Co-opting the Cell Wall in Fighting Methicillin-Resistant Staphylococcus aureus: Potent Inhibition of PBP 2a by Two Anti-MRSA β-Lactam Antibiotics

Abstract

The second generation of penicillins (including methicillin) was introduced to the clinic in 1959, and by 1961 strains of methicillin-resistant Staphylococcus aureus (MRSA) had emerged. 1 β-Lactam antibiotics were the standard treatment for staphylococcal infections prior to the emergence of MRSA. MRSA is now a global health threat, 2 and community-acquired MRSA has become a significant challenge in the clinic. 2 β-Lactams, which include penicillins, cephalosporins, and carbapenems, target the transpeptidase activity of penicillin-binding proteins (PBPs) involved in cell wall biosynthesis. The molecular basis for resistance to β-lactams in MRSA is complex and a full discussion is outside of the scope of this report; however, the key component is the presence in MRSA isolates of a gene, mecA, absent in methicillin susceptible strains, which encodes a novel PBP (PBP 2a). 3-5 Whereas other S. aureus PBP transpeptidases are susceptible to inhibition by β-lactams, the strains that possess PBP 2a are able to perform the critical cell wall cross-linking reaction even in the presence of β-lactam antibiotics. 3,6,7 PBP 2a escapes inhibition by β-lactams because they fail to readily gain access to the active site of this enzyme.The crystal structure of PBP 2a reveals it to have a closed active site. 8 This is a paradox, as the enzyme must bind to the peptidoglycan to carry out the cross-linking reaction. We previously disclosed that the two strands of peptidoglycan occupy in excess of 1000 Å 3 of volume, 9 hence the X-ray structure does not reveal how peptidoglycan could bind the active site. We presented evidence that interactions of PBP 2a with the peptidoglycan at an allosteric site trigger a conformational change that leads to accessibility to the active site, an event that should play a critical role in the physiological function of this important enzyme. 10,11 In this report we characterize the mode of action of two new anti-MRSA β-lactam antibiotics from Cerexa, Inc., ceftaroline (CPT) a cephalosporin and ME1036 (ME) a carbapenem, which are currently undergoing clinical trials. Both compounds are broad-spectrum antibiotics, but their activities against MRSA and multidrug resistant streptococci are especially noteworthy. In contrast to the commercially available β-lactam antibiotics, CPT and ME are exquisite inhibitors of PBP 2a of MRSA.The backbone of the peptidoglycan is made up of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A peptide stem-terminating with two D-Ala residues and typically a pentapeptide-is appended to the NAM unit. Compound 1 is a synthetic surrogate for the repeat unit of the peptidoglycan, whose synthesis was reported recently. 12 © 2008 American Chemical Society E-mail: mobashery@nd.edu. The penultimate D-Ala residue of the peptidoglycan acylates an active site serine in PBP 2a resulting in an acyl-enzyme species, with concomitant departure of the terminal D-Ala as the leaving group. This acyl-enzyme species reacts with the second strand of peptidoglycan in the ...

One ring to rule them all: Current trends in combating bacterial resistance to the β‐lactams

“…However, the lack of a Michaelis complex makes it difficult to decipher the exact role of R1 and R2 during the conformational rearrangements leading to acylation. A S. aureus PBP2a allosteric binding site has been inferred from kinetic studies, 50,62 and was recently supported by X-ray crystallography to be an 60 Å from the PBP2a TPase active site. 63 The allosteric site is located at the intersection of Lobe 1 (residues 166-240), Lobe 2 (residues 258-277), Lobe 3 (residues 364-390), and the N-terminal extension domain (residues 27-138).…”

Affinity of Ceftaroline and Other β-Lactams for Penicillin-Binding Proteins from Staphylococcus aureus and Streptococcus pneumoniae

“…These strains may well represent heterogeneous groups, each with its own specific resistance mechanisms. Villegas-Estrada et al (61), using different determination and calculation methodology, tested the same (Hershey) VRSA strain examined in the current study as well as one different linezolid-resistant MRSA strain. Ceftaroline was found to be very active, with MICs of 0.25 to 2 g/ml.…”

“…Differences between our results and those reported previously (27) may be explained at least partially by differences in strains and culture conditions. The higher affinity of ceftaroline for PBP2a than those of other ␤-lactams may be explained by the presence of longer side chains in the chemical structure, which increase interactions with the active-site groove of PBP2a (6,37) and/or facilitate allosteric interactions that promote access to the active site (61). In MRSA strains, it is known that PBP2a may replace the transpeptidase function of PBP2, although the transglycosylase function of the latter becomes critical for MRSA growth in the presence of ␤-lactams (51).…”

Evaluation of Ceftaroline, Vancomycin, Daptomycin, or Ceftaroline plus Daptomycin against Daptomycin-Nonsusceptible Methicillin-Resistant Staphylococcus aureus in an In Vitro Pharmacokinetic/Pharmacodynamic Model of Simulated Endocardial Vegetations

“…Ceftaroline (CPT), the active metabolite of the prodrug CPTfosamil (CPT-F), demonstrates in vitro bactericidal activity against MRSA, including hVISA, VISA, and DNS strains (11)(12)(13)(14). Furthermore, some in vitro data suggest that CPT is more active against MRSA with reduced susceptibility to lipo-and glycopeptides than fully susceptible strains due to the phenomenon referred to as the "seesaw" effect (14)(15)(16).…”