The crystal structure of a penicillin-binding protein 1B (PBP1b) has been obtained by a team in Taiwan. The research could represent a major step forward in the development of novel antibiotics against resistant strains of bacteria.

Writing in the Proceedings of the National Academy of Sciences (USA), researchers from the Genomics Research Center at the Academia Sinica in Taipei and colleagues, remind readers how drug-resistant bacteria, such as MRSA (methicillin resistant Staphylococcus aureus and vancomycin-resistant enterococci (VRE) have caused serious medical problems in recent years. The need to find novel antibacterial agents that can defeat such microbes is undisputed, they say.

Genomics Research Center scientists Minag-Ta Sung, Yen-Ting Lai, Chia-Ying Huang, Lien-Yang Chou, Hao-Wei Shih, and Wei-Chieh Cheng, Chi-Huey Wong, and Che Ma, also affiliated with National Yang-Ming University and National Taiwan University further explain that the bacterial enzyme transglycosylase, a multidomain membrane protein essential for cell wall synthesis, represents a potential target for such novel antibiotics.

The bacterial cell wall can be described as a mesh of cross-linked peptidoglycan, which acts as a scaffold for the membrane and maintains structural integrity of the cell. Transpeptidases and transglycosylases carry out the main functions of cell wall synthesis at the membrane surface but inhibitors that block only transpeptidases are currently available to medicine. "The most commonly used antibiotics available to medicine only exist for transpeptidase, but not transglycosylase," Ma told SpectroscopyNOW, "This is why we aim to use transglycosylase as a new target."

For decades transpeptidase has been the main target of the beta-lactam antibiotics, such as penicillin and methicillin, and glycopeptides, including vancomycin. Unfortunately, strains of bacteria that resist these drugs evolved modified versions of these enzymes almost as soon as bacteria were first exposed to the drugs. The enzyme active sites no longer receive the drugs as substrates and so the bacteria can carry on building their cell walls uninhibited. This was not a significant problem until very recently as these resistant forms of the bacteria have spread across the globe.

Intriguingly, no resistant strains against moenomycin from the microbial Streptomyces fungi, the only natural inhibitor to transglycosylase have rarely been found. As such, this enzyme would be an obvious target for the development of novel antibiotics.

Previous researchers have determined two crystal structures of transglycosylase, a bifunctional transglycosylase from S. aureus (SaPBP2) and a transglycosylase from Aquifex aeolicus (AaPGT). For both of these studies, the transmembrane and transpeptidase domains were removed. While such studies offered useful insights into the workings of this class of enzyme, and led to the development of antibiotics, it remains worthwhile to determine the structure of intact transglycosylase enzymes. Such a structure might provide even more sophisticated targets from which the next generation of antibiotic molecule might emerge.

The Taiwanese team has now determined the X-ray crystal structure of the bifunctional transglycosylase penicillin-binding protein 1b (PBP1b) from Escherichia coli in complex with its inhibitor moenomycin to 2.16 Ångstrom resolution.

The crystal structure not only reveals details of the transglycosylase and transpeptidase domains but also offers a complete visualization of this target for drug designers. Additionally, the study exposes a domain for protein-protein interaction and a transmembrane helix domain essential for substrate binding, enzymatic activity, and membrane orientation, the team says.

"The crystal structure of E. coli PBP1b represents a structural platform of transglycosylase," explain the researchers, "in particular for Gram-negative bacterial pathogens, for the development of antibiotics." They point out that several compounds with lower molecular weights than moenomycin are known, and so are easier to synthesise. These compounds compete with moenomycin and bind directly to the transglycosylase domain, although they are not such potent inhibitors. Nevertheless, "the structural information between these compounds and transglycosylase can be studied via molecular modeling or by X-ray crystallography using the current E. coli PBP1b structure as a template for structure-based drug design," the team adds.

Related links:

• Proc Natl Acad Sci, 2009, in press
• Ma Page 

Article by David Bradley