Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions.

In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products. Almost all processes in a biological cell need enzymes in order to occur at significant rates.

Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.

References

Smith AD (Ed) et al. (1997) Oxford Dictionary of Biochemistry and Molecular Biology Oxford University Press ISBN 0-19-854768-4