Question

In Krebs’ cycle, the FAD participates as an electron acceptor during the conversion of
A. Succinic acid to fumaric acid
B. Fumaric acid to malic acid
C. α-ketoglutarate to succinyl coA
D. Succinyl coA to succinic acid

Answer 1

Hint: Krebs’ cycle is the process through which Acetyl coA produced in the mitochondria is fully oxidized in the mitochondria through a series of metabolic reactions and produces Carbon dioxide, water, and ATP and reduces the hydrogen acceptor such as $NADH_2$ and $FADH_2$. FAD is an electron carrier because it forms $FADH_2$ by accepting two electrons.

Complete answer:
Succinic acid undergoes a dehydrogenation reaction by the enzyme succinic dehydrogenase which uses FAD as the hydrogen acceptor to produce $FADH_2$. Hence the option A. Succinic acid to fumaric acid is correct.
Option:B - Fumaric acid is transformed into Malic acid under the action of the Fumarase enzyme and one mole of water is utilized in this process. This step is also known as hydration. No involvement of electron acceptor is seen in this process hence option B. Fumaric acid to malic acid is incorrect.
Option:C - α-ketoglutarate is oxidatively decarboxylated to succinyl coA with the help of the enzyme α-ketoglutaric acid dehydrogenase. One molecule of CoA is utilized in this process and the electron acceptor here is NAD. Hence, the option C. α-ketoglutarate to succinyl coA is incorrect.
Option:D - Succinyl coA is converted to succinic acid in the presence of the enzyme succinate thiokinase. In this reaction, CoA-SH is cleaved, and GDP and Pi form GTP which in turn forms ATP. No involvement of FAD is seen in this process. Hence the option D. Succinyl coA to succinic acid is incorrect.

Hence option C. Succinic acid to fumaric acid is the correct answer.

Note :
Flavin Adenine Dinucleotide is the full form of FAD. Krebs’ cycle is a catabolic pathway. The total number of ATPs produced in one Krebs’ cycle is. Krebs’ cycle is also known as a citric acid cycle (CAC) or Tricarboxylic Acid Cycle (TCA cycle). The TCA cycle occurs exclusively in aerobic conditions. Succinic dehydrogenase enzyme is found on the inner mitochondrial membrane.