Substrate specificity of penicillin amidase from E. coli.

1. The kinetic parameters of 12 substrates of penicillin amidase (penicillin amidohydrolase, EC 3.5.1.11) from E. coli have been determined. Most of the penicillin amidase amide substrates containing a phenylacetyl group in the acyl moiety have been shown to have similar catalytic constants of 50 s-1. Substitution of the phenylacetyl group b 2-thienylacetyl group (cephalothin, cephaloridine) having a similar structure leads to a slight decrease in kcat. 2. Nonspecific penicillin amidase substrates, which contain a free amino group in their acyl moiety, are characterized by a strong dependence of kcat, on the structure of the leaving group with Km being constant. To investigate the free amino group influence on the reaction kinetics, pH-dependences of kcat/Km of enzymatic hydrolysis of phenylacetic and D-(-)-alpha-aminophenylacetic acid p-nitroanilides have been studied. It has been shown that enzyme binds the deprotonated form of the substrate only. 3. Under thermodynamically favourable conditions for the synthesis of beta-lactam antibiotics (at low pH), a concentration of the deprotonated substrate form is very low, and the reaction proceeds in the bimolecular regime. The value of the second-order rate constant for the substrate having a free amino group is small even at pH 7.5, and sharply decreases as does the pH. Hence, despite the favourable thermodynamic conditions for the production of all beta-lactam antibiotics, low reaction rate is the basic hindrance for enzymatic synthesis of penicillins and cephalosporins having a free amino group in the acyl moiety.