Final answer:
Noncompetitive inhibition occurs when an inhibitor binds to an allosteric site and changes the enzyme's shape, affecting both Km and Vmax. In this type of inhibition, Km remains the same, while Vmax decreases.
Step-by-step explanation:
Noncompetitive inhibition occurs when an inhibitor binds to an allosteric site on the enzyme, changing its shape and preventing the substrate from properly binding to the active site. As a result, both Km (the substrate concentration at which the reaction rate is half of the maximum) and Vmax (the maximum reaction rate) are affected.
In noncompetitive inhibition, Km remains the same because it measures how easily a substrate can bind to the active site, which is not affected by the inhibitor's binding at the allosteric site. However, Vmax decreases because the inhibitor alters the enzyme's shape and reduces its catalytic activity. Therefore, the correct answer is (a) Binds to the active site; Km increases, Vmax decreases.