Figure 5.

Stereo view of the PBP6 acyl-enzyme complex compared to that of the deacylation transition state. A PBP5 transition-state analog structure (PDB Code 1Z6F) is superimposed onto the PBP6 structure with ampicillin (PDB Code 3ITA), showing active site residues 39, 40, 43, 108 and 210. The transition-state analog and the catalytic serine of PBP5 are shown in blue, whereas ampicillin and PBP6 are shown in pink. Water molecules are represented as red spheres. Key distances are displayed as dashed lines. The amine group of Lys43 engages in hydrogen bonds with two water molecules in the PBP6 acyl-enzyme complex. The distance between the β-lactam-thiazolidine bridging carbon atom of ampicillin, and the boronic acid oxygen of the superimposed transition state analog, is only 1.97 Å. This carbon presents a steric impediment to hydrolytic deacylation.⁴⁷,⁵⁷