A NEW CRYSTAL FORM OF BOVINE HEART UBIQUINOL - CYTOCHROME-C OXIDOREDUCTASE - DETERMINATION OF SPACE GROUP AND UNIT-CELL PARAMETERS

Ubiquinol:cytochrome c oxidoreductase, the middle segment of the mitochondrial respiratory chain, is a multi-subunit transmembrane redox enzyme. The purified protein from beef heart mitochondria has been crystallized by three groups in three different forms, but progress toward a structure has been hampered by the limited order (resolution) of the crystals. It has been found that under certain conditions the enzyme crystallizes in a new form suitable for X-ray diffraction studies. These crystals belong to space group C222(1) in the orthorhombic system. The cell dimensions are a = 384, b = 118 and c = 177 Angstrom. These new crystals at present diffract to 3.8 Angstrom at best. This is not significantly better than hexagonal [P6(1(5))22] crystals grown, but the new crystals have the advantage of less spot overlap because of face-centered packing which results in systematic extinctions. More importantly, the availability of the same enzyme in multiple crystal forms may allow phase refinement and extension by the method of molecular replacement.