Unit 6 - Vitamins & Coenzymes
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
There are two types of cofactors:
1) essential ions - metal ions -inorganic
2) coenzymes - +rganic mo-ecules 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:
1) 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
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 B vitamins and their coenzymes
Niacin (nicotinic acid) nicotinamide
Get niacin in enriched cereals, meat, legumes.
NAD an+ NADP are the coenzymes (cosubstrates).
NAD consists of 2 5’ribonucleotides (AMP and nicotinamide monomucleotide) joined by a
For NADP , have a phosphoryl group on 2’-oxygen.
Both coenzymes act as cosubstrates for dehydrogenases catalyze the oxidation of substrates
by transfer of 2e and 1H NADH and NADPH.
Vitamin B (2iboflavin)
Coenzymes are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).
Riboflavin found in milk, whole grains, liver.
The coenzymes serve as prosthetic groups involved in 1e or 2e transfers.
FAD + 2e + 2H ---> FADH 2
FMN + 2e + 2H --> FMNH
Enzymes that require FAD or FMN are called flavoenzymes or flavoproteins.
Can actually donate 1 or 2 e at a time form partially oxidized compound when only 1e is
donated relatively stable.
Vitamin B (1hiamine)
Structure: pyrimidine ring and positively charged thiazolium ring.
Found in husks of rice and other cereals, liver, meat, particularly pork.
Deficiency in thiamine causes beriberi - extensive nervous system and circulatory system
damage, muscle wasting, edema.
Coenzyme form is thiamine pyrophosphate (TPP) - synthesized by transfer of pyrophosphoryl
group from ATP via thiamine pyrophosphate synthetase.
Used primarily in decarboxylases as a coenzyme.
Vitamin B6 fa