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 S-adenosylmethionine ATP can donate phosphoryl group S-adenosylmethionine donates methyl groups in many biosynthesis reactions

Cofactor of nitric oxide synthase 5,6,7,8 - Tetrahydrobiopterin

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

Nicotinic acid (niacin) an nicotinamide are precursor of NAD and NADP Lack of niacin causes the disease pellagra NAD + and NADP + NAD and NADP are coenzymes for dehydro- genases

FAD and FMN Flavin adenine dinucleotide (FAD) and Flavin mononucleotide (FMN) are derived from riboflavin (Vit B 2 ) Flavin coenzymes are involved in oxidation-reduction reactions FMN (black), FAD (black/blue)

Thiamine Pyrophosphate (TPP) TPP is a derivative of thiamine (Vit B 1 ) TPP participates in reactions of: (1) Oxidative decarboxylation (2) Transketo- lase enzyme reactions

Pyridoxal Phosphate (PLP) PLP is derived from Vit B 6 family of vitamins PLP is a coenzyme for enzymes catalyzing reactions involving amino acid metabolism (isomerizations, decarboxylations, transamination)

Enzymes active sites Active site – specific region in the enzyme to which substrate molecule is bound Substrate usually is relatively small molecule Enzyme is large protein molecule Therefore substrate binds to specific area on the enzyme