Any of a number of freely diffusing organic compounds that function as cofactor s with enzyme s in promoting a variety of metabolic reactions.
Coenzymes participate in enzyme-mediated catalysis in stoichiometric ( mole -for-mole) amounts, are modified during the reaction, and may require another enzyme-catalyzed reaction to restore them to their original state. Examples include nicotinamide adenine dinucleotide (NAD), which accepts hydrogen (and gives it up in another reaction), and ATP , which gives up phosphate groups while transferring chemical energy (and reacquires phosphate in another reaction). Most of the B vitamins (see vitamin B complex ) are coenzymes and are essential in facilitating the transfer of atoms or groups of atoms between molecules in the formation of carbohydrates, fats, and proteins. See also metabolism ; stoichiometry .