The citric acid cycle (Krebs cycle or TCA cycle) is the most important metabolic pathway for energy production in the body. It occurs in the mitochondrial matrix and involves the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins to produce carbon dioxide, water, and the electron carriers NADH and FADH2 to drive ATP production. The cycle generates 12 ATP per acetyl-CoA molecule oxidized and also provides precursors for various biosynthetic pathways. Key steps include the condensation of acetyl-CoA with oxaloacetate to form citrate, followed by oxidative decarboxylations and hydrations that regenerate oxaloacetate and release carbon dioxide

1. CITRIC ACID CYCLE
Mrs. Prajakta B. Kothawade
Assistant Professor,
PES Modern College of Pharmacy, for ladies, Moshi, Pune

2. Citric acid cycle
• The citric acid cycle (Krebs cycle or tricarboxylic acid-TCA 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. Citric acid cycle essentially involves the
oxidation of acetyl CoA to CO2 and H2O.
• The name TCA cycle is used, since, at the outset of the cycle, tricarboxylic acids
(citrate, cisaconitate and isocitrate) participate.

3. TCA cycle –the central metabolic pathway
• Krebs cycle is the most important central pathway connecting almost all the
individual metabolic pathways (either directly or indirectly).
• The citric acid cycle was proposed by Hans Adolf Krebs in 1937, based on the
studies of oxygen consumption in pigeon breast muscle. The cycle is named in his
honour (Nobel Prize for Physiology and Medicine in 1953.)

4. Location of TCA cycle
• The enzymes of TCA cycle are located in mitochondrial matrix, in close
proximity to the electron transport chain. This enables the synthesis of ATP
by oxidative phosphorylation without any hindrance.

5. TCA cycle -Overview
• Krebs cycle basically 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 are oxidized to CO2 and
oxaloacetate is regenerated and recycled. Oxaloacetate is considered to play a
catalytic role in citric acid cycle

6. TCA cycle: an Open cycle
• Krebs cycle is a cyclic process. However, it should not be viewed as a closed circle,
since many compounds enter the cycle and leave. TCA cycle is comparable to a
heavy traffic circle in a national highway with many connecting roads. Each
intermediate of the cycle connecting another pathway is a road!

7. Reactions of citric acid cycle
1. Formation of citrate : Krebs cycle proper starts with the condensation of
acetyl CoA and oxaloacetate, catalysed by the enzyme citrate synthase.

9. • 2. and 3. Citrate is isomerized to isocitrate by the enzyme aconitase. This is achieved
in a two stage reaction of dehydration followed by hydration through the formation
of an intermediate cis-aconitate.
• 4. and 5. Formation of a-ketoglutarate : The enzyme isocitrate dehydrogenase (lCD)
catalyses the conversion (oxidative decarboxylation) of isocitrate to oxalosuccinate
and then to o-ketoglutarate. The formation of NADH and the liberation of CO2
occur at this stage.

10. • 6. Conversion of cl-ketoglutarate to succinyl CoA occurs through oxidative
decarboxylation, catalyzed by o-ketoglutarate dehydrogenase complex. This enzyme
is dependent on five cofactors-TPP, lipoamide, NAD+, FAD and CoA. The
mechanism of the reaction is analogous to the conversion of pyruvate to acetyl CoA
. At this stage of the TCA cycle, second NADH is produced and the second CO2 is
liberated.

11. • 7. Formation of succinate : Succinyl CoA is converted to succinate by succinate
thiokinase. This reaction is coupled with the phosphorylation of CDP to CTP. This
is a substrate level phosphorylation. GTP is converted to ATP by the enzyme
nucleoside diphosphate kinase. CTP + ADP

12. • 8. Conversion of succinate to fumarate : Succinate is oxidized by succinate
dehydrogenase to fumarate. This reaction results in the production of FADH2 and
not NADH. 9. Formation of malate : The enzyme fumarase catalvses the conversion
of fumarate to malate with the addition of H2O.

13. • 10. Conversion of malate to oxaloacetate : Malate is then oxidized to oxaloacetate by
malate dehydrogenase. 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.

14. 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 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.

16. Inhibitors of Krebs cycle

17. Amphibolic nature of the citric acid cycle
• The citric acid cycle provides various intermediates for the synthesis of many
compounds needed by the body. Krebs cycle is both cataholic and anaholic in
nature, hence regarded as amphibolic. TCA cycle is actively involved in
gluconeogenesis, transamination and deamination

18. • The most important synthetic (anabolic) reactions connected with TCA cycle are,
• 1 . Oxaloacetate and o-ketoglutarate, respectively, serve as precursors for the synthesis of
aspartate and glutamate which, in turn, are required for the synthesis of other non-essential
amino acids, purines and pyrimidines.
• 2. Succinyl CoA is used for the synthesis of porphyrins and heme.
• 3. Mitochondrial citrate is transported to the cytosol, where it is cleaved to provide acetyl
CoA for the biosynthesis of fatty acids, sterols etc.

19. References
• Biochemistry ,U. Satyanarayana, U Chakrapani, Page number-254 to 258.