - C O O -

C H 2 - C O O -

+ C o A - S H

Coenzyme A

c itrate synthase

Citrate (3 carboxyl groups)

O

O

19

19-13

The Citric Acid Cycle • Step 2: dehydration and rehydration gives

isocitrate; catalyzed by aconitase

• citrate is achiral; it has no stereocenter

• isocitrate is chiral; it has 2 stereocenters and 4

stereoisomers are possible

• only one of the 4 stereoisomers of isocitrate is formed

in the cycle

C - C O O - H O

C H 2 - C O O -

C H 2 - C O O -

Citrate

C - C O O -

C H 2 - C O O -

C - C O O - H

C H - C O O -

C H 2 - C O O -

Aconitate

H O

Isocitrate (3 carboxyl groups)

C H - C O O -

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19-14

The Citric Acid Cycle

• Step 3: oxidation of isocitrate followed by

decarboxylation

• isocitrate dehydrogenase is an allosteric enzyme; it is

inhibited by ATP and NADH, activated by ADP and

NAD+

C - C O O - H

C H - C O O -

C H 2 - C O O -

H O

Isocitrate

C - C O O - H

C - C O O -

C H 2 - C O O -

C - H H

C - C O O -

C H 2 - C O O -

N A D H N A D +

a -Ketoglutarate (2 carboxyl groups)

C O 2

isocitrate dehydrogenase

O O

Oxalosuccinate

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19-15

The Citric Acid Cycle

• Step 4: oxidative decarboxylation of

a-ketoglutarate to succinyl-CoA

• like pyruvate dehydrogenase, this enzyme is a

multienzyme complex and requires coenzyme A,

thiamine pyrophosphate, lipoic acid, FAD, and NAD+

• G0’ = -33.4 kJ•mol-1

C H 2

C - C O O -

C H 2 - C O O -

a -Ketoglutarate

O

C o A - S H

N A D H N A D +

a -ketoglutarate

dehydrogenase complex

C H 2

C

C H 2 - C O O -

S C o A O

Succinyl-CoA (1 carboxyl groups)

+ C O 2

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19-16

The Citric Acid Cycle

• Step 5: formation of succinate

• The two CH2-COO- groups of succinate are equivalent

• This is the first energy-yielding step of the cycle

• The overall reaction is slightly exergonic

Succinyl-CoA + H2 O Succinate + CoA-SH

GDP + Pi GTP + H2 O

G0'

(kJ•mol -1)

-33.4

+30.1

-3.3Succinyl-CoA + GDP + Pi Succinate + CoA-SH + GTP

C H 2

C

C H 2 - C O O -

S C o A O

Succinyl-CoA

G D P + P i C o A - S H

Succinate (2 carboxyl groups)

+ G T P +

succinyl-CoA synthetase

C H 2 - C O O -

C H 2 - C O O - +

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19-17

The Citric Acid Cycle

• Step 6: oxidation of succinate to fumarate

F A D F A D H 2

C H 2 - C O O -

C H 2 - C O O -

Succinate

succinate Dehydrogenase +Fe, - heme

C

C H

H

C O O -

- O O C

Fumarate (2 carboxyl groups)

• Note: succinate dehydrogenase is the only TCA

enzyme that is located in the inner mitochondrial

membrane and linked directly to ETC

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19-18

The Citric Acid Cycle

• Step 7: hydration of fumarate

• Step 8: oxidation of malate

C - C O O -

C H 2 - C O O -

Oxaloacetate (2 carboxyl groups)

