Cofactors are small molecules or compounds that bind to enzymes and allow the enzymes to function. Most of the enzymes require the presence of cofactors in order to be active. Inactive enzymes without a cofactor attached are referred to as apoenzymes. Conversely, enzymes that are complete with cofactors and ready to catalyze reactions are called holoenzymes.

There are two categories of cofactors: metals and coenzymes. Metal cofactors are essentially metal ions, such as Zn2+, Cu2+, Mg2+, and Ni2+. Ag is non-essential element in most of the organisms.