Fluorescence quenching of reconstituted NCD-4-labeled cytochrome c oxidase complex by DOXYL-stearic acids

It has been known for some time that dicyclohexylcarbodiimide (DCCD) inhibits the proton translocation function of the cytochrome c oxidase complex (CcO) and that there is one major site in subunit III which is modified upon reaction with DCCD (Glu-90 for the bovine enzyme). We have examined the reaction of bovine CcO with N-cyclohexyl-N'-(4-dimethylamino-alpha-napthyl)carbodiimide (NCD-4), a fluorescent analog of DCCD. NCD-4 labeling of CcO is strongly inhibited by DCCD implicating Glu-90 of subunit III as the site of chemical modification by NCD-4. The fluorescence of reconstituted NCD-4-labeled bovine CcO is strongly quenched by hydrophobic nitroxides, whereas hydrophilic nitroxides and iodide ions have a reduced quenching ability. It is concluded that the Glu-90 of subunit III resides near the protein-lipid interface of the membrane spanning region of the enzyme. Different quenching abilities of 5-, 7-, 10-, 12-, and 16-4,4-dimethyl-3-oxazolinyloxy-stearic acids suggest that the NCD-4 label is located in the membrane bilayer in the region near the middle of the hydrocarbon tail of stearic acid. In light of these results, it is unlikely that Glu-90 is part of a proton channel that is associated with the proton pumping machinery of the enzyme but the outcome of this study does not eliminate an allosteric regulatory role for this residue.