Question

Which statement is wrong for Kreb’s cycle?
A) The cycle starts with condensation of acetyl group (Acetyl-CoA) with pyruvic acid to yield citric acid.
B) There are three points in the cycle where $NAD^+$ is reduced to NADH + $H^+$
C) There is one point in the cycle where $FAD^+$ is reduced to $FADH_2$
D) During conversion of Succinyl CoA to succinic acid, a molecule of GTP Is synthesised.

Answer 1

Hint: Totally 12 molecules of ATP are synthesised in one round of the Citric Acid Cycle. These are produced using $FADH_2$, NADH + $H^+$, and GTP. The ATP is produced when these molecules enter oxidative phosphorylation.

Complete Answer:
- In the Krebs cycle, citric acid is produced in the first step. However, this is not by condensation of acetyl CoA with pyruvic acid, but of acetyl CoA with oxaloacetic acid. Oxaloacetate is produced in the last step of the cycle, and then comes back to join a fresh molecule of Acetyl CoA. Because of the production of Citric acid in this step, it is also known as the citric acid cycle.
- For the final production of ATP, several intermediates are produced in this cycle. There are three steps where NADH + $H^+$ is produced. These are in the production of α-ketoglutaric acid, Succinyl CoA, and oxaloacetic acid.
- $FADH_2$ and GTP are produced once each. That is, in the conversion of Succinate to Fumaric acid, and the conversion of Succinyl CoA to Succinic acid respectively.
- Each molecule of NADH + $H^+$ gives rise to 3 ATP, $FADH_2$ gives rise to 2 ATP, and from GTP one ATP is produced. The citric acid cycle is therefore responsible for the production of 12 molecules of ATP for every turn through the cycle.

Hence the correct answer is option A.

Note: The Krebs cycle is also known as the TCA or tricarboxylic acid cycle. This is because citric acid, that is formed right in the first step of the cycle, has three carboxylic groups, and these are used and regenerated at the end.