Enzyme inhibition refers to the decrease in the activity of an enzyme due to the interaction of certain molecules called inhibitors. Competitive and uncompetitive inhibition are two important types of reversible enzyme inhibition. They differ in their mechanism of action, binding site, effect on enzyme kinetics, and influence on reaction rate. In competitive inhibition, the inhibitor resembles the substrate in structure and competes with the substrate for binding at the active site of the enzyme. Since both substrate and inhibitor cannot bind simultaneously to the active site, the inhibitor blocks substrate binding and prevents formation of the enzyme-substrate complex. Competitive inhibition is therefore dependent on the relative concentrations of substrate and inhibitor. If the substrate concentration is increased sufficiently, it can outcompete the inhibitor and restore enzyme activity. A common example of competitive inhibition is the inhibition of succinate dehydrogenase by malonate. Malonate has a structure similar to succinate and competes for the active site of the enzyme. Another example is methotrexate inhibition of dihydrofolate reductase. In uncompetitive inhibition, the inhibitor does not bind to the free enzyme. Instead, it binds only to the enzyme-substrate complex at a site different from the active site. This binding forms an inactive enzyme-substrate-inhibitor complex and prevents the reaction