Another example centers on the key determinant of broad-spectrum beta-lactam resistance in the notorius clinical superbug methicillin-resistant Staphylococcus aureus (MRSA), the membrane-spanning, penicillin-binding protein 2a (PBP2a), a transpeptidase component of the multi-enzyme bacterial cell wall-synthesizing assembly. Transpeptidases (the target of beta-lactam antibiotics) are required for production of peptide cross-links that give the cell wall its necessary strength and rigidity. Because of its low affinity for beta-lactams, PBP2a provides cross-linking transpeptidase activity at beta-lactam concentrations that inhibit the other cell wall transpeptidases; such peptidases are normally produced by S. aureus and other pathogenic bacteria. We have determined, to high resolution, the crystal structures of native MRSA PBP2a as well as those of acyl-enzyme complexes with various beta-lactam antibiotic substrates. Analysis of PBP2a’s active site reveals the structural basis of its broad-spectrum resistance to the clinically used beta-lactam antibiotics and identifies features important for high-affinity binding. This information is being used in structure-based inhibitor design strategies aimed at combating MRSA resistance, as highlighted by recent analysis of PBP2a with new generations of cephalosporin compounds such as Ceftobiprole.

We have also determined the structure of the first bifunctional glycosyltransferase/transpeptidase enzyme component of the cell wall assembly, PBP2 from S. aureus, which polymerizes/cross-links the lipidated glyco-peptide-containing building blocks of the cell wall. We have analyzed the structure of this membrane-anchored enzyme in the presence of the substrate analogue inhibitor moenomycin, a highly potent natural product antibiotic commonly used to treat livestock. Our structures provided new insight into the molecular basis of processive polymerization/cross-linking by these enzymes as well as the essential features of glycosyltransferase-targeted inhibition by moenomycin. We are using information from these structural findings to create modified forms of moenomycins and to pursue leads for new antibiotic treatment of human infections.