Final answer:
In noncompetitive inhibition, Vmax is reduced but Km remains unchanged because the inhibitor binds to the enzyme away from the active site, altering its structure without affecting the substrate affinity.
Step-by-step explanation:
When noncompetitive inhibition occurs, Vmax decreases but Km stays the same. In noncompetitive inhibition, an inhibitor can bind to either the free enzyme or the enzyme-substrate complex, since its binding site is distinct from the enzyme's active site. This binding alters the enzyme's structure and affects the active site's configuration. The result is that the maximum reaction rate is lowered regardless of an increase in substrate concentration, whereas Km, the substrate concentration at which the reaction rate is half of Vmax, remains unchanged because the affinity between the enzyme and substrate is not affected.