SPECTROSCOPIC IDENTIFICATION OF THE AXIAL LIGANDS OF CYTOCHROME B(560) IN BOVINE HEART SUCCINATE-UBIQUINONE REDUCTASE

The axial ligands of low potential cytochrome b(560) in the five subunit bovine heart succinate-ubiquinone reductase complex and in the isolated quinone binding proteins have been investigated using EPR and near-infrared magnetic circular dichroism spectroscopies. The results are consistent with bis-histidine ligation with near-perpendicular imidazole rings for cytochrome b(560) in the four-subunit complex. The pronounced changes in EPR properties that accompany isolation of the cytochrome-b(560) containing quinone binding proteins, are attributed to perturbation of the orientation of the imidazole rings of the heme bis-histidine ligands, rather than a change in axial ligation.