Enzymes are catalysts that speed up chemical reactions in the body without being changed themselves. Each enzyme only works on specific substrates that fit into its active site like a key in a lock. The enzyme attaches to the substrate and causes a reaction that changes the substrate into a product, before detaching. Factors like temperature, pH levels, activators, and inhibitors can impact an enzyme's performance by changing its shape or binding to its active site. Reversible inhibitors are especially important as they are the basis for many drugs and can be overcome when the inhibitor is removed.