Cofactor

The terminology distinguishes several overlapping categories. An organic cofactor is termed a coenzyme, and where it binds so tightly that it does not dissociate during catalysis it is called a prosthetic group; haem in cytochromes is the standard example. Loosely bound cosubstrates such as NAD+ and coenzyme A shuttle between enzymes, being chemically altered in one reaction and regenerated in another. Inorganic cofactors are metal ions, with zinc, magnesium, iron, copper and manganese the most frequently encountered, and they serve variously to polarise substrates, stabilise developing negative charge, or transfer electrons. The enzyme without its cofactor is an inactive apoenzyme; the assembled, catalytically competent form is the holoenzyme. Many water-soluble vitamins are precursors from which coenzymes are synthesised, which is the biochemical basis of the corresponding deficiency diseases.