Enzymes are biological catalysts made by living cells.
A typical human cell contains several thousand enzymes.
They are used to catalyse a vast number of chemical reactions at temperatures suitable for living organisms, i.e. between 5 and 40°C.
Enzymes are vitally important because in their absence reactions in the cell would be too slow to sustain life.
The chemical (or chemicals) which an enzyme works on is called its substrate.
An enzyme combines with its substrate to form a short-lived enzyme/substrate complex. This proximity of the enzyme with the substrate in the complex greatly increases the chances of a reaction occurring.
Once a reaction has occurred, the complex breaks up into products and enzyme.
The enzyme remains unchanged at the end of the reaction and is free to interact again with more substrate.
Substrate + enzyme enzyme / substrate complex enzyme/product complex enzyme + product(s).
E + S ES EP E + P