Enzymes are biological catalysts that catalyze biochemical reactions, take place inside the cell. A particular enzyme only becomes active when the cell needs to perform the biochemical reaction catalyzed by that particular enzyme. Apoenzyme and holoenzyme are two states of the enzymes.
Apoenzyme is the catalytically-inactive, protein component of the enzyme. The holoenzyme is the catalytically-active form of the enzyme, consisting of the apoenzyme and cofactor. Cofactor can be either a metal ion or a small organic molecule. The main function of the cofactor is to bind with the structure of the apoenzyme to assist the function of the enzyme.