Answer:
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.
I think the correct answer from the choices listed above is option B. A modification of the leaves into the spine in a cactus plant can be seen The spines are modified leaves. It helps defend the plant and also reduce the surface area which helps limit loss of water.
