Crystallization and preliminary X-ray characterization of cytochrome c" from the obligate methylotroph Methylophilus methylotrophus.

Cytochrome c" from the obligate methylotroph Methylophilus methylotrophus is a 15 kDa monohaem protein which has a c-type haem covalently linked to the protein chain. Two histidine residues are the axial ligands of the Fe atom in the oxidized form. This cytochrome is one of the few known haem proteins which undergoes a change of spin state of the Fe atom upon reduction, with the detachment of an axial histidine ligand. Initial crystallization conditions involved the utilization of cadmium chloride as an additive and resulted in highly mosaic crystals with poor diffraction properties. Optimization of the crystallization conditions was achieved by slowing the nucleation process utilizing agarose gels and viscous additives such as PEG, ethylene glycol and glycerol. Addition of glycerol to the crystallization buffer produced crystals suitable for X-ray diffraction, with a reduced solvent content and mosaicity, which diffracted to a maximum resolution of 1.19 A using synchrotron radiation. The crystals obtained under these conditions were employed for structure solution using the multiwavelength anomalous dispersion method at the Fe K edge.