Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. There are three common types of enzyme inhibition - competitive, uncompetitive and mixed inhibition (or non-competition inhibition). Competitive inhibition involves inhibitor binding at an enzyme's substrate binding site. Uncompetitive inhibition involves inhibitor binding to enzyme-substrate complex. Mixed inhibition involves inhibitor binding to both the free enzyme and the enzyme-substrate complex.