Crystallization and preliminary X-ray crystallographic analysis of a periplasmic tetrahaem flavocytochrome c₃ from Shewanella frigidimarina NCIMB400 which has fumarate reductase activity

The fumarate reductase of Escherichia coli and other bacteria is a membrane-bound enzyme consisting of four subunits. A soluble periplasmic 64 kDa tetrahaem flavocytochrome c₃ from Shewanella frigidimarina NCIMB400 which possesses a catalytic fumarate reductase activity has been crystallized. The crystals belong to space group P2₁2₁2₁ with unit-cell parameters a = 72.4, b = 110.1, c = 230.2 Å. Assuming a molecular dimer in the asymmetric unit, the crystals contain 65% solvent and, when cryocooled to 100 K, the crystals diffract to at least 3.0 Å resolution. The crystals, however, display an inherent lack of isomorphism and the plausibility of a MAD phasing experiment has therefore been investigated by measuring the iron K absorption edge from a single crystal.