1. The Citric Acid Cycle
Hans Krebs, 1900–1981

2. Learning Objectives
What Role Does the Citric Acid Cycle Play in Metabolism?
What Is the Overall Pathway of the Citric Acid Cycle?
How Is Pyruvate Converted to Acetyl-CoA?
What Are the Individual Reactions of the Citric Acid Cycle?
What Are the Energetics of the Citric Acid Cycle, and How Is It Controlled?
What Is the Glyoxylate Cycle?
What Role Does the Citric Acid Cycle Play in Catabolism?
What Role Does the Citric Acid Cycle Play in Anabolism?

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

4. Mitochondrion

5. The Citric Acid Cycle TCA cycle= Krebs cycle= citric acid cycle
In eukaryotes, cycle occurs in the mitochondrial matrix
mitochondrion

6. The Citric Acid Cycle Pyruvate Acetyl -CoA GDP GTP F A D H N C O2 N + C O
Citric
acid
cycle
(8 steps)
Coenzyme A
CO2

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°’ = kJ•mol-1

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)

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."

12. The Citric Acid Cycle
Step 1: Formation of citrate by condensation of acetyl-CoA with oxaloacetate; G°’= kJ•mol-1
citrate synthase (condensing E) is an allosteric enzyme, inhibited by NADH, ATP, and succinyl-CoA C H 3 - S o A
Acetyl-CoA O 2
Oxaloacetate +
Coenzyme A c
itrate
synthase
Citrate
(3 carboxyl groups)

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 - O H 2
Citrate
Aconitate
Isocitrate
(3 carboxyl groups)

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 - O H 2
Isocitrate N A D + a
-Ketoglutarate
(2 carboxyl groups)
isocitrate
dehydrogenase
Oxalosuccinate

15. The Citric Acid Cycle Step 4: oxidative decarboxylation of
-ketoglutarate to succinyl-CoA
like pyruvate dehydrogenase, this enzyme is a multienzyme complex and requires coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD+
DG0’ = kJ•mol-1 C H 2 - O a
-Ketoglutarate o A S N D +
-ketoglutarate
dehydrogenase
complex
Succinyl-CoA
(1 carboxyl groups)

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 C H 2 - O S o A
Succinyl-CoA G D P + i
Succinate
(2 carboxyl groups) T
succinyl-CoA
synthetase

17. The Citric Acid Cycle Step 6: oxidation of succinate to fumarate2 C - O
Succinate
succinate
Dehydrogenase
+Fe, - heme
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

18. The Citric Acid Cycle Step 7: hydration of fumarate
Step 8: oxidation of malate C H O -
Fumarate 2
L-Malate
(2 carboxyl groups)
fumarase C - O H 2
Oxaloacetate
(2 carboxyl groups) N A D +
malate
dehydrogenase
L-Malate

19. From Pyruvate to CO2

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

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
-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

23. Control of the CA Cycle
Conversion of pyruvate to acetyl-CoA

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

25. Why Is the Oxidation of Acetate So Complicated?

26. Because ……………
Citric Acid Cycle Components Are Important Biosynthetic Intermediates
Besides its role in the oxidative catabolism of carbohydrates, fatty acids, and amino acids, the cycle provides precursors for many biosynthetic pathways.
It is also important for plants and bacteria

27. Glyoxalate Cycle…..

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.

30. Biosynthetic Roles of the Citric Acid Cycle
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.

31. End
Chapter 19