What effect do positive allosteric ligands have on Km?

Positive allosteric ligands enhance the activity of an enzyme by binding to an allosteric site, which is distinct from the active site where substrate molecules bind. This interaction leads to a conformational change in the enzyme that typically increases its affinity for the substrate.

Km, or the Michaelis constant, reflects the affinity of an enzyme for its substrate; a lower Km indicates a higher affinity. When a positive allosteric ligand binds to the enzyme, the altered conformation makes it easier for the substrate to bind. Consequently, the effective concentration of substrate required to reach half-maximal velocity decreases, resulting in a decrease in Km.

Thus, in the presence of a positive allosteric modulator, the enzyme's affinity for its substrate is enhanced, which is why the correct answer indicates that Km decreases. Understanding this concept is critical in biochemical applications, as it illustrates the regulatory mechanisms enzymes use to respond to different metabolic conditions.