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

摘要

It is largely accepted that serine ss-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 ss-lactam antibiotics such as penicillins. On the other hand, ss-lactamases react similarly with these antibiotics but the acyl-enzymes are unstable and rapidly hydrolysed. Besides, all known PBPs and ss-lactamases share very low sequence similarities, thus rendering it difficult to understand how a PBP could evolve into a ss-lactamase. In this study, we identified a new family of cyanobacterial PBPs featuring the highest sequence similarity with the most widespread class A ss-lactamases. Interestingly, the O-loop, which, in the ss-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 characterised 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 ss-lactamase activity, a 90 fold increase in deacylation rate was obtained by introducing a glutamate in the shorter O-loop.