A Ligand-Exchange Mechanism of Proton Pumping Involving Tyrosine-422 of Subunit I of Cytochrome Oxidase Is Ruled Out

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• 作者：David M. Mitchell ; Pia Ä ; delroth ; Jonathan P. Hosler ; John R. Fetter ; Peter Brzezinski ; Michelle A. Pressler ; Roland Aasa ; Bo G. Malmströ ; m ; James O. Alben ; Gerald T. Babcock ; Robert B. Gennis
• 刊名：Biochemistry
• 出版年：1996
• 出版时间：January 23, 1996
• 年：1996
• 卷：35
• 期：3
• 页码：824 - 828
• 全文大小：386K
• 年卷期：v.35,no.3(January 23, 1996)
• ISSN：1520-4995

文摘

The molecular mechanism by which proton pumping iscoupled to electron transfer incytochrome c oxidase has not yet been determined.However, several models of this process have beenproposed which are based on changes occurring in the vicinity of theredox centers of the enzyme. Recently,a model was described in which a well-conserved tyrosine residue insubunit I (Y422) was proposed toundergo ligand exchange with the histidine ligand (H419) of thehigh-spin heme a₃ during thecatalyticcycle, allowing both residues to serve as part of a proton transportingsystem. Site-directed mutants ofY422 have been constructed in the aa₃-typecytochrome c oxidase of Rhodobacter sphaeroidesto testthis hypothesis (Y422A, Y422F). The results demonstrate that Y422is not an essential residue in theelectron transfer and proton pumping mechanisms of cytochromec oxidase. However, the results supportthe predicted proximity of Y422 to heme a₃, asnow confirmed by crystal structure. In addition, itisshown that the pH-dependent reversed electron transfer between hemea and heme a₃ is normal intheY422F mutant. Hence, these data also demonstrate that Y422 is notthe residue previously postulated tointeract electrostatically with heme a₃, nor isit responsible for the unique EPR characteristics of hemeain this bacterial oxidase.