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.