Question

If pyruvate-2-14C (pyruvate with the middle carbon atom radioactively labeled) is provided to actively respiring mitochondria, most of the radioactivity will be incorporated into citrate. Trace the route whereby radioactively labeled carbon atoms are incorporated into citrate, and indicate where in the citrate molecule the label will first appear.

Answer 1

Answer:

The middle carbon atom of pyruvate in the TCA cycle becomes the carboxyl carbon of acetate and hence the newly added (upper) carboxyl group in citrate

Explanation:

Before entering the TCA cycle, pyruvate is converted into Acetyl-CoA by  oxidative decarboxylation of pyruvate. 1st two carbons of glucose to be fully oxidized. This reaction is catalyzed by Pyruvate dehydrogenase.

The middle carbon atom of the pyruvate in the TCA cycle becomes the carboxyl carbon of the acetate as the carboxyl group is removed from the pyruvate , releasing CO2. NAD is reduced to NADH. Acetyl group is transferred to coenzyme-A , resulting in acetyl coA