Introduction
(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. 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 - ceftobiprole and ceftaroline - that bind and inhibit PBP2a have been
developed.
Cell Wall Synthesis
β-Lactam antibiotics stop the production of the cell wall by targeting bacterial
PBPs. The cell wall, which is composed of peptidoglycan and surrounds the cell
membrane, is crucial for maintaining the structural integrity of the bacterium.
The cell wall is composed of rows of peptidoglycan cross-linked together with
pentaglycine chains. Peptidoglycan consists of N-acetylmuramic Acid (NAM) and
N-acetylglucosamine (NAG) polymers. The NAM residues have a five amino acid side
chain that terminates with two D-Alanine (D-Ala) residues. MRSA is resistant to all
β-lactams because it acquires an alternative PBP, PBP2a, that is not bound or
inhibited by any β-lactams. Recently, two cephalosporins - ceftobiprole and
ceftaroline - that bind and inhibit PBP2a have been developed.
How does PBP2a works?
PBP2a is composed of two domains: a (NPB) domain and a
domain. 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 () which catalyzes the cross-linking of the peptidoglycan rows with pentaglycine cross-links.
Catalytic Mechanism of PBP2a
The D-Ala-D-Ala side-chain substrate of the peptidoglycan accesses the active site of the PBP2a.
Ser403 nucleophilically attacks the peptide bond of the terminal D-Ala residues of the substrate. The terminal D-Ala residue then exits the active site.
The now terminal D-Ala residue forms a covalent bond to Ser403, while a cross-linking pentaglycine chain enters the active site.
A covalent bond forms between the pentaglycine chain and the terminal D-Ala residue, regenerating the active site serine residue.
How does Ceftobiprole work?
MRSA becomes resistant to β-lactams by acquiring an alternative PBP, PBP2a, that is neither bound nor inhibited by β-lactams. Recently, two cephalosporins – and ceftaroline – that have anti-MRSA activity have been developed. Ceftobiprole is able to inhibit PBP2a because additional chemical groups at the position of the cephalosporin backbone are able to interact with additional amino acid residues in PBP2a; specifically . As a result of its tighter binding to PBP2a,
is able to more efficiently react with the serine active site residue and therefore inhibit the activity of PBP2a.
