title: Altering conserved lipid binding sites in cytochrome c oxidase of Rhodobacter sphaeroides perturbs the interaction between subunits I and III and promotes suicide inactivation of the enzyme.
abstract: Subunit III of the three-subunit catalytic core of cytochrome c oxidase (CcO) contains no metal centers, but it does bind two lipids, within a deep cleft, in binding sites conserved from bacteria to humans. Subunit III binds to subunit I, where it prevents the spontaneous suicide inactivation of CcO by decreasing the probability of side reactions at the heme-Cu O2 reduction site in subunit I. Subunit III prevents suicide inactivation by (1) maintaining adequate rates of proton delivery to the heme-Cu active site and (2) stabilizing the structure of the active site during turnover [Mills and Hosler (2005) Biochemistry 44, 4656]. Here, we first show that mutating several individual residues of the conserved lipid binding sites in subunit III disturbs the subunit I-III interface. Then, two lipid binding site mutants were constructed with an affinity tag on subunit III such that the mutant CcOs could be isolated with 100% subunit III. R226A eliminates an ion pair to the phosphate of the outermost lipid of the cleft, while W59A-F86A disrupts interactions with the fatty acid tails of both lipids. Once these mutant CcOs are placed into soybean phospholipid vesicles, where extensive exchange of bacterial for soybean lipids takes place, it is shown that altering the lipid binding sites mimics a major loss of subunit III, even though subunit III is completely retained, in that suicide inactivation becomes much more probable. The rate of proton delivery to the active site remains rapid, ruling out slow proton uptake as the primary reason for increased suicide inactivation upon alteration of the lipid binding sites. We conclude that altering the lipid binding sites of subunit III may promote side reactions leading to suicide inactivation by allowing greater movement to occur in and around the O2 reduction site of subunit I during the catalytic cycle.
authors.affiliation: Department of Biochemistry, University of Mississippi Medical Center, 2500 North State Street, Jackson, Mississippi 39216, USA.
authors.forename: Lakshman
authors.initials: L
authors.lastname: Varanasi
authors.forename: Denise
authors.initials: D
authors.lastname: Mills
authors.forename: Anna
authors.initials: A
authors.lastname: Murphree
authors.forename: Jimmy
authors.initials: J
authors.lastname: Gray
authors.forename: Chris
authors.initials: C
authors.lastname: Purser
authors.forename: Rodney
authors.initials: R
authors.lastname: Baker
authors.forename: Jonathan
authors.initials: J
authors.lastname: Hosler
mesh_terms: D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics; D002384:Catalysis; D003300:Copper / Q000737:chemistry / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004789:Enzyme Activation; D008055:Lipids / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000235:genetics; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D008055:Lipids; D021122:Protein Subunits; D003300:Copper; D003576:Electron Transport Complex IV; D010100:Oxygen