Enzymes, which are made up of mostly proteins, are catalysts. A catalyst is a substance that modifies and increases the rate of a reaction without being consumed in the process.

Enzymes bind, temporarily, to one or more of the reactants of the reaction they catalyze. This lowers the level of activation energy required, thus increasing reaction activity.

Many enzymes consist of a protein and a non-protein (called the cofactor). The proteins in enzymes are usually globular. The functioning of the enzyme is determined by the shape of the protein.