Although enzymes are the main biological catalysts, several types of RNA molecules known as ribosomes can also speed up reactions by lowering the activation energy. Enzymes are released completely unchanged after catalyzing the conversion of reactants to products. Enzymes catalyze only a single type of biochemical reaction and combine with only one single type of molecules or group of closely related molecules. This characteristic is called specificity . Enzymes are saturated by high substrate concentrations.
Many enzymes contain cofactors , which are nonprotein groups that contribute to their function.
Organic cofactors, which are called coenzymes , are all complex chemical groups of various kinds, many are derived from vitamins. Also, when tightly linked to an enzyme by a covalent bond, these coenzymes are sometimes referred to as prosthetic groups . The part of the enzyme which combines with a substrate is the active site . The active site takes up a very small portion of the total structure in most enzymes. In most, this site is located in a cavity or a pocket ton the enzyme surface.