Competitive Inhibition

A competitive inhibitor is a molecule with a similar shape to the substrate. It can fit into and bind to the active site, but no reaction takes place (or it reacts very slowly). While the inhibitor occupies the active site, the substrate cannot bind, so fewer enzyme-substrate complexes form and the rate falls.The inhibitor and the substrate compete for the same active sites. Which one binds depends on their relative concentrations:
• increasing inhibitor concentration increases the inhibition
• increasing substrate concentration reduces the effect, because a substrate molecule is more likely than an inhibitor molecule to collide with any free active siteAt very high substrate concentration, almost all active sites are occupied by substrate, so the same maximum rate (V_max) can still be reached; it just needs more substrate. Most competitive inhibitors are reversible.Examples:
• malonate inhibits succinate dehydrogenase, a respiratory enzyme, because it has a similar shape to its substrate, succinate
• in methanol poisoning, patients may be given ethanol, which competes with methanol for the active site of alcohol dehydrogenase, reducing the formation of toxic productsWorked example: at a fixed inhibitor concentration, the substrate concentration is raised. The proportion of active sites occupied by inhibitor gets smaller, so the rate rises towards the uninhibited rate.