This slide contains details of Citric Acid Cycle along with its pathway.

2. Citric Acid Cycle (Krebs Cycle or
Tricarboxylic Acid—TCA cycle)
• The citric acid cycle is the most important metabolic
pathway for the energy supply to the body. About 65-
70% of the ATP is synthesized in Krebs cycle.70% of the ATP is synthesized in Krebs cycle.
• Citric acid cycle essentially involves the oxidation of
acetyl CoA to CO2 and H2O.
• This cycle utilizes about two-thirds of total oxygen
consumed by the body. The name TCA cycle is used,
since, at the outset of the cycle, tricarboxylic acids
(citrate, cisaconitate and isocitrate) participate.

3. TCA cycle—an overview
• It involves the combination of a two carbon
acetyl CoA with a four carbon oxaloacetate to
produce a six carbon tricarboxylic acid, citrate. In
the reactions that follow, the two carbons arethe reactions that follow, the two carbons are
oxidized to CO2 and oxaloacetate is regenerated
and recycled.
• Oxaloacetate is considered to play a catalytic
role in citric acid cycle.

4. (Citric Acid Cycle) Step I
• Formation of citrate : Krebs cycle proper
starts with the condensation of acetyl CoA and
oxaloacetate, catalysed by the enzyme citrate
synthase.synthase.

5. (Citric Acid Cycle) Step I - Continued

6. (Citric Acid Cycle) Step II & III
• Citrate is isomerized to isocitrate by the
enzyme aconitase.
• This is achieved in a two stage reaction of• This is achieved in a two stage reaction of
dehydration followed by hydration through
the formation of an intermediate—cis-
aconitate.

7. (Citric Acid Cycle) Step II & III - Continued

8. (Citric Acid Cycle) Step IV & V
• Formation of -ketoglutarate : The enzyme
isocitrate dehydrogenase (ICD) catalyses the
conversion (oxidative decarboxylation) of
isocitrate to oxalosuccinate and then to -isocitrate to oxalosuccinate and then to -
ketoglutarate.
• The formation of NADH and the liberation of
CO2 occur at this stage.

9. (Citric Acid Cycle) Step IV & V - Continued

10. (Citric Acid Cycle) Step VI
• Conversion of -ketoglutarate to succinyl CoA
occurs through oxidative decarboxylation,
catalysed by -ketoglutarate dehydrogenase
complex.complex.
• The mechanism of the reaction is analogous to
the conversion of pyruvate to acetyl CoA

11. (Citric Acid Cycle) Step VI - Continued

12. (Citric Acid Cycle) Step VII
• Formation of succinate : Succinyl CoA is
converted to succinate by succinate thiokinase.
This reaction is coupled with the phosphorylation
of GDP to GTP.of GDP to GTP.
• This is a substrate level phosphorylation.
• GTP is converted to ATP by the enzyme
nucleoside diphosphate kinase.
• GTP + ADP  ATP + GDP

13. (Citric Acid Cycle) Step VII - Continued

14. (Citric Acid Cycle) Step VIII
• Conversion of succinate to fumarate : Succinate
is oxidized by succinate dehydrogenase to
fumarate.
• This reaction results in the production of FADH2
and not NADH.

15. (Citric Acid Cycle) Step VIII - Continued

16. (Citric Acid Cycle) Step IX
• Formation of malate : The enzyme fumarase
catalyses the conversion of fumarate to
malate with the addition of H2O.

17. (Citric Acid Cycle) Step IX - Continued

18. (Citric Acid Cycle) Step X
• Conversion of malate to oxaloacetate : Malate is
then oxidized to oxaloacetate by malate
dehydrogenase.
• The third and final synthesis of NADH occurs at• The third and final synthesis of NADH occurs at
this stage.
• The oxaloacetate is regenerated which can
combine with another molecule of acetyl CoA,
and continue the cycle.

