IMAT 2016 Q34 [Respiratory Enzymes]

Which of the following are respiratory enzymes?

  1. acetyl coenzyme A
  2. FAD
  3. reduced NAD

A. 1 and 2 only
B. 2 and 3 only
C. 1, 2 and 3
D. none of them
E. 1 and 3 only

The correct answer is D.

Acetyl CoA

Acetyl coenzyme A is, like the name suggests, a coenzyme. It plays a key role in the metabolism of proteins, lipids and carbohydrates.

One of its purposes is to be a reaction intermediate to ultimately enable the production of ATP through the Krebs cycle (or citrate cycle).

It is NOT a respiratory enzyme.


FAD, or flavin adenine dinucleotide, helps catalyze redox reactions. It plays a key role in the processes of DNA repair, photosynthesis, and has an antioxidant role (removal of toxic free radicals).

More specific uses for FAD are its role as cofactor :

  • for cytochrome B5 reductase, which helps maintain hemoglobin in its reduced state (which is its functional state)
  • for glutathione reductase, which helps prevent red blood cells from oxidative stress.

It is NOT a respiratory enzyme.

Reduced NAD

Reduced NAD or NADH, is an electron and hydrogen donor. It is obtained by the processes of glycolysis and/or the Krebs cycle.

It helps with oxidative reactions such as the ones in the electron transport chain that ultimately produces ATP (by oxidative phosphorylation).

It is NOT a respiratory enzyme.