The catalytic activity of many enzymes depends on the presence of components called cofactors. Cofactors generally are either metal ions or small (relative to the enzymes) organic molecules called coenzymes.

Metal ions may assist in stabilizing the active conformation of the enzyme, or may play more specific roles in binding substrates and catalyzing reactions. Coenzymes fall into two groups:

These species are too numerous for us to consider all of them, so we will simply look at a few examples.

Metalloenzymes based on iron, zinc, copper, and cobalt are widespread, and a few enzymes have been found that contain nickel.

Of the metabolic coenzymes, the most widely used is adenosine triphosphate, ATP, which undergoes four types of reaction:

Line Structure of ATP ATP in a Jmol Window

Notice that several different fragments of the ATP are transferred in these reactions: a phosphoryl, an AMP unit, and an adenosyl unit.

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