Citric Acid Cycle (Kreb's Cycle)

a) Formation of acetyl CoA
The raw material for citric acid cycle is acetyl CoA. It is formed from fatty acids by b-oxidation and from pyruvic CoA to form Acetyl Co A. This reaction is catalysed by a set of enzymes called pyruvic acid dehydrogenase. In this reaction, 2 hydrogen atoms and one CO2 molecule are removed. Hence this reaction is called oxidative decarboxylation. The 2 hydrogen atoms are accepted by NAD and NAD is converted into NADH.

formation of acetyl coa in krebs cycle


b) Formation of citric acid
The acetyl- CoA combines with oxaloacetic acid to form citric acid. It contains 6 carbon atoms. This reaction is catalysed by an enzyme called citric acid synthetase.

c) Dehydration
Citric acid undergoes dehydration and forms cis-aconitic acid. This reaction is catalysed by the enzyme aconitase.


d) Hydration
The aconitic acid is hydrated and it forms isocitric acid. This reaction is catalysed by the enzyme aconitase.


e) Dehydrogenation I
Isocitric acid undergoes dehydrogenation in the presence of isocitric acid dehydrogenase to form oxalo succinic acid. In this reaction 2 hydrogen atoms are released. They are accepted by NAD+ to form NADH.

formation of nadh in citric acid cycle


f) Decarboxylation
The oxalo succinic acid undergoes decarboxylation to form a-ketoglutaric acid. This reaction is catalysed by decarboxylase. In this reaction one CO2 is eliminated. Hence the a-ketoglutaric acid has only 5 carbon atoms.


g) Oxidative decarboxylation
During oxidative decarboxylation a-ketoglutaric acid is converted into succinyl CoA. This reaction is catalysed by a-ketoglutaric acid dehydrogenase. Two hydrogen atoms are released and they are transferred to NAD. The NAD is converted into NADH.
In the next step, the succinyl CoA is decarboxylated to succinic acid. This step is catalysed by succinic acid thiokinase. CoA is liberated.

h) Oxidation
Succinic acid is oxidised to fumaric acid by the removal of 2 hydrogen atoms. The reaction is catalysed by succinic acid dehydrogenase. The hydrogen atoms are accepted by FAD and it forms FADH2.


i) Hydration
Fumaric acid undergoes hydration to form malic acid. This reaction is catalysed by fumarase.


j) Dehydrogenation
It is the final step in Krebs cycle. Oxalo acetic acid is regenerated from malic acid by a process of dehydrogenation. This reaction is catalysed by malic acid dehydrogenase in the presence of NAD. The 2 hydrogen atoms removed are accepted by NAD and it forms NADH.
The oxaloacetic acid formed in the above reaction condenses with the acetyl CoA to form citric acid again and thus the cycle is repeated.