/koh"fak'teuhr/, n.
1. Biochem. any of various organic or inorganic substances necessary to the function of an enzyme.
2. Math.
a. a prefactor or postfactor.
b. the product of the minor of a given element of a matrix times -1 raised to the power of the sum of the indices of the row and column crossed out in forming the minor.
[1935-40; CO- + FACTOR]

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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.

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      a component, other than the protein portion, of many enzymes. If the cofactor is removed from a complete enzyme (holoenzyme), the protein component (apoenzyme) no longer has catalytic activity. A cofactor that is firmly bound to the apoenzyme and cannot be removed without denaturing the latter is termed a prosthetic group; most such groups contain an atom of metal such as copper or iron. A cofactor that is bound loosely to the apoenzyme and can be readily separated from it is called a coenzyme. Coenzymes take part in the catalyzed reaction, are modified during the reaction, and may require another enzyme-catalyzed reaction for restoration to their original state.

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Universalium. 2010.

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