Enzymes are biological catalysts. A catalyst is defined as a substance that increases the rate of a chemical reaction without itself undergoing any permanentchange. Enzymes do precisely this in living things. Without them, the rate of the reactions would be so slow as to cause serious, if not fatal, damage. Enzymes have two main functions: To act as highly specific catalysts, and also to provide a way of controlling reactions, the amount of enzyme determines how quickly the reaction can proceed.
Enzymes are usually globular proteins (some have been found to be RNA molecules that can act as enzymes) and have a specific three-dimensional shape. Enzyme molecules have a complicated three-dimensional shape due to the particular way the amino acid chain that makes up the protein is folded. An enzymes three-dimensional shape is called its tertiary structure. A few of the amino acids on the surface of the molecule fold inwards to make a specific indentation, called the active site, into which a particular substrate can fit, this is the where the reactions occur.
Enzymes are substrate specific. The active site of each different enzyme has its own particular tertiary structure, so only a substrate with a complementary shape will fit; we say it has a highly specific shape. In order to work, an enzyme must temporarily bond with a substrate; the substrate binds to the active site mainly with hydrogen bonds. Binding to the active site makes the substrate react more quickly. The substance produced at the end of a reaction between an enzyme and a substrate, is called the product. There may be more than one substrate or more than one product. The explanation of how enzymes catalyse reactions is called the lock and key hypothesis. The enzyme is the 'lock' and the substrate is the 'key'.