table 8-3 International Classification of enzymes No Class Type of reaction catalyzed Oxidoreductases Transfer of electrons(hydride ions or H atom Transferases Group-transfer reactions Hydrolase Hydrolysis reactions(transfer of functional groups to water) 4 Lyases Addition of groups to double bonds, or format of double bonds by removal of groups Isomerases Transfer of groups within molecules to yield isomeric forms Ligases Formation of c-CC-Sc-0, and C-N bonds by condensation reactions coupled to ATP cleavage Most enzymes catalyze the transfer of electrons, atoms, or functional groups. They are theretore classified, given code numbers, and assigned names according to the type of transfer reactio the group donor, and the group acceptor Lehninger
Lehninger
ENZYMES ARE HIGHLY SPECIFIC Enzymes are highly specific both in the reaction catalyzed and in their choice of reactants, which are called substrates. An enzyme usually cata- lyzes a single chemical reaction or a set of ciosely related reactions. Side reactions leading to the wasteful formation of by-products rarely occur in enzyme-catalyzed reactions, in contrast with uncatalyzed ones. The de gree of specificity for substrate is usually high and sometimes virtually absolute
Let us consider proteolytic enzymes as an example. The reaction catalyzed by these enzymes is the hydrolysis of a peptide bond H O H O N-C—C—N—C-C—+H2O 一N=C-C++H2N-C H Peptide Carboxyl Amino component component Most proteolytic enzymes also catalyze a different but related reaction. namely, the hydrolysis of an ester bond R1-C-0-R2+H2O-=R1 +Ho-R,+H+ Ester Acid Alcohol
H O H O H O T N—C—C一N-C—C-N—C—C H2O N-C-C一NCC+H2N-C H H R H Lysine Lysine arginine arginine Figure 3-21 Trypsin hydrolyzes polypeptide on the car boxyl side of arginine and lysine residues
Hydrolysis site H N一C—C+N-C-C H Lysine or argInine Hydrolysis site H O H N—C—C+N—C-C H H B Arginine Glycine Figure 8-1 Comparison of the specificities of (A) trypsin and (B)thrombin, Trypsin cleaves on the carboxyl side of argi nine and lysine residues. Thrombin cleaves Arg-Gly bonds in particular sequences only