The orientation of the three haems of the 'in situ' ubiquinol oxidase, cytochrome bd, of Escherichia coli

The Esherichia coli cytochrome bd complex incorporates three haems as prosthetic groups. In the ferric form these are a predominantly high-spin chlorin (haem d), a high-spin haem b (b₅₉₅) and a low-spin haem b (b₅₅₈). The orientations of these three haems have been determined by e.p.r. studies on oriented multilayer preparations of cytoplasmic membrane fragments. The low-spin haem b (b₅₅₈) and the high-spin haem dare oriented with their haem planes perpendicular to the membrane plane. The high-spin haem b₅₉₅ is oriented with its haem plane at approx. 55% to the membrane plane. A minor low-spill component, attributable to a low-spin subpopulation of the haem d, is also oriented with its haem plane perpendicular to the membrane plane.