Enzymes are proteins that catalyze biochemical reactions . A catalyst is a substance that greatly accelerates the rate of a particular reaction without being used up or permanently altered . In the real world , most catalysts eventually deteriorate and no longer function as a catalyst . In the cell , all enzymes are eventually degraded and converted back to their constituent amino acids plus , in some cases , byproducts of oxidation or other types of damage . Proteins do not have some unique magical property that allows them to function as enzymes . For certain activities nucleic acids also participate in the chemistry of catalysis . For example , mRNA can catalyze certain types of RNA splicing .

The specific order of the amino acids in an enzyme governs the reaction it catalyzes . The polypeptide chain folds , largely spontaneously to form a definite but somewhat flexible three - dimensional structure . The region of the enzyme directly involved in catalysis is called the active site . This region contains amino acid R groups that are arranged to bind specific substrates or reactants . In some enzymes , the active site functions by promoting bending of the substrate in such a way that the rate of a specific reaction is enhanced greatly . In other enzymes , the active site contains amino acid R groups that attack or chemically react with the substrate and thus enhance the rate of a specific reaction . Following the reaction , the products are released from the surface of the enzyme . Each cell of the body contains several thousand different types of enzymes . Enzymes occur in the cytosol at concentrations between 0.01 and 10 µM .