Structure of a pseudomerohedrally twinned monoclinic crystal form of a pyridoxal phosphate-dependent catalytic antibody

The purification, crystallization and structure determination at 2.3 Å resolution of the complex of the pyridoxal-5′-phosphate (PLP) dependent catalytic antibody 15A9 with a phosphopyridoxyl-L-alanine (PPL-L-alanine) substrate analogue are described. The crystal belongs to space group P2₁, with two molecules in the asymmetric unit related by non-crystallographic symmetry. The unit-cell parameters are a = 63.5, b = 81.7, c = 79.3 Å and β is fortuitously 90°. Refinement of the structure converged at unacceptably high R factors. Although the traditional analysis of intensity distribution did not indicate twinning, pseudomerohedral twinning was revealed by a newer test based on local intensity differences [Padilla & Yeates (2003), Acta Cryst. D59, 1124–1130]. When the potential twinning operator was included in SHELX, the structure could be satisfactorily refined with a twinning fraction of 0.46, indicating a nearly perfect hemihedrally twinned crystal. One of the active sites is occupied by the phosphopyridoxyl-L-alanine ligand, while one iodide ion mimics the cofactor phosphate group in the other. Four other iodide ions are present in the structure: two are involved in specific intermolecular contacts and two dictate the conformation of the CDRH3 loop in each molecule.