The Tipper–Strominger Hypothesis and Triggering of Allostery in Penicillin-Binding Protein 2a of Methicillin-Resistant Staphylococcus aureus (MRSA)

Abstract

The transpeptidases involved in the synthesis of the bacterial cell wall (also known as penicillin-binding proteins, PBPs) have evolved to bind the acyl-d-Ala-d-Ala segment of the stem peptide of the nascent peptidoglycan for the physiologically important crosslinking of the cell wall. The Tipper-Strominger hypothesis stipulates that β-lactam antibiotics mimic the acyl-d-Ala-d-Ala moiety of the stem, and thus are recognized by the PBPs with bactericidal consequences. We document that this mimicry exists also at the allosteric site of PBP2a of methicillin-resistant Staphylococcus aureus (MRSA). Interactions of different classes of β-lactam antibiotics, as mimics of the acyl-d-Ala-d-Ala moiety at the allosteric site, lead to a conformational change, across a distance of 60 Å to the active site. We directly visualize this change using an environmentally sensitive fluorescent probe affixed to the protein loops that frame the active site. This conformational mobility, documented in real time, allows antibiotic access to the active site of PBP2a. Furthermore, we document that this allosteric trigger enables synergy between two different β-lactam antibiotics, wherein occupancy at the allosteric site by one facilitates occupancy by a second at the transpeptidase catalytic site, thus lowering the minimal-inhibitory concentration. This synergy has important implications for the mitigation of facile emergence of resistance to these antibiotics by MRSA.

The Road to Avibactam: The First Clinically Useful Non-β-Lactam Working Somewhat Like a β-Lactam

“…In this regard, the Tipper-Strominger hypothesis [67], in other words, that β-lactam antibacterials mimic the conformation of the D-Ala-D-Ala component of the PBP bacterial cell wall substrate, may be relevant [68], in that in order to get to the stage where reaction to the acylenzyme complex can occur the β-lactam antibacterial must efficiently compete with the natural substrate for binding [69]. Following from biophysical evidence that some nucleophilic serine enzymes may bind their substrates in a high-energy conformation which strains the amide bond [70][71][72], a refinement of the Tipper-Strominger hypothesis suggests that (some) β-lactam antibacterials may mimic a strained conformation of the D-Ala-D-Ala entity (Figure 2) [73][74][75].…”

Antimicrobial activity of ceftaroline against methicillin-resistant Staphylococcus aureus (MRSA) isolates collected in 2013–2014 at the Geneva University Hospitals

“…While high-level ceftaroline resistance can be explained by mutations in the transpeptidase domain that alter the active site geometry, low-level resistance associated with mutations within the allosteric site was explained by a novel allosteric modulation of the active site [15, 18–20]. Ceftaroline binds to the PBP2a allosteric site at therapeutic concentrations and triggers the opening and acylation of the active site by a second ceftaroline molecule [7, 20].…”

“…Ceftaroline binds to the PBP2a allosteric site at therapeutic concentrations and triggers the opening and acylation of the active site by a second ceftaroline molecule [7, 20]. Mutations within the PBP2a allosteric domain alter the triggering mechanism and possibly affect protein–protein interactions needed for peptidoglycan biosynthesis [7, 14, 19, 21–23].…”

Fighting bacterial resistance: approaches, challenges, and opportunities in the search for new antibiotics.Part 1. Antibiotics used in clinical practice: mechanisms of action and the development of bacterial resistance

“…New or improved mechanism of action; b. New physicochemical properties that makes it impossible for bacterial drug efflux pumps to recognize them and to remove them from bacterial cell; c. Ability to withstand the action of bacterial deactivating enzymes; d. Ability to bind to the modified target in bacteria [41][42][43][116][117][118][119].…”

“…Fishovitz et al reported that the triggering of the allosteric cite of PBP2 enzyme by one β-lactam antibiotic enables another β-lactam drug to interact with the active cite of that protein. Therefore, they report that there is synergy in the action of ceftaroline and some other β-lactams like oxacillin, imipenem, and meropenem against the MRSA bacterial strains [43].…”