A series of enzymes catalyze the reactions in the metabolic pathway X → Y → Z → A. Product A binds to the enzyme that converts X to Y at a position remote from its active site. This binding decreases the activity of the enzyme. What is substance X?
A) an allosteric inhibitor
B) a substrate
C) an intermediate
D) the product

Question

Quizplus · Accepted Answer

Substance X is a substrate because it is being converted into product Y by an enzyme. The fact that Product A binds to the enzyme at a remote site and decreases its activity indicates that it is an allosteric inhibitor, but this is not the identity of Substance X. Substance X could potentially be an intermediate in the pathway, but it is described as being converted into Y, suggesting that it is a substrate. The product has already been formed by the time the enzyme that converts Z to A comes into play.

A Series of Enzymes Catalyze the Reactions in the Metabolic