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]

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