Which statement best defines a non-competitive inhibitor?

A non-competitive inhibitor is characterized by its ability to bind to an enzyme at a site other than the active site, which alters the enzyme's shape and thereby reduces its activity. This mechanism differs from that of competitive inhibitors, which bind directly to the active site and prevent substrate binding. By changing the shape of the enzyme, a non-competitive inhibitor can decrease the overall number of available functional enzymes, which prevents the conversion of substrate to product, regardless of the concentration of the substrate present. This means that, even if the substrate is abundant, the presence of the non-competitive inhibitor can still inhibit the reaction, making this definition accurate and relevant in enzymatic regulation contexts.

The other options do not accurately describe non-competitive inhibition: binding to the active site refers to competitive inhibition, having no effect on enzyme activity implies no inhibition occurs, and enhancement of reaction rates contradicts the fundamental nature of inhibition itself.