title: Rotational motion of yeast cytochrome oxidase in phosphatidylcholine complexes studied by saturation-transfer electron spin resonance.
abstract: Cytochrome oxidase from yeast has been covalently labeled with a nitroxide derivative of maleimide and reconstituted in lipid-substituted complexes with dimyristoyl-, dioleoyl-, or dielaidoyl-phosphatidylcholine. The rotational mobility of the enzyme in the complexes has been studied as a function of temperature and time, and of lipid/protein ratio, using saturation-transfer electron spin resonance spectroscopy. For complexes with dimyristoylphosphatidylcholine, the rotational mobility of the protein decreases abruptly below the gel-to-fluid-phase transition. This change is accompanied by a lateral segregation of the protein, as seen by freeze-fracture electron microscopy, and by an increase in the activation energy for the enzymatic activity. A time-dependent decrease in the rotational motion of the protein is observed on incubating at temperatures in the fluid phase of the lipid. This corresponds with a time-dependent loss of enzyme activity observed on incubation at temperatures in the fluid phase, but not at temperatures in the gel phase, over a period of 3 h. The rotational mobility decreases with increasing protein concentration in the complexes, both in the fluid and in the gel phases. The dependence of the protein mobility on lipid/protein ratio can be interpreted quantitatively in terms of the effect of increased random protein-protein contacts in the fluid phase. The maximum limiting rotational correlation time for the protein diffusion at high lipid/protein ratios in the fluid phase is tau R[[ approximately equal to 25 microseconds, suggesting that the protein is present as either a monomer or more probably a dimer in the reconstituted membrane.
authors.affiliation: Max-Planck-Institut für biophysikalische Chemie, Abteilung Spektroskopie, Göttingen, Federal Republic of Germany.
authors.forename: P
authors.initials: P
authors.lastname: Fajer
authors.forename: P F
authors.initials: PF
authors.lastname: Knowles
authors.forename: D
authors.initials: D
authors.lastname: Marsh
mesh_terms: D004134:Dimyristoylphosphatidylcholine; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D003576:Electron Transport Complex IV / Q000378:metabolism*; D005614:Freeze Fracturing; D007700:Kinetics; D008081:Liposomes; D008854:Microscopy, Electron; D008928:Mitochondria / Q000201:enzymology; D010713:Phosphatidylcholines; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013113:Spin Labels; D013329:Structure-Activity Relationship
chemical_list: D008081:Liposomes; D010713:Phosphatidylcholines; D013113:Spin Labels; D003576:Electron Transport Complex IV; C017251:1,2-oleoylphosphatidylcholine; D004134:Dimyristoylphosphatidylcholine