Enzymes are sometimes considered under two broad categories:

1. Endozymes or intracellular enzymes – act within the cells in which they are produced.Example: All metabolic enzymes.
2. Exozymes or extracellular enzymes – produced in the cells and act outside the cell.Example: Digestive enzymes.

Chemically, the enzymes may be divided into two categories:

1. Simple protein enzymes: They contain simple proteins only.
2. Complex protein enzymes: They contain conjugated proteins.

The functional unit of enzyme is known as holoenzyme, which is oft en made up of apoenzyme (the protein part) and coenzyme (non-protein part).

The term prosthetic group is used when the non-protein part (tightly) covalently binds with the apoenzyme. The coenzyme or cofactors can be easily separated from apoenzyme.

Coenzyme: Examples: NAD, NADP, FAD, FMN, BIOTIN, and TPP

Cofactors: Examples: Mg²⁺, Mn, Ca, Cu, Co, and Fe

The word monomeric enzyme is used when if it’s made up of single polypeptide. Examples: ribonuclease and trypsin.

Oligomeric enzymes possess more than one polypeptide unit. Examples: Lactate dehydrogenase and aspartate transcarbamoylase.

Multienzyme complex possesses specific sites to catalyse different reactions in sequence. Examples: Pyruvate dehydrogenase and fatty acid synthase.