Protein rotational mobility and lipid fluidity of purified and reconstituted cytochrome c oxidase.

The rotational mobility of spin-labeled bovine heart mitochondrial cytochrome e oxidase in purified form, and incorporated into lipid vesicles was studied. A rigidly attached short chain maleimide spin label permitted the measurement of the protein’s overall rotational mobility by saturation transfer electron paramagnetic resonance. A long chain maleimide spin label was used to detect he fluidity of the lipid hydrocarbon region adjacent to the protein by conventional EPR. One method of preparing the purified enzyme resulted in a high degree of protein rotational mobility at 4°C both in the purified detergent-solubilized enzyme (effective rotational correlation time of 100 ns) and in reconstituted membranes (correlation time of 40 ps). By contrast, another purification procedure resulted in little or no submillisecond protein rotational mobility both in purified form and in reconstituted membranes, suggesting the presence of large protein aggregates. Thus, the state of aggregation of cytochrome oxidase in membranes appears to depend on the state of aggregation prior to reconstitution. The mobile and immobile enzymes had the same high activity. In both reconstituted preparations, the bulk of the lipid was quite fluid at 4”C, as probed by a free fatty acid spin label. The lipid hydrocarbon region adjacent to the protein, as probed by the long chain maleimide spin label, was also quite fluid in the membranes containing mobile enzyme, but was strongly immobilized in the membranes containing immobile enzyme. Thus, the strong immobilization of lipid in our preparations of cytochrome oxidase is apparently caused by proteinprotein interactions, not by rigidity at the protein-lipid boundary.