A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

A non-competitive inhibitor is characterized by its ability to bind to a site other than the active site on the enzyme. This binding occurs at an allosteric site, which means that the inhibitor can affect the enzyme's activity without directly competing with the substrate for the active site. When a non-competitive inhibitor binds, it leads to a conformational change in the enzyme, thereby reducing its activity regardless of whether the substrate is present or not. This means that increasing substrate concentration does not overcome the inhibition effect caused by a non-competitive inhibitor.

The other characteristics described in the choices highlight different features of enzyme inhibitors. Binds to the active site pertains to competitive inhibitors, which compete with the substrate. An increase in enzyme activity is a characteristic contrary to any form of inhibition. Permanent modification suggests that the enzyme undergoes irreversible changes, which is not true for non-competitive inhibitors as their effect can often be reversed if the inhibitor is removed. Thus, the correct characteristic of a non-competitive inhibitor is its binding to a site other than the active site.