Based on their chemical composition, the enzymes have been classified into three categories:

1. Enzyme molecule containing protein only. Example: Pepsin, trypsin, urease, and papain.
2. Enzyme molecule containing a protein and a cation. Example: Carbonic anhydrase (Zn²⁺ as cation) and arginase (Zn²⁺ as cation).
3. Enzyme molecule containing a protein and a non-protein and organic compound known as prosthetic group. Example: Iron phorphyrin enzyme – catalase, cytochrome c peroxidase, flavoprotein enzyme – glycine oxidase, and coenzymes – phosphorylase.