Structure of alanine racemase from S. pneumoniae. (A) Ribbon diagram of the alanine racemase monomer with β-sheets colored green and α-helices colored gold. (B) Ribbon diagram of the alanine racemase dimer where one monomer is colored blue and the opposite monomer red. The N'-pyridoxyl-lysine-5'-monophosphate or LLP residue (PLP cofactor covalently bound to lysine; black or grey spheres) resides in the α/β barrel domain of the active site. The active site is composed of residues from the α/β barrel domain of one monomer and residues from the β-strand domain of the other monomer.