There are other groups that contribute to the reactivity of enzymes beside amino acid residues. These groups are called cofactors - chemicals required by apoenzymes (inactive) to become holoenzymes (active). There are two types of cofactors: essential ions - metal ions -inorganic, coenzymes - organic molecules that act as group-transfer reagents (accept or donate groups)- can also be h+ and/or e- Both provide reactive groups not found on a. a. side chains. Coenzymes can be either cosubstrates (loosely bound to enzyme; is altered, then regenerated) or prosthetic groups (tightly bound to enzyme). Coenzymes can be classified by their source: metabolite coenzymes synthesized by common metabolites include nucleoside triphosphates. Most abundant is atp, but also include uridine diphosphate glucose (udp-glucose) and. Atp can donate all of its three phosphoryl groups in group-transfer reactions. S-adenosylmethionine can donate its methyl group in biosynthetic reactions. Udp-glucose is a source of glucose for synthesis of glycogen in animals and starch in plants: vitamin-derived coenzymes.