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 aminoacid. 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 S-adenosylmethionine
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
Vitamins are required for coenzyme synthesis and must be supplied in the diet
Lack of particular vitamins causes disease
There are two catagories of vitamins:
water-soluble - B vitamins and vit. C required daily in diet, excess excreted in urine
lipid-soluble - vitamins A, D, E, K Intake must be limited, Stored in fat
