0-Lactamase catalytic properties

Some cephalosporins can be both substrates and inhibitors of /3-lactamases. The acyl-enzyme intermediate can undergo either rapid deacylation (Fig. 5.4, Pathway a) or elimination of the leaving group at the 3 -position to yield a second acyl-enzyme derivative (Fig. 5.4, Pathway b), which hydrolyzes very slowly [35][53], Thus, cephalosporins inactivate /3-lactamases by a mechanism similar to that described above for class-II inhibitors. It has been hypothesized that differences in the rate of deacylation of the acyl-enzyme intermediates derive from their different abilities to form H-bonds. A H-bond to NH in Fig. 5.4, Pathway a, may be necessary to assure a catalytically essential conformation of the enzyme, whereas the presence of a H-bond acceptor in Fig. 5.4, Pathway b, may drive the enzyme to an unproductive conformation. The ratio between hydrolysis and elimination, and, consequently, the relative importance of substrate and inhibitor behaviors of cephalosporins, is determined by the nature of the leaving group at C(3 ). An appropriate substitution at C(3 ) of cephalosporins may, therefore, increase the /3-lactamase inhibitory properties and yield potentially better antibiotics [53]. 

Earlier investigations at the molecular level, which have been reviewed in previous editions of this book (19,20), have been recently expanded to cover additional /8-lactamase preparations. The picture which emerges indicates an unexpected diversity of size, structure, and molecular properties. Even more surprising is the variety of catalytic activities, which became increasingly evident as more /8-lactamase preparations were iso-... 

A comparison of spontaneously and experimentally released -lactamase preparations of B. licheniformis (40) indicates that the ends of the polypeptide chain can vary without affecting the properties of the enzyme. Although such preparations differ in the C- and N-terminus, they are serologically and catalytically indistinguishable (40,63). Similarly, B. licheniformis 749/C isozymes, separable by starch gel electrophoresis or DEAE-chromatography and believed to differ at the C- or N-terminus (40), have identical substrate specificities (Table V). 

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