A Novel β-Lactamase Activity from a Penicillin-binding Protein of Treponema pallidum and Why Syphilis Is Still Treatable with Penicillin

Abstract

Treponema pallidum, the causative agent of syphilis, is sensitive to penicillins. Yet, an abundant membrane-bound protein of this organism, Tp47, turns over penicillins. It is shown herein that the turnover process is a hydrolytic reaction that results in the corresponding penicilloates, products that have their β-lactam bonds hydrolyzed. This is the reaction of β-lactamases, bona fide resistance enzymes to β-lactam antibiotics. Remarkably, the x-ray structure of Tp47 bears no resemblance to any other β-lactamases or the related penicillin-binding proteins. Furthermore, evidence is presented that the reaction of Tp47 takes place in the absence of the zinc ion and does not involve intermediary acyl enzyme species. Hence, the β-lactamase activity of Tp47 is the fifth known mechanism for turnover of β-lactam antibiotics. Tp47 also exhibits a penicillin binding reaction, in the process of which the enzyme is covalently modified in the active site. The two reactions take place in two different active sites, and the events of the β-lactamase activity are over 2,000-fold more rapid than the penicillin binding reaction. The level of β-lactamase activity is high and is held back only by a strong product-inhibition component to the catalytic process. If natural selection would result in a mutant variant of Tp47 that overcomes product inhibition for the β-lactamase activity, a novel bona fide resistance to penicillins will emerge in Treponema, which will be a disconcerting clinical development. The physiological functions of Tp47 are not known, but it is likely that this is at least a bifunctional enzyme involved in the processing of the Treponema peptidoglycan as a substrate.