An atom, organic molecule, or molecular group that is necessary for the catalytic activity (see catalysis) of many enzymes. A cofactor may be tightly bound to the protein portion of an enzyme and thus be an integral part of its functional structure, or it may be only loosely associated and free to diffuse away from the enzyme. Cofactors of the integral kind include metal atoms—such as iron, copper, or magnesium—or moderately sized organic molecules called prosthetic groups; many of the latter contain a metal atom, often in a coordination complex (see transition element). Removal of the cofactor from the enzyme's structure causes loss of its catalytic activity. Loosely associated cofactors are called coenzymes; examples include most members of the vitamin B complex. Rather than directly contributing to the catalytic ability of an enzyme, coenzymes participate with the enzyme in the catalytic reaction. Sometimes this distinction in definition is no longer made, and coenzyme is used in the broader sense of cofactor.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on cofactor, visit

Seen & Heard

What made you look up cofactor? Please tell us what you were reading, watching or discussing that led you here.

Get Our Free Apps
Voice Search, Favorites,
Word of the Day, and More
Join Us on FB & Twitter
Get the Word of the Day and More