Simultaneous release of penicilloic acid and phenylacetyl glycine by penicillin-binding proteins 5 and 6 of Escherichia coli

A new family of cyanobacterial penicillin-binding proteins. A missing link in the evolution of class A beta-lactamases

It is largely accepted that serine beta-lactamases evolved from some ancestral DD-peptidases involved in the biosynthesis and maintenance of the bacterial peptidoglycan. DD-peptidases are also called penicillin-binding proteins (PBPs), since they form stable acyl-enzymes with beta-lactam antibiotics, such as penicillins. On the other hand, beta-lactamases react similarly with these antibiotics, but the acyl-enzymes are unstable and rapidly hydrolyzed. Besides, all known PBPs and beta-lactamases share very low sequence similarities, thus rendering it difficult to understand how a PBP could evolve into a beta-lactamase. In this study, we identified a new family of cyanobacterial PBPs featuring the highest sequence similarity with the most widespread class A beta-lactamases. Interestingly, the Omega-loop, which, in the beta-lactamases, carries an essential glutamate involved in the deacylation process, is six amino acids shorter and does not contain any glutamate residue. From this new family of proteins, we characterized PBP-A from Thermosynechococcus elongatus and discovered hydrolytic activity with synthetic thiolesters that are usually good substrates of DD-peptidases. Penicillin degradation pathways as well as acylation and deacylation rates are characteristic of PBPs. In a first attempt to generate beta-lactamase activity, a 90-fold increase in deacylation rate was obtained by introducing a glutamate in the shorter Omega-loop.

Cytoplasmic high-level expression of a soluble, enzymatically active form of the Escherichia coli penicillin-binding protein 5 and purification by dye chromatography

High-level expression of a soluble form of penicillin-binding protein 5 (PBP5), called PBP5s, and translocation across the cytoplasmic membrane results in lysis of Escherichia coli cells. The detrimental effect of increased amounts of this D,D-carboxypeptidase on the stability of murein polymer can be avoided by accumulation of the overexpressed protein in the cytoplasm. The signal peptide of the structural gene dacAs, coding for PBP5s was deleted by creating a BamHI site at the site of processing and the truncated gene dacAsc was cloned under the control of the lambda PR promoter. Temperature induction resulted in a 200-fold overproduction of the mature PBP5s in the cytosol (PBP5sc) which is no longer harmful to the cells. PBP5sc could quantitatively be recovered in the soluble fraction after disrupting the cells. The protein retained full enzymatic activity as measured by the release of D-alanine from bisacetyl-L-Lys-D-Ala-D-Ala and formation of [14C]penicillin-protein complex at a 1:1 stoichiometry. A one-step purification procedure using the immobilized dye Procion rubine MX-B resulted in homogeneous preparations of both wild-type and mutated forms of PBP5sc.

Penicillin-degrading activities of peptides from pneumococcal penicillin-binding proteins

Trypsin treatment of native penicillin-binding proteins (PBPs) 1a, 2b and 3 from Streptococcus pneumoniae resulted in the formation of stable peptides containing the beta-lactam-binding site with molecular masses ranging from 26 kDa to 36 kDa. Whereas the PBP 1a peptide (Ia) was enzymatically rather unstable, the PBP 2b peptide (IIb) and the PBP 3 peptide (III) were able to bind and release beta-lactams with similar rates compared to the intact PBP, the turnover rate of fragment II b was even twice as fast as that observed with PBP 2b. Analysis of the turnover products by thin-layer chromatography revealed that PBP 2b and 3 produced penicilloic acid as well as phenylacetylglycine. On the other hand, with the corresponding tryptic fragments only the hydrolysis product penicilloic acid was obtained.

Penicillin-binding site on the Escherichia coli cell envelope

The binding of 35S-labeled penicillin to distinct penicillin-binding proteins (PBPs) of the "cell envelope" obtained from the sonication of Escherichia coli was studied at different pHs ranging from 4 to 11. At low pH, PBPs 1b, 1c, 2, and 3 demonstrated the greatest amount of binding. At high pH, these PBPs bound the least amount of penicillin. PBPs 1a and 5/6 exhibited the greatest amount of binding at pH 10 and the least amount at pH 4. With the exception of PBP 5/6, the effect of pH on the binding of penicillin was direct. Experiments distinguishing the effect of pH on penicillin binding by PBP 5/6 from its effect on beta-lactamase activity indicated that although substantial binding occurred at the lowest pH, the amount of binding increased with pH, reaching a maximum at pH 10. Based on earlier studies, it is proposed that the binding at high pH involves the formation of a covalent bond between the C-7 of penicillin and free epsilon amino groups of the PBPs. At pHs ranging from 4 to 8, position 1 of penicillin, occupied by sulfur, is considered to be the site that establishes a covalent bond with the sulfhydryl groups of PBP 5. The use of specific blockers of free epsilon amino groups or sulfhydryl groups indicated that wherever the presence of each had little or no effect on the binding of penicillin by PBP 5, the presence of both completely prevented binding. The specific blocker of the hydroxyl group of serine did not affect the binding of penicillin. These observations suggest that a molecule of penicillin forms simultaneous bonds between its S at position 1 and sulfhydryl groups of PBP 5 and between its C-7 and free epsilon amino groups of PBP 5.