Question

FAD⁺ is reduced during the conversion of
A) Citric acid to isocitric acid
B) isocitric acid to alpha ketoglutaric acid
C) alpha-ketoglutaric acid to succinyl CoA
D) succinic acid to fumaric acid

Answer 1

NAD and FAD are two cofactors which are involved in cellular respiration. Cellular respiration is the process in which energy is derived from the blood that is glucose. Cellular respiration includes the following steps- glycolysis, Krebs cycle, electron transport chain. The electron and hydrogen atoms are released from glycolysis and Krebs cycle combine with energy NAD and FAD molecules to form NADH and FADH₂.

Complete answer: The condensation takes place in Krebs cycle. Acetyl group is converted into the citric acid by the reaction with water and fatty acids. The enzyme hydrate synthesis is used for the catalyzation. The citric acid obtained from the previous step is converted into isocitrate. Then the next process is decarboxylation which field ketoglutarate acid and succinyl-CoA. The succinyl-CoA converts into succinic acid in which one molecule of GTP is produced. In the next reaction, ATP is produced from the ADP, and NAD is reduced to NADH, and FAD is reduced to FADH. Then the acetic acid is converted to oxalate which is again converted into nitrate and hence the process is repeated. In the third step of the citric acid cycle in which the citrate is converted to ketoglutarate, energy is reduced to NADH₂ and in the fourth step where ketoglutarate is converted into succinyl-CoA in which FAD is reduced to FADH. In the last step of the Krebs cycle, NAD is converted into NADH.
FAD⁺ is the hydrogen acceptor in this reaction as there is very little free change. In this reaction, succinic acid is oxidized to fumaric acid. At the same time, FAD⁺ is reduced to FADH₂.
Thus, the answer is option D: succinic acid to fumaric acid.

Note: This Kreb’s cycle or citric acid cycle takes place two times for each molecule of the glucose and it is the cycle of closed-loop.