BREAKDOWN OF THE STEREOSPECIFICITY OF DD-PEPTIDASES AND BETA-LACTAMASES WITH THIOLESTER SUBSTRATES

With peptide analogues of their natural substrates (the glycopeptide u nits of nascent peptidoglycan), the DD-peptidases exhibit a strict pre ference for D-Ala-D-Xaa C-termini. Gly is tolerated as the C-terminal residue, but with a significantly decreased activity. These enzymes we re also known to hydrolyse various ester and thiolester analogues of t heir natural substrates. Some thiolesters with a C-terminal leaving gr oup that exhibited L stereochemistry were significantly hydrolysed by some of the enzymes, particularly the Actinomadura R39 DD-peptidase, b ut the strict specificity for a D residue in the penultimate position was fully retained. These esters and thiolesters also behave as substr ates for beta-lactamases. In this case, thiolesters exhibiting L stere ochemistry in the ultimate position could also be hydrolysed, mainly b y the class-C and class-D enzymes. However, more surprisingly, the cla ss-C Enterobacter cloacae P99 beta-lactamase also hydrolysed thioleste rs containing an L residue in the penultimate position, sometimes with a higher efficiency than the D isomer.