Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Enzymes
Allosteric enzymes are inhibited or activated by their modulators

When a specific inhibitory, or negative, modulator binds to the allosteric site—which happens when the modulator concentration increases within The Cell—the enzyme shifts into a less active or completely inactive form, effectively "turning off." Conversely, when the modulator concentration in the cell drops, the inhibitor leaves the allosteric site and the enzyme "turns on" again, reverting to its active form.

However, There are also allosteric Enzymes that are activated by modulator molecules. In this case, the activating, or positive, modulator function is performed not by the end product of a given enzymatic pathway, but by another metabolite that serves as a molecular signal indicating that the enzyme needs to "speed up" (Fig. 9-20). Often, the substrate molecule itself acts as this type of activating modulator. Allosteric enzymes of this Class, known as homotropic (since the modulator and the substrate are the same compound), possess two or more binding sites for the substrate. These binding sites frequently serve a dual function, acting as both catalytic and regulatory sites. Allosteric enzymes of this type respond to situations where the substrate accumulates in excess and must be depleted through downstream reactions.

Fig. 9-20. MECHANISM OF ACTION of inhibitory and activating Allosteric regulators.

Thus, There are two types of allosteric enzymes. Enzymes of the first type are inhibited by their modulators, which typically differ in chemical Structure from the substrate (hence these enzymes are called heterotropic). Enzymes of the second type are activated by their modulators, frequently by the substrates themselves. The "turning on" and "turning off" of allosteric enzymes is largely reminiscent of the "switching" behavior of Hemoglobin by 2,3-bisphosphoglycerate (Chapter 8).

Some allosteric enzymes are subject to regulation by two or more modulators that can exert opposite effects on the enzyme—that is, one modulator (or more) activates the enzyme, while another (or others) inhibits it. In such more complex enzymes, each modulator has its own specific allosteric site; by binding to this site, the modulator signals the enzyme to increase or decrease its catalytic activity.