Question

Acceptor of acetyl Co-A in the Krebs cycle is
A. Malic acid
B. Fumaric acid
C. α-ketoglutaric acid
D. Oxaloacetic acid

Answer 1

Hint: Krebs’ cycle or tricarboxylic acid cycle occurs inside the mitochondrial matrix of the inner chamber. Its details were given by Sir Hans Krebs' pigeon muscles. It is also called Citric acid cycle as the first product formed is citric acid.

Complete answer: Krebs’ cycle acts as a common pathway for metabolism of carbohydrates, fats and proteins and it acts as a metabolic sink to receive the intermediates of fats and proteins for metabolism for their final oxidation. Oxaloacetic acid combines with acetyl-CoA and enters Krebs’ cycle and repeats the process. In Krebs’ cycle, four pairs of hydrogen are removed from the cycle intermediates by enzymatic dehydrogenation catalyzed by dehydrogenases or oxido-reductases. Three pairs of hydrogen are used to reduce three molecules of NAD and one molecule FAD. Krebs’ also receives the intermediates of fat and protein oxidation. As two molecules of acetyl-CoA are formed from one glucose molecule, Krebs’ cycle must rotate twice for each molecule to respire.
The Citric acid cycle or Krebs’ cycle has the following steps- condensation, rearrangement, dehydrogenation, decarboxylation, oxidative decarboxylation, dehydrogenation, hydration, and dehydrogenation again.
In eukaryotes, all enzymes of Krebs’ cycle occur in the matrix of mitochondria but in prokaryotes, Krebs’ cycle occurs in cytoplasm. One turn of Krebs’ cycle produces twelve Adenosine triphosphate.
As it serves in both catabolic and anabolic processes, it is an amphibolic pathway.
So, the correct answer is “Option D”.

Note: This is a cycle because the last step regenerates the reactants of the first step. Krebs’ cycle requires continuous supply of NAD and FAD as it occurs only under aerobic conditions. It is a low energy producing process.