Molecular surface representations of the entryway to the active site of alanine racemase from S. pneumoniae. (A) The surface of three layers of entryway residues: residues comprising the inner layer are pink (here, the constricting Tyr352 and Tyr263' residues can be seen), the middle layer residues are orange, and the outer layer residues are blue. The PLP cofactor is colored green. Primed numbers denote residues from the second monomer. (B) Surface of the entryway colored by electrostatic potential (same view as in A).