Isoenzymes. Role of cofactors and coenzyme vitamins in the catalytic action of enzymes.

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Isoenzymes. Role of cofactors and coenzyme
vitamins in the catalytic action of enzymes.
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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
• Vitamin-derived coenzymes - derivatives of
  vitamins
• Vitamins cannot be synthesized by mammals, but
  must be obtained as nutrients

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Examples of metabolite coenzymes
ATP can donate phosphoryl group
ATP
S-adenosylmethionine donates methyl groups in
many biosynthesis reactions
S-adenosylmethionine
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5,6,7,8 - Tetrahydrobiopterin
Cofactor of nitric oxide synthase
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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

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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 dehydro-genases
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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)
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Thiamine Pyrophosphate (TPP)

• TPP is a derivative of thiamine (Vit B1)
• TPP participates in reactions of (1)
  Oxidative decarboxylation(2) Transketo-lase
  enzyme reactions

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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)

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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