N A D + N A D H

malate dehydrogenase

C H - C O O - H O

C H 2 - C O O -

L-Malate

O

C

C H

H

C O O -

- O O C

Fumarate

H 2 O C H - C O O - H O

C H 2 - C O O -

L-Malate (2 carboxyl groups)

fumarase

19

19-19

From Pyruvate to CO2

CoA-SH +

Pyruvate dehydrogenase compl ex

Citric acid cycle

Pyruvate + NAD +

Acetyl -CoA + NADH + CO2 H++

Acetyl -CoA + 3NAD + + FAD GDP+ + Pi

2 CO2 + CoA-SH + 3NADH 3H++ + FADH2 + GTP

Pyruvate 4NAD + + FAD GDP+ + Pi

2 H2 O+

2 H2 O+

3 CO2 + 4NADH + FADH2 + GTP 4H++

+

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19-20

Summary • The two-carbon unit needed at the start of the

citric acid cycle is obtained by converting

pyruvate to acetyl-CoA

• This conversion requires the three primary

enzymes of the pyruvate dehydogenase complex,

as well as, the cofactors TPP, FAD, NAD+, and

lipoic acid

• The overall reaction of the pyruvate

dehydogenase complex is the conversion of

pyruvate, NAD+, and CoA-SH to acetyl-CoA,

NADH + H+, and CO2

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19-21

Pyruvate

Acetyl -CoA +

+ CoA-SH + NAD+

Acetyl -CoA + NADH + CO2 + H+

Citrate Isoci trate

Isoci trate

+ Oxaloacetate H2 O

Citrate + CoA-SH + H+

+ NAD+

a-Ketoglutarate + NADH + CO2

1.

2.

G°' (kJ•mol -1)

-33.4

-32.2

+6.3

-7.13.

4 NAD+

GTPSucci nate

FAD FADH2

4.

CoA-SH

5.

6.

7.

a-Ketoglutarate + NAD+ + CoA-SH

Succi nyl-CoA + NADH + CO2 + H+

Succi nyl-CoA + GDP + Pi

+ +

Succi nate + Fumarate +

Fumarate + H2 O Malate

8. Malate

+

Oxaloacetate + NADH+ NAD+

+ FAD + GDP + Pi

3 CO2 4 NADH+ + FADH2 + GTP + 4 H+

Pyruvate

-33.4

-3.3

~0

-3.8

+29.2

-77.7

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19-22

Control of the CA Cycle

• Three control points within the cycle

• citrate synthase: inhibited by ATP, NADH, and succinyl

CoA; also product inhibition by citrate

• isocitrate dehydrogenase: activated by ADP and NAD+,

inhibited by ATP and NADH

• a-ketoglutarate dehydrogenase complex: inhibited by

ATP, NADH, and succinyl CoA; activated by ADP and

NAD+

• One control point outside the cycle

• pyruvate dehydrogenase: inhibited by ATP and NADH;

also product inhibition by acetyl-CoA

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19-23

Control of the CA Cycle

Conversion of pyruvate to acetyl-CoA

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19-24

Cells in a resting

metabolic state

Cells in an active

metabolic state

need and use

comparatively little energy

need and use more energy

than resting cells

high ATP, low ADP imply

high ATP/ADP ratio

low ATP, high ADP imply

low ATP/ADP ratio

high NADH, low NAD+

imply high NADH/NAD+

ratio

low NADH, high NAD +

imply low NAHDH/NAD+

ratio

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19-25

Why Is the Oxidation of Acetate

So Complicated?

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19-26

Because …………… 1. Citric Acid Cycle Components Are

Important Biosynthetic Intermediates

2. Besides its role in the oxidative

catabolism of carbohydrates, fatty acids,

and amino acids, the cycle provides

precursors for many biosynthetic

pathways.

3. It is also important for plants and bacteria

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19-27

Glyoxalate Cycle…..

19

19-28

Glyoxalate Cycle

Bacteria and plants can synthesize acetyl CoA

from acetate and CoA by an ATP-driven

reaction that is catalyzed by

acetyl CoA synthetase.

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19-29

19

19-30

Biosynthetic Roles of the Citric Acid Cycle. Intermediates

drawn off for biosyntheses (shown by red arrows) are

replenished by the formation of oxaloacetate from

pyruvate.

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19-31

End

Chapter 19