1.3.98.1 hanging drop vapour diffusion method, with 30% PEG 6000, 1 mM dithiothreitol, 0.2 M sodium acetate, and 0.1 M Tris-HCl at pH 8.5 685158 1.3.98.1 in complex with product orotate 657355 1.3.98.1 in ligand-free form and in complexes with inhibitor oxonate, physiological substrates and products of the first and second half-reactions. Ligands bind to the same active site of enzyme, consistent with one-site ping-pong Bi-Bi mechanism. The binding of ligands does not cause any significant structural changes, and both reduced and oxidized FMN cofactors are in planar conformation. Resiude C130 is well located for abstracting a proton from dihydroorotate C5 and transferring it to outside water molecules. The bound fumarate is in a twisted conformation, which induces partial charge separation. The thermodynamically favorable reduction of fumarate with reduced FMN seems to proceed in the way that its C2 accepts a proton from C130 and C3 a hydride or a hydride equivalent from reduced FMN N5 696221 1.3.98.1 to 2.4 A resolution, space group C2221. Class 1A family enzyme, the monomer folds as a alpha/beta barrel consisting of a core of central eight parallel beta-strands surrounded by a ring of eight alpha-helices. The asymmetric unit of the crystal structure contains four monomers that are arranged as two distinct homodimers that are nearly perpendicular to each other, i.e. the tetramer has the shape of the letter L 723848 1.3.98.1 wild-type enzyme and mutants N67A, K213E, P56A, R57A, K136E 656148