An allosteric protein demonstrates a decrease in its Kg for molecule Y as the concentration of molecule X increases. Based on this information which of the following statements is true?

a. X is a homotropic positive modulator.
b. X is a heterotropic positive modulator.
c. X is a homotropic negative modulator.
d. X is a homotropic negative modulator.

X is negative heterotropic modulator

Explanation:

In allosteric regulations, modulators are molecules that causes a change in the conformation of an enzyme, hence, resulting a change in enzyme activity. It can lead to a decrease or an increase of the enzyme.  When a molecule decreases the enzyme activity it called a negative modulator, when it increases or activates the enzyme activity, it is called a positive modulator.

A positive or negative modulator can either be homotrophic (substrate acts as modulator) or heterotropic (another ligand acts as modulator).

In the example above, X is negative heterotropic modulator, because the modulator is different from the substrate and it also decreases the enzyme activity.

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