1: J Biol Chem. 2008 Mar 12 [Epub ahead of print] Restoration of susceptibility of methicillin-resistant staphylococcus aureus (MRSA) to beta -lactam antibiotics by acidic pH: Role of penicillin-binding protein 2A (PBP 2A). Lemaire S, Fuda C, Van Bambeke F, Tulkens PM, Mobashery S. Unité de pharmacologie cellulaire et moléculaires, Université catholique de Louvain, Brussels B-1200. Methicillin-resistant Staphylococcus aureus (MRSA) is a global scourge, and treatment options are becoming limited. The MRSA phenotype reverts to that of beta-lactam sensitive S. aureus when bacteria are grown at pH 5.0 in broth and, more importantly from a medical perspective (protracted, relapsing infections), after phagocytosis by macrophages, where the bacteria thrive in the acidic environment of phagolysosomes. The central factor for the MRSA phenotype is the function of the penicillin-binding protein 2a (PBP 2a), which maintains transpeptidase activity while being poorly inhibited by beta-lactams because of a closed conformation of its active site. We document herein by binding, acylation/deacylation kinetics, and circular dichroism spectroscopy with purified PBP 2a that at acidic pH (i) beta-lactams interact with PBP 2a more avidly; (ii) the non-covalent pre-acylation complex exhibits a lower dissociation constant and an increased rate of acyl-enzyme formation (first-order rate constant) without change in hydrolytic deacylation rate; and (iii) PBP 2a undergoes a conformational change in the presence of the antibiotic consistent with the opening of the active site from the closed conformation. These observations argue that PBP 2a most likely evolved for its physiological function at pH 7 or higher by adopting a closed conformation, which is not maintained at acidic pH. While at the organism level the effect of acidic pH on other biological processes in MRSA could not be discounted, our report should provide the impetus for closer examination of the properties of PBP 2a at low pH, and thereby, identifying novel points of intervention in combating this problematic organism. PMID: 18337244 [PubMed - as supplied by publisher] Related Links Mechanistic basis for the action of new cephalosporin antibiotics effective against methicillin- and vancomycin-resistant Staphylococcus aureus. [J Biol Chem. 2006] PMID:16459335 Activation for catalysis of penicillin-binding protein 2a from methicillin-resistant Staphylococcus aureus by bacterial cell wall. [J Am Chem Soc. 2005] PMID:15713078 Role of acidic pH in the susceptibility of intraphagocytic methicillin-resistant Staphylococcus aureus strains to meropenem and cloxacillin. [Antimicrob Agents Chemother. 2007] PMID:17307986 The basis for resistance to beta-lactam antibiotics by penicillin-binding protein 2a of methicillin-resistant Staphylococcus aureus. [J Biol Chem. 2004] PMID:15226303 Penicillin-binding protein 2a from methicillin-resistant Staphylococcus aureus: kinetic characterization of its interactions with beta-lactams using electrospray mass spectrometry. [Biochemistry. 1999] PMID:10350472