citric acid cycle or TCA cycle. krebs cycle is amphibolic in nature and its important reactions. occurs in mitochondrial matrix in close proximity to ETC. 5 types of vitamins are involved in this cycle. also inhibitors are present . regulation of TCA cycle is governed by mainly 3 enzymes and there is mention the energies of every step that takes place in citric acid cycle. citric acid cycle produces 24 molecules of ATP in every cycle

1. PRESENTED BY:
SALEHA DASTGIR (6607)
PHARM-D (2ND SEMESTER)

2. Citric acid cycle
 Other names
i. Kreb’s cycle
ii. Tricarboxylic acid cycle (TCA)
 Definition
It is a cyclic process which involves a sequence of compounds interrelated by oxidation-
reduction and other reactions which finally produces CO2 and H2O and it is final common
pathway of breakdown of fats, carbohydrates and proteins.

3. Location
The whole process is aerobic and enzymes are located in
mitochondrial matrix, either free or attached to inner
surface of mitochondrial membrane which facilitates
transfer of reducing equivalents to adjacent enzymes of
respiratory chain

4. Biomedical importance of citric
acid cycle
 Final common pathway for carbohydrates, fats and proteins through formation
of 2-carbon unit acetyl CoA.
 Acetyl CoA is oxidized to CO2 and H2O giving out energy - catabolic role
 Intermediates of TCA cycle plays important role in synthesis also like heme
formation, formation of non essential amino acids, FA synthesis, cholesterol and
steroid synthesis - anabolic role.

5. Reactions of citric acid cycle
 Reactions of TCA cycle are arbitrarily divided into four stages:
Stage -I
 Formation of citrate
Acetyl group of acetyl-coA is transferred to OAA, no oxidation or decarboxylation is involved. A
molecule of H2O is required to hydrolyze high energy bond linkage between acetyl group and
coA, the energy released is used for citrate condensation.
Energetics: No ATP is required.

6. Formation of cis-aconitate and
isocitrate
 Both processes are catalyzed by the same enzyme Aconitase which requires Fe++. Formation
of cis-aconitate from citrate as a result of dehydration and formation of isocitrate from cis-
aconitate as a result of rehydration.
 Energetics: No ATP formed at this stage

7. Stage - II
 Formation of oxalosuccinate and α-ketoglutarate
Since it is not possible to separate dehydrogenase from decarboxylase activity, it is concluded
that the two reactions are catalyzed by single enzyme Isocitrate dehydrogenase.

8. Oxidative decarboxylation of
α - ketoglutarate to succinyl-CoA
 This reaction is analogous to oxidative decarboxylation of pyruvic acid to acetyl-coA. Enzyme is
alpha-ketoglutarate dehydrogenase complex.
 Energetics: NADH produced is oxidized in respiratory chain yielding 3ATP, from 2NADH 6ATP
will be produced.

9. Stage III
 Formation of succinate
The product of preceding stage succinyl-CoA is converted to succinate to continue the cycle.
Enzyme catalyzing this reaction is Succinate thiokinase.
Release of free energy from oxidative decarboxylation of α -ketoglutarate is sufficient to generate
high energy bond in addition to formation of NADH. ATP is produced at substrate level without
participation of ETC. this is the only example of substrate level phosphorylation in TCA cycle.
 Energetics: One ATP is produced in this reaction at substrate level. So, from two succinyl-CoA
2ATP will be produced.

10. Stage - IV
 Oxidation of succinate to fumarate
It is a dehydrogenation reaction catalyzed by Succinate dehydrogenase; hydrogen acceptor is FAD+.
This is the only dehydrogenation in TCA cycle which involves direct transfer of hydrogen from
substrate to a flavoprotein without participation of NAD+.
 Energetics: Oxidation of FADH2 through ETC yields 2ATP. Hence 2 molecules of succinate will give
4ATP.

11. Formation of malate
Fumarate is hydrated to malate in a freely reversible reaction catalyzed by fumarase. Fumarase
catalyzes the addition of the elements of water to the double bond of fumarate.

