How many times decarboxylation occurs during each TCA cycle ?

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Many times

Twice

Thrice

Once

answer is D.

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Detailed Solution

During each turn of the citric acid cycle (also known as the tricarboxylic acid cycle or TCA cycle), decarboxylation occurs twice. Decarboxylation is the process of removing a carboxyl group (-COOH) from a molecule, resulting in the release of carbon dioxide (CO₂).

The two decarboxylation reactions in the TCA cycle occur during the following steps:

(1) Isocitrate to $α$ -ketoglutarate: Isocitrate is first converted to $α$ -ketoglutarate by the enzyme isocitrate dehydrogenase.
In this reaction, a carboxyl group is removed from isocitrate, and carbon dioxide is released.

(2) $α$ -ketoglutarate to succinyl-CoA: $α$ -ketoglutarate is then converted to succinyl-CoA by the enzyme $α$ -ketoglutarate dehydrogenase complex. Similar to the previous step, a carboxyl group is removed from $α$ -ketoglutarate, resulting in the release of carbon dioxide.