Isoenzymes. Role of cofactors and
coenzyme vitamins in the catalytic action
of enzymes.
Coenzymes
• Coenzymes act as group-transfer reagents
• Hydrogen, electrons, or groups of atoms can be
transferred
Coenzyme classification
(1) Metabolite coenzymes - synthesized from
common metabolites
(2) Vitamin-derived coenzymes - derivatives of
vitamins
Vitamins cannot be synthesized by mammals, but
must be obtained as nutrients
Examples of metabolite coenzymes
ATP can donate
phosphoryl group
ATP
S-adenosylmethionine
donates methyl groups
in many biosynthesis
reactions
S-adenosylmethionine
5,6,7,8 - Tetrahydrobiopterin
Cofactor of nitric oxide synthase
Vitamin-Derived Coenzymes
• Vitamins are required for coenzyme synthesis
and must be obtained from nutrients
• Most vitamins must be enzymatically
transformed to the coenzyme
• Deficit of vitamin and as result correspondent
coenzyme results in the disease
NAD+ and NADP+
• Nicotinic acid (niacin) an nicotinamide are precursor of
NAD and NADP
• Lack of niacin causes the disease pellagra
NAD and
NADP are
coenzymes
for
dehydrogenases
FAD and FMN
• Flavin adenine dinucleotide (FAD) and Flavin
mononucleotide (FMN) are derived from riboflavin (Vit B2)
• Flavin coenzymes are involved in oxidation-reduction
reactions
FMN (black), FAD (black/blue)
Thiamine Pyrophosphate (TPP)
• TPP is a
derivative of
thiamine (Vit B1)
• TPP participates
in reactions of:
(1) Oxidative
decarboxylation
(2) Transketolase enzyme
reactions
Pyridoxal Phosphate (PLP)
• PLP is derived from Vit B6 family of vitamins
PLP is a coenzyme for enzymes catalyzing reactions involving amino
acid metabolism (isomerizations, decarboxylations, transamination)
Enzymes active sites
Substrate usually is relatively small
molecule
Enzyme is large protein molecule
Therefore substrate binds to specific
area on the enzyme
Active site – specific region in the
enzyme to which substrate molecule is
bound