19. (Citric Acid Cycle) Step X - Continued

20. Pathway of Citric acid cycle:
Diagram/Pictorial RepresentationDiagram/Pictorial Representation

21. Sample 1

22. Sample 2

23. Energetics of TCA cycle
Step
No.
Enzyme Coenzyme ATPs generated
4 Isocitrate dehydrogenase NADH 3
6 -Ketoglutarate NADH 36 -Ketoglutarate
dehydrogenase
NADH 3
7 Succinic thiokinase GTP 1
8 Succinate dehydrogenase FADH2 2
10 Malate dehydrogenase NADH 3
Total ATPs generated 12

24. Energetics of TCA cycle
• During the process of oxidation of acetyl CoA via
citric acid cycle, 4 reducing equivalents (3 as NADH
and one as FADH2) are produced.
• Oxidation of 3 NADH by electron transport chain
coupled with oxidative phosphorylation results in
• Oxidation of 3 NADH by electron transport chain
coupled with oxidative phosphorylation results in
the synthesis of 9 ATP, whereas FADH2 leads to the
formation of 2 ATP. Besides, there is one substrate
level phosphorylation.
• Thus, a total of twelve ATP are produced from one
acetyl CoA.

25. Significance
 TCA cycle is the final common oxidative pathway for all
the major ingredients of food stuffs. carbohydrates enter
via pyruvate and acetyl CoA. ratty acids are broken down
into acetyl CoA. Amino acids, after transamination enter
in this cycle.in this cycle.
 Without carbohydrates, fat cannot be metabolised,
because for complete oxidation of fat, oxaloacetate is
required, which is formed from pyruvate. Oxaloacetate
acts as a true catalyst, as it enters in the cycle but is
regenerated at the end.

26. Significance
 Excess carbohydrate is converted to neutral fats which are
stored in the body. The pathway for the formation of
neutral fat is: Glucose  Pyruvate Acetyl CoA Fatty
acid. The conversion of pyruvate to acetyl CoA is catalysed
by pyruvate dehydrogenase, which is an irreversible
reaction. Hence, fat cannot be converted to glucose.
by pyruvate dehydrogenase, which is an irreversible
reaction. Hence, fat cannot be converted to glucose.
 Most amino acids enter in TCA cycle after deamination. For
example, glutamic acid enters at -ketoglutarate whereas
aspartate enters at oxaloacetate. These amino acids which
are converted as members of the cycle can also enter in
gluconeogenesis via oxaloacetate. Such amino acids are
called gluconeogenic amino acids.

27. Significance
 The amino acids like leucine, are metabolised to acetyl
CoA. Acetyl CoA either enters TCA cycle and gets
completely oxidised or is channeled to ketone body
formation. Such amino acids are called as ketogenic.
 All other pathways are either catabolic or anabolic, but All other pathways are either catabolic or anabolic, but
TCA cycle is purely amphibolic i.e. catabolic + anabolic.
TCA cycle acts as a source for the precursors of
biosynthetic pathways, e.g. heme is synthesized from
succinyl CoA whereas aspartate forms oxaloacetate.
This is also called as anapleorotic role of TCA cycle.

28. Regulation of citric acid cycle
• Three enzymes—namely citrate synthase,
isocitrate dehydrogenase and -ketoglutarate
dehydrogenase - regulate citric acid cycle.

29. Regulation of TCA cycle
 Citrate synthase is inhibited by ATP, NADH, acetyl CoA and succinyl CoA.
 Isocitrate dehydrogenase is activated by ADP, and inhibited by ATP and
NADH.
  Ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH.
 Availability of ADP is very important for the citric acid cycle to proceed. Availability of ADP is very important for the citric acid cycle to proceed.
This is due to the fact that unless sufficient levels of ADP are available,
oxidation (coupled with phosphorylation of ADP to ATP) of NADH and
FADH2 through electron transport chain stops. The accumulation of NADH
and FADH2 will lead to inhibition of the enzymes (as stated above) and
also limits the supply of NAD+ and FAD which are essential for TCA cycle to
proceed.

30. References:
• Biochemistry 5th Edition by Dr. U. Satyanarayana & Dr. U. Chakrapani
• Harper’s Illustrated Biochemistry, twenty-sixth edition by Robert K. Murray, Daryl K. Granner, Peter A. Mayes &
Victor W. Rodwell
• Lehninger, Principle of Biochemistry, Fourth edition by David L. Nelson and Michael M. Cox