Peptidase activity of beta-lactamases

Although beta-lactamases have generally been considered as being devoid of peptidase activity, a low but significant hydrolysis of various N-acylated dipeptides was observed with representatives of each class of beta-lactamas es. The k(cat)/K-m values were below 0.1 M-1.s(-1), but the enzyme rate enh ancement factors were in the range 5000-20000 for the best substrates. Not unexpectedly, the best 'peptidase' was the class C beta-lactamase of Entero bacter cloacae P99, but, more surprisingly, the activity was always higher with the phenylacetyl- and benzoyl-D-Ala-D-Ala dipeptides than with the dia cetyl- and alpha-acetyl-L-Lys-D-Ala-D-Ala tripeptides, which are the prefer red substrates of the low-molecular-mass, soluble DD-peptidases. A comparis on between the beta-lactamases and DD-peptidases showed that it might be as difficult for a DD-peptidase to open the beta-lactam ring as it is for the beta-lactamases to hydrolyse the peptides, an observation which can be exp lained by geometric and stereoelectronic considerations.