19

19-1

The Citric

Acid Cycle

19

19-2

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

19-3

Mitochondrion

The Powerhouse of the Cell

19

19-4

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

• In Eukaryotes, cycle occurs in the mitochondrial matrix

• In Prokaryotes CAC occurs in the cytosol

mitochondrion

19

19-5

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

19-6

Summary Of

Reactions Of CAC

Can I Keep Selling

Sex For

Money Officer

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

Congenital Lactic Acidosis

• Absence of pyruvate decarboxylase activity in

man ( first enzyme ).

• characterised by progressive neuromuscular

deterioration and accumulation of lactate and

hydrogen ions in blood, urine and/or

cerebrospinal fluid, frequently resulting in early

death.

• It is an x-linked dominant disease affecting both

sexes.

19

19-12

Arsenic poisoning • It inhibits lipoic acid.

• Epidemiological evidence shows an association

between inorganic arsenic in drinking water and

increased risk of skin, lung and bladder cancers

19

19-13

Beriberi

A vitamin-deficiency disease first described in 1630

by Jacob Bonitus, a Dutch physician working in

Java

The term beriberi is derived from the Sinhalese

word meaning “extreme weakness.”

19

19-14

Beriberi • It is a nutritional disorder caused by a deficiency

of thiamin (vitamin B1) and characterized by

impairment of the nerves and heart.

• In the form known as dry beriberi, there is a

gradual degeneration of the long nerves, first of

the legs and then of the arms, with associated

atrophy of muscle and loss of reflexes.

• In wet beriberi, a more acute form, there is

edema resulting largely from cardiac failure and

poor circulation. In infants breast-fed by mothers

who are deficient in thiamin, beriberi may lead to

rapidly progressive heart failure.

19

19-15

Wernicke–Korsakoff syndrome

• This is found predominantly in alcoholics.

Chronic alcohol consumption can result in

thiamine deficiency by causing inadequate

nutritional thiamine intake, decreased absorption

of thiamine from the gastrointestinal tract, and

impaired thiamine utilization in the cells.

• People differ in their susceptibility to thiamine

deficiency, however, and different brain regions

also may be more or less sensitive to this

condition.

19

19-16

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

19-17

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

isocitrate; catalyzed by aconitase(-by flouroacetate

rat poison).

• 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 -

19

19-18

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

19

19-19

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

19

19-20

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

Su ccin yl-CoA+ H2 O Su ccin ate + CoA-SH

GDP + Pi GTP + H2 O

G0'

(k J•mol-1)

-33.4

+30.1

-3.3Su ccin yl-CoA+ GDP + Pi Su ccin ate+ 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 - +

19

19-21

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

19

19-22

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

19-23

From Pyruvate to CO2

CoA-SH +

Pyruvate dehydrogenase complex

Citric acid cycle

Pyruvate + NAD +

Acetyl-CoA + NADH + CO2 H++

Acetyl-CoA + 3NAD + + FAD G DP+ + Pi

2 CO2 + CoA-SH + 3NADH 3H++ + FADH2 + G TP

Pyruvate 4NAD + + FAD G DP+ + Pi

2 H2 O+

2 H2 O+

3 CO2 + 4NADH + FADH2 + G TP 4H++

+

19

19-24

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

19

19-25

Pyruvate

Acetyl-CoA +

+ CoA-SH + NAD+

Acetyl-CoA + NADH + CO2 + H+

Citrate Isocitrate

Isocitrate

+ 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+

G TPSuccinate

FAD FADH2

4.

CoA-SH

5.

6.

7.

a-Ketoglutarate + NAD+ + CoA-SH

Succinyl-CoA + NADH + CO2 + H+

Succinyl-CoA + G DP + Pi

+ +

Succinate + Fumarate +

Fumarate + H2 O Malate

8. Malate

+

Oxaloacetate + NADH+ NAD+

+ FAD + G DP + Pi

3 CO2 4 NADH+ + FADH2 + G TP + 4 H+

Pyruvate

-33.4

-3.3

~0

-3.8

+29.2

-77.7

19

19-26

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

19

19-27

Control of the CA Cycle

Conversion of pyruvate to acetyl-CoA

19

19-28

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

19

19-29

Why Is the Oxidation of Acetate

So Complicated?

19

19-30

Because …………… 1. Besides its role in the oxidative

catabolism of carbohydrates, fatty acids,

and amino acids, the cycle provides

precursors for many biosynthetic

pathways.

2. It is also important for plants and bacteria

19

19-31

Glyoxalate Cycle…..

19

19-32

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.

19

19-33

19

19-34

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.

19

19-35

End

Chapter 19