Background
Transpeptidases (TP), also known as penicillin-binding proteins (PBP), catalyze the cross-linking of peptidoglycan polymers during bacterial cell wall synthesis. The natural transpeptidase substrate is the D-Ala-D-Ala peptidoglycan side chain terminus. Beta-lactam (β-lactam) antibiotics, which include penicillins, cephalosporins and carbapenems, bind and irreversibly inhibit transpeptidases by mimicking the D-Ala-D-Ala substrate, resulting in the inhibition of cell wall synthesis and ultimately bacterial cell growth.
Figure 1. A.Bacterial Cell Wall B.Peptidoglycan with D-Ala-D-Ala substrate
Overuse and misuse of β-lactams has led to the generation of methicillin- resistant Staphylococcus aureus (MRSA) isolates that have acquired an alternative transpeptidase, PBP2a, which is neither bound nor inhibited by β- lactams. MRSA isolates are resistant to all β-lactams, and are often only susceptible to “last resort antibiotics”, such as vancomycin. Recently, two cephalosporins - and ceftaroline - that bind and inhibit PBP2a have been developed.
How does PBP2a work
PBP2a is composed of two domains: a The NBP domain of PBP2a is anchored in the cell membrane, while the TP domain “sits” in the periplasm with its active site facing the inner surface of the cell wall. The active site contains a serine residue at position 403 (Ser403) which catalyzes the cross-linking of the peptidoglycan rows with pentaglycine cross-links.
How does Ceftobiprole Work
is able to inhibit PBP2a because additional chemical groups at are able to interact with additional amino acid residues in PBP2a; specifically and Met641. As a result of its tighter binding to PBP2a, ceftobiprole is able to more efficiently react with the serine active site residue and therefore inhibit the activity of PBP2a.
