FIG. 2.

PBP 2X-carbapenem complex structures. (A and B) F_o − F_c omit electron density maps calculated for the structures omitting the carbapenem moiety covalently attached to Ser337 in molecule 1 of PBP 2X in complex with biapenem and tebipenem. The maps, depicted as magenta meshes, are contoured at 2.8 σ. Carbapenem-acylated Ser residues are shown as stick models, with carbon atoms colored green, oxygen atoms red, nitrogen atoms blue, and sulfur atoms yellow. (C) Superpositioned active-site regions of the uncomplexed (magenta) and biapenem (light blue)- and tebipenem (orange)-complexed structures. The carbapenems and side chains of selected residues are shown as thick sticks. Atoms are colored as described for panel A, except that the carbon atoms of the carbapenems and selected residues in the uncomplexed and complexed structures are magenta, light blue, and orange, respectively. (D and E) Stereo view of the PBP 2X active site complexed with biapenem and tebipenem, respectively. Atoms are colored as described for panel A, except that the carbon atoms of PBP 2X are orange. The positions of hydrogen atoms of the C-2 side chain were calculated by DS Modeling (Accelrys). The hydrogen atoms that form CH/π interactions are shown as balls (light blue), and other hydrogen atoms are shown as thick sticks (white). Hydrogen bonds are represented by thin lines (magenta). (F) Schematic diagram of the carbapenem binding mode. Hydrogen bonds are represented by broken lines (magenta), and hydrophobic contacts are shown as arcs (green). The atomic numbering scheme of the carbapenem skeleton is also shown.