Enzymes are globular proteins whose catalytic role is to increase the rate of reactions in living systems. They are not consumed or permanently changed by the reactions they catalyse, so an enzyme can be used again.
An enzyme's catalytic activity depends on its three-dimensional protein structure. Part of this structure forms an active site, where substrate molecules can bind.
Enzymes in living organisms
Metabolism consists of many linked reactions, and enzymes control the individual steps within these pathways. Enzyme-controlled reactions therefore contribute to structures and functions from the cellular level to the whole organism.
Intracellular enzymes
These are produced and function inside cells, where they catalyse cellular reactions.
Extracellular enzymes
These are secreted by cells and catalyse reactions outside cells. Digestive enzymes in the gut are examples.
Because an enzyme is a protein, conditions that disturb its tertiary structure can also alter its active site and prevent normal catalytic activity.
Exam Tip: When defining an enzyme, explain that it increases the rate of a biological reaction and is not used up by that reaction.