Question

Determine the correct answer and explain why it is correct. Identify why the other options are not correct.

1. One of the major roles of the TCA cycle is to generate reduced cofactors for ATP production from oxidative phosphorylation. The compound donating the net eight electrons to the cofactors is which one of the following?

1. Pyruvate
2. Acetyl-CoA
3. Lactate
4. Oxaloacetate
5. Phosphoenolpyruvate

Answer 1

Answer: TCA cycle or Krebs cycle serves as a common oxidative pathway for carbohydrates, fats and proteins. It is the final common oxidative pathway that oxidizes acetyl CoA to CO₂. It is the source of reduced co-enzymes that provide the substrate for the respiratory chain and forms the link between the catabolic and anabolic pathways. The central molecule which forms the integration of major metabolic pathways is acetyl CoA which donate 8 electrons, 3 pairs to NAD⁺ forming 3 NADH, another pair of electrons to FAD forming 1 FADH₂.

Pyruvate undergoes oxidative decarboxylation to give 2 molecules of acetyl CoA, along with 2 NADH + H⁺, so a total of 4 electrons are donated by pruvate to the cofactor. Lactate is formed anaerobically by the reduction of pyruvate in the presence of reducing agent NADH + H⁺ which involves 2 electrons. Oxaloacetate is an important junction point in metabolisms, it enters the TCA cycle causing complete oxidation of acetyl CoA and comes out of it without any change. Phosphoenolpyruvate forms pyruvate in the last step of glycolysis with the formation of ATP via substrate level phosphorylation and does not involves the donation of electrons to the cofactors.

So the correct option is (b).