cofactor
Summary
A cofactor is a nonprotein helper, either a metal ion or a small organic molecule (a coenzyme, often vitamin-derived), that an enzyme needs to be active. The enzyme without it is an apoenzyme; with it, a holoenzyme. Testable point: losing the cofactor, such as by chelating a metal ion or by vitamin deficiency, lowers activity.
Detail
Meaning: A cofactor is a nonprotein helper that an enzyme needs for full activity. There are two groups: metal ions (Zn²⁺, Mg²⁺, Fe²⁺) and small organic molecules called coenzymes (NAD⁺, FAD, coenzyme A), many of which are made from vitamins. A protein lacking its cofactor is an apoenzyme and is inactive; with the cofactor bound it is a holoenzyme. A cofactor held very tightly or covalently is called a prosthetic group. Cofactors take part in catalysis, for example by carrying electrons or chemical groups.
On the MCAT: A passage may show an enzyme that loses activity after dialysis or treatment with a chelator such as EDTA, with activity restored when the metal ion is added back. Another may link a dietary vitamin deficiency to reduced activity of enzymes that need the matching coenzyme. Predict that the reaction rate drops without the cofactor, and that Vmax falls because less enzyme is active.
Common confusion: Some books reserve "cofactor" for metal ions and "coenzyme" for organic molecules; the MCAT usually treats coenzymes as one kind of cofactor. A cofactor is also not a regulator: it is required for activity, whereas an allosteric effector only modulates it.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.