12. Oxidation of malate to oxaloacetate
 Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid
cycle by malate dehydrogenase. During this oxidation, NAD+ is reduced to NADH + H+.
 Oxaloacetate produced acts catalytically combines with a fresh molecule of acetyl- CoA and the
whole process is repeated
 Energetics: Oxidation of NADH2 through ETC produces 3ATP. Hence 2 molecules of NADH+ will give 6ATP

14. Energetics of citric acid cycle

15. Inhibitors of citric acid cycle
Fluoroacetate → Aconitase enzyme
Malonate → Succinate dehydrogenase enzyme
Arsenite → α -ketoglutarate dehydrogenase enzyme

16. Role of vitamins in citric acid cycle
Five B vitamins are associated with TCA cycle for yielding energy.
Riboflavin → In the form of FAD-a cofactor for succinate
dehydrogenase enzyme.
Niacin → In the form of NAD-the electron acceptor for isocitrate
dehydrogenase, alpha ketoglutarate dehydrogenase and malate
dehydrogenase.
Thiamine → As thiamine diphosphate-required as coenzyme for
decarboxylation in alpha ketoglutarate dehydrogenase reaction.
Lipoic acid → Required as coenzyme for α -ketoglutarate
dehydrogenase reaction.
Pantothenic acid → As a part of coenzyme A, the cofactor attached to
active carboxylic acid residue such as acetyl-CoA and succinyl-CoA.

17. Citric acid cycle is Amphibolic in nature
Citric acid cycle has dual role
i. Catabolic
ii. Anabolic
 Catabolic role
The two-carbon compound acetyl-CoA produced from metabolism of carbohydrates, lipids and
proteins are oxidized in this cycle to produce CO2, H2O and energy as ATP.
 Anabolic or synthetic role
Intermediates of TCA cycle are utilized for synthesis of various compounds.

18. Examples
 Transamination: synthesis of non-essential amino acids
Transaminase reactions produce ketoacids, PA, OAA and alpha-ketoglutarate from alanine,
aspartate and glutamate respectively. Because these reactions are reversible, TCA cycle serves as a
source of C-skeletons for synthesis of non-essential amino acids.
Aspartate + PA → OAA + Alanine
Glutamate + PA → α -ketoglutarate + Alanine
 Formation of glucose
Other amino acids contribute to gluconeogenesis because all or part of their C-skeleton enter
TCA cycle after deamination or transamination.

19. Examples
 Fatty acid synthesis
Acetyl-CoA formed from PA by the action pf PDH complex is starting material for long chain fatty
acid synthesis. But this synthesis is extramitochondrial whereas acetyl-CoA is formed in mitochondria.
Acetyl-CoA is impermeable to mitochondrial membrane and hence it has to be transported out. This
is achieved by “citric acid” an intermediate of TCA cycle which is permeable to mitochondrial
membrane.
 Synthesis of cholesterol and steroids
Acetyl-CoA is used for synthesis of cholesterol, which in turn is required for synthesis of steroids.
 Haem synthesis
Succinyl-CoA produced in TCA cycle takes part in haem synthesis

21. Regulation of citric acid cycle
1. As primary function of TCA cycle is to provide energy, respiratory control via ETC and oxidative
phosphorylation exerts the main control.
2. In addition to this overall and coarse control, several enzymes of TCA cycle are also important in
the regulation.
Three key enzymes are:
 Citrate synthase
 Isocitrate dehydrogenase (ICD)
 α -ketoglutarate dehydrogenase
These enzymes are responsive to the energy status as expressed by ATP/ADP ratio and NADH/NAD+
ratio.

22. “
”i. Citrate synthase enzyme is allosterically inhibited by ATP and long chain acyl-CoA.
ii. NAD+- dependant mitochondrial isocitrate dehydrogenase is activated allosterically by ADP and is inhibited
by ATP and NADH.
iii. α-ketoglutarate dehydrogenase regulation is analogous to pyruvate dehydrogenase complex.

23. References
 Denise R. Ferrier, “Lippincott’s Illustrated Reviews: Biochemistry”, Sixth
Edition
 MN Chatterjee & Rana Shinde, “Textbook of Medical Biochemistry”, Eighth
Edition