Redox-linked transient deprotonation at the binuclear site in the aa(3)-type quinol oxidase from Acidianus ambivalens: Implications for proton translocation

The hyperthermophilic archaeon Acidianus ambivalens expresses a membrane-bo und aa(3)-type quinol oxidase, when grown aerobically, that we have studied by resonance Raman spectroscopy, The purified aa(3) oxidase, which does no t contain bound quinol, undergoes a reversible slow conformational change a t heme a(3) upon reduction, as indicated by a change in the frequency of it s heme formyl stretching mode, from 1,660 cm(-1) to 1,667 cm(-1). In contra st, upon reduction of the integral membrane enzyme or the purified enzyme p reincubated with decylubiquinol, this mode appears at 1,667 cm(-1) much mor e rapidly, suggesting a role of the bound quinol in controlling the redox-l inked conformational changes. The shift of the formyl mode to higher freque ncy is attributed to a loss of hydrogen bonding that is associated with a g roup having a pKa of approximate to 3.8. Based on these observations, a cru cial element for proton translocation involving a redox-linked conformation al change near the heme a(3) formyl group is postulated.