Crystallization and X-ray structure determination of cytochrome c2 from Rhodobacter sphaeroides in three crystal forms.

Cytochrome c(2) serves as the secondary electron donor that reduces the photo-oxidized bacteriochlorophyll dimer in photosynthetic bacteria. Cytochrome c(2) from Rhodobacter sphaeroides has been crystallized in three different forms. At high ionic strength, crystals of a hexagonal space group (P6(1)22) were obtained, while at low ionic strength, triclinic (P1) and tetragonal (P4(1)2(1)2) crystals were formed. The three-dimensional structures of the cytochrome in all three crystal forms have been determined by X-ray diffraction at resolutions of 2.20 A (hexagonal), 1.95 A, (triclinic) and 1.53 A (tetragonal). The most significant difference observed was the binding of an imidazole molecule to the iron atom of the heme group in the hexagonal structure. This binding displaces the sulfur atom of Met l00, which forms the axial ligand in the triclinic and tetragonal structures.