CRYSTALLIZATION AND X-RAY STRUCTURE DETERMINATION OF CYTOCHROME-C(2) FROM RHODOBACTER-SPHAEROIDES IN 3 CRYSTAL FORMS

Cytochrome c2 serves as the secondary electron donor that reduces the photo-oxidized bacteriochlorophyll dimer in photosynthetic bacteria. C ytochrome c2 from Rhodobacter sphaeroides has been crystallized in thr ee different forms. At high ionic strength, crystals of a hexagonal sp ace group (P6(1)22) were obtained, while at low ionic strength, tricli nic (P1) and tetragonal (P4(1)2(1)2) crystals were formed. The three-d imensional structures of the cytochrome in all three crystal forms hav e been determined by X-ray diffraction at resolutions of 2.20 angstrom (hexagonal), 1.95 angstrom (triclinic) and 1.53 angstrom (tetragonal) . The most significant difference observed was the binding of an imida zole molecule to the iron atom of the heme group in the hexagonal stru cture. This binding displaces the sulfur atom of Met100, which forms t he axial ligand in the triclinic and tetragonal structures.