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Structural analysis of an “open” form of PBP1B from Streptococcus pneumoniae

The class A PBP1b from Streptococcus pneumoniae is responsible for glycosyltransferase and transpeptidase (TP) reactions, forming the peptidoglycan of the bacterial cell wall. The enzyme has been produced in a stable, soluble form and undergoes time‐dependent proteolysis to leave an intact TP domain. Crystals of this TP domain were obtained, diffracting to 2.2 Å resolution, and the structure was solved by using molecular replacement. Analysis of the structure revealed an “open” active site, with important conformational differences to the previously determined “closed” apoenzyme. The active‐site nucleophile, Ser460, is in an orientation that allows for acylation by β‐lactams. Consistent with the productive conformation of the conserved active‐site catalytic residues, adjacent loops show only minor deviation from those of known acyl‐enzyme structures. These findings are discussed in the context of enzyme functionality and the possible conformational sampling of PBP1b between active and inactive states.