Question

Enzymes (or their production) are often regulated in a negative or positive way, depending on whether they are related to anabolic (building) or catabolic (breaking down) reactions. Please describe, in your own words, why excess of products might inhibit enzymatic activity for anabolic reactions but why excess of reactants might stimulate enzymatic activity for catabolic reactions. This question is pretty complex, so take your time to develop your thoughts prior to typing your answer. The best answers should incorporate aspects of bioenergetics, genetic expression and cell biology.

Answer 1

In general, the reverse is true.

Excess of products might inhibit enzymatic activity for catabolic reactions but an excess of reactants stimulate enzymatic activity for anabolic reactions.

Examples:

1. n Glucose-6-phosphate -----> -(-glucose-alpha1,4-glucose-)-n

It is an anabolic reaction.

Glycogen synthase is activated by Glucose-6-phosphate.

It is an anabolic reaction.

Here, increased availability of glucose activates glycogen synthase so that the blood glucose levels are reduced rapidly.

If the substrate concentration is less, the enzyme becomes less functional as there is no need for the rapid synthesis of glycogen.

2. Glucose ------> Glucose-6-phosphate

It is a catabolic reaction.

Hexokinase is inhibited by its product Glucose-6-phosphate. This ensures that rapid degradation of glucose does not occur even if there is no/limited energy requirement.