Calculated Proton Uptake on Anaerobic Reduction of Cytochrome c Oxidase: Is the Reaction Electroneutral?

详细信息    查看全文

• 作者：Yifan Song ; Ekaterina Michonova-Alexova ; and M. R. Gunner
• 刊名：Biochemistry
• 出版年：2006
• 出版时间：July 4, 2006
• 年：2006
• 卷：45
• 期：26
• 页码：7959 - 7975
• 全文大小：419K
• 年卷期：v.45,no.26(July 4, 2006)
• ISSN：1520-4995

文摘

Cytochrome c oxidase is a transmembrane proton pump that builds an electrochemical gradientusing chemical energy from the reduction of O₂. Ionization states of all residues were calculated withMulti-Conformation Continuum Electrostatics (MCCE) in seven anaerobic oxidase redox states rangingfrom fully oxidized to fully reduced. One long-standing problem is how proton uptake is coupled to thereduction of the active site binuclear center (BNC). The BNC has two cofactors: heme a₃ and Cu_B. If theprotein needs to maintain electroneutrality, then 2 protons will be bound when the BNC is reduced by 2electrons in the reductive half of the reaction cycle. The effective pK_as of ionizable residues around theBNC are evaluated in Rhodobacter sphaeroides cytochrome c oxidase. At pH 7, only a hydroxidecoordinated to Cu_B shifts its pK_a from below 7 to above 7 and so picks up a proton when heme a₃ andCu_B are reduced. Glu I-286, Tyr I-288, His I-334, and a second hydroxide on heme a₃ all have pK_asabove 7 in all redox states, although they have only 1.6-3.5 mages/gifchars/Delta.gif" BORDER=0 >pK units energy cost for deprotonation.Thus, at equilibrium, they are protonated and cannot serve as proton acceptors. The propionic acids nearthe BNC are deprotonated with pK_as well below 7. They are well stabilized in their anionic state and donot bind a proton upon BNC reduction. This suggests that electroneutrality in the BNC is not maintainedduring the anaerobic reduction. Proton uptake on reduction of Cu_A, heme a, heme a₃, and Cu_B showsmages/entities/ap.gif">2.5 protons bound per 4 electrons, in agreement with prior experiments. One proton is bound by ahydroxyl group in the BNC and the rest to groups far from the BNC. The electrochemical midpointpotential (E_m) of heme a is calculated in the fully oxidized protein and with 1 or 2 electrons in the BNC.The E_m of heme a shifts down when the BNC is reduced, which agrees with prior experiments. If theBNC reduction is electroneutral, then the heme a E_m is independent of the BNC redox state.