What do competitive inhibitors do?

• A. They bind to the enzyme outside the active site
• B. They compete with natural substrate for binding site
• C. They are always reversible
• D. They bind covalently to the active site

Answer: (B)

Competitive inhibitors mimic the substrate and bind to the enzyme at the active site, so they compete with the natural substrate for binding. This means they temporarily block substrate access by occupying the same spot the substrate would use. Because of this competition, you can overcome the inhibition by increasing the substrate concentration, allowing the enzyme to bind substrate more often and reach its maximum rate. The presence of a competitive inhibitor raises the apparent substrate concentration needed (the Km appears higher) but does not change the maximum rate (Vmax stays the same).

The other ideas don’t fit this mechanism: binding outside the active site would be a different type of inhibition (allosteric or noncompetitive), and while many competitive inhibitors are reversible, saying they are "always reversible" isn’t a defining feature, nor would covalent binding to the active site describe this competitive scenario.