Question

Write with structural formulas and cofactors a reaction which is inhibited by arsenite

and a reaction which is inhibited by NADH+H⁺

Answer 1

Ans. #1. Arsenite acts as the inhibitor of pyruvate dehydrogenase complex that catalyzes the conversion of pyruvate into acetyl-CoA.

#2. NADH is required as coenzyme for several enzymes.

NADH itself does not act as inhibitor of any reaction in the biological system.

However, NAD⁺/ NADH ratio may be the driving factor for various reactions.

For example, consider step 6 of glycolysis-

            G-3-P + NAD⁺ + Pi ----G-3-P dehydrogenase-------> 1,3-bisPG

The cytoplasm has a limited pool of NAD⁺/NADH. During glycolysis, NAD⁺ is reduced into NADH. Thus, an increase in cytoplasm [NADH] means a decrease in cytoplasmic [NAD⁺]. In unavailability of NAD⁺ in cytoplasm, the enzyme glyceraldehyde-3-phosphat dehydrogenase becomes catalytically inactive.

NAD⁺ is regenerated from NADH in cytoplasm through malate-aspartate shuttle during aerobic respiration and through lactate fermentation during anaerobic respiration.