An alkaline D-stereospecific endopeptidase with β-lactamase activity from Bacillus cereus

Abstract

We purified a novel extracellular D-stereospecific endopeptidase, alkaline D-peptidase (D-stereospecific peptide hydrolase, EC 3.4.11.-), to homogeneity from the culture broth of the soil bacterium Bacillus cereus strain DF4-B. The M(r) of the enzyme was 37,952, and it was composed of a single polypeptide chain. The optimal pH for activity was ~10.3. The enzyme was strictly D-stereospecific toward oligopeptides composed of D- phenylalanine such as (D-Phe)3 and (D-Phe)4. The enzyme also acted to a lesser extent on (D-Phe)6, Boc-(D-Phe)4 (where Boc is tert-butoxycarbonyl), Boc-(D-Phe)4 methyl ester, Boc-(D-Phe)3 methyl ester, Boc-(D-Phe)2, (D- Phe)2, and others, but not upon their corresponding peptides composed of L- Phe, (D-Ala)(n) (n = 2-5), (D-Val)3, and (D-Leu)2. The mode of action of the enzyme was clarified with synthetic substrates ((D-Phe)2-D-Tyr and D- Tyr-(D-Phe)2) and eight stereoisomers of (Phe)3. The enzyme had β- lactamase activity toward ampicillin and penicillin G, although carboxypeptidase DD and D-aminopeptidase activities were undetectable. The gene coding for alkaline D-peptidase (adp) was cloned into plasmid pUC118, and a 1164-base pair open reading frame consisting of 388 codons was identified as the adp gene. The predicted polypeptide was similar to carboxypeptidase DD from Streptomyces R61, penicillin-binding proteins from Streptomyces lactamdurans and Bacillus subtilis, and class C β-lactamases. Thus, the enzyme was categorized as a new 'penicillin-recognizing enzyme'.