Lesson 15 : Enzymes- Coenzymes and Co-Factors


  1. 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).
  2. 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-
  3. Both provide reactive groups not found on aminoacid. side chains.
  4. 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:


  1. 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.

  2. 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:

    1. water-soluble - B vitamins and vit. C required daily in diet, excess excreted in urine
    2. lipid-soluble - vitamins A, D, E, K Intake must be limited, Stored in fat
Last modified: Friday, 20 January 2012, 6:08 AM