title: Role of aspartate 132 at the orifice of a proton pathway in cytochrome c oxidase.
abstract: Proton transfer across biological membranes underpins central processes in biological systems, such as energy conservation and transport of ions and molecules. In the membrane proteins involved in these processes, proton transfer takes place through specific pathways connecting the two sides of the membrane via control elements within the protein. It is commonly believed that acidic residues are required near the orifice of such proton pathways to facilitate proton uptake. In cytochrome c oxidase, one such pathway starts near a conserved Asp-132 residue. Results from earlier studies have shown that replacement of Asp-132 by, e.g., Asn, slows proton uptake by a factor of ∼5,000. Here, we show that proton uptake at full speed (∼10(4) s(-1)) can be restored in the Asp-132-Asn oxidase upon introduction of a second structural modification further inside the pathway (Asn-139-Thr) without compensating for the loss of the negative charge. This proton-uptake rate was insensitive to Zn(2+) addition, which in the wild-type cytochrome c oxidase slows the reaction, indicating that Asp-132 is required for Zn(2+) binding. Furthermore, in the absence of Asp-132 and with Thr at position 139, at high pH (>9), proton uptake was significantly accelerated. Thus, the data indicate that Asp-132 is not strictly required for maintaining rapid proton uptake. Furthermore, despite the rapid proton uptake in the Asn-139-Thr/Asp-132-Asn mutant cytochrome c oxidase, proton pumping was impaired, which indicates that the segment around these residues is functionally linked to pumping.
authors.affiliation: Department of Biochemistry and Biophysics, Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Ann-Louise
authors.initials: AL
authors.lastname: Johansson
authors.forename: Martin
authors.initials: M
authors.lastname: Högbom
authors.forename: Jens
authors.initials: J
authors.lastname: Carlsson
authors.forename: Robert B
authors.initials: RB
authors.lastname: Gennis
authors.forename: Peter
authors.initials: P
authors.lastname: Brzezinski
mesh_terms: D000042:Absorption; D001224:Aspartic Acid / Q000378:metabolism*; D018360:Crystallography, X-Ray; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011487:Protein Conformation; D017494:Proton Pumps / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018892:Proton-Motive Force / Q000235:genetics / Q000502:physiology*; D012242:Rhodobacter sphaeroides; D015032:Zinc
chemical_list: D017494:Proton Pumps; D001224:Aspartic Acid; D003576:Electron Transport Complex IV; D015032:Zinc
keywords: cytochrome aa3; electron transfer; metalloprotein; respiration; transporter