Biochemistry

Table 3.1 - Classification of coenzymes Coenzyme name The initial substance for the formation of coenzyme Vitamin coenzymes Thiamine (TDF) Flavin (FMF, FAD) Pantothenic (CoA) Nicotinamide (NAD, NADP) Pyridoxine (pyridoxalphosphate) Folic (THFK) Cobalamide Biotin Thiamine (vitamin Bl) Riboflavin (vitamin B2) Pantothenic acid (vitamin B3) Nicotinamide (vitamin PP) Pyridoxine (vitamin B6) Folic acid (vitamin Be) Vitamin bl 2 Biotin (vitamin H) Non-Vitamin coenzymes Containing metal Metalloporphyrin Peptide Carbohydrate Nucleotide (UDFG) (not derived from vitamins) Mn, Mg, Fe, Zn, Mo, Ca Gems, chlorophyll Glutathione Monosaccharide phosphates Nucleoside mono- and diphosphates Complex enzyme systems are known containing several enzyme proteins and coenzymes. The interaction of the enzyme with the substance (substrate) undergoing change is ensured by the presence on the surface of the enzyme of a certain region, the structure of which is complementary to the structure of the substrate, i.e., approaches it like a key to a lock. In simple protein enzymes, the catalytic function is performed by a part of the protein molecule, called the active (catalytic) center, which is a unique combination of certain amino acid residues that make up the enzyme's peptide chain. The architectonics of the center provides interaction with certain groups of the substrate. The most frequently occurring residues in the active sites are serine, tyrosine, histidine, tryptophan, arginine, cysteine, aspartic and glutamic acids (Fig. 3.1). Amino acid residues responsible for the catalytic activity of a simple enzyme are located in different parts of the polypeptide chain. In a series of bicomponent enzymes, two active centers are found: substrate ­ binding and allosteric. 82