The orientation of iron-sulfur clusters and a spin-coupled ubiquinone pair in the mitochondrial membrane.

Oriented multilayers made from beef heart and yeast mitochondria and submitochondrial particles were studied using electron paramagnetic resonance. EPR signals from membrane-bound iron-sulfur clusters and from a spin-coupled ubiquinone pair are highly orientation dependent, implying that these redox centers are fixed in the membrane at definite angles relative to the membrane plane. Typically the iron-iron axis (gz) of the binuclear iron-sulfur clusters is in the membrane plane. This finding is discussed in terms of the protein structure. The tetranuclear iron-sulfur clusters can have their gz axis either perpendicular or parallel to the membrane plane, but intermediate orientation was not observed.