Streptomyces albus G serine β-lactamase. Probing of the catalytic mechanism via molecular modelling of mutant enzymes

In previous studies, several amino acids of the active site of class A , β-lactamases have been modified by site-directed mutagenesis. On the basis of the catalytic mechanism proposed for the Streptomyces albus G , β-lactamase [Lamotte- Brasseur, Dive, Dideberg, Charlier, Frere & Ghuysen (1991) Biochem. J. 279, 213-221], the influence that these mutations exert on the hydrogen-bonding network of the active site has been analysed by molecular mechanics. The results satisfactorily explain the effects of the mutations on the kinetic parameters of the enzyme's activity towards a set of substrates. The present study also shows that, upon binding a properly structured ,β-lactam compound, the impaired cavity of a mutant enzyme can readopt a functional hydrogen-bonding-network configuration.