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The Proton-Driven Rotor of ATP Synthase: Ohmic Conductance (10 fS), and Absence of Voltage Gating

Boris A. Feniouk ^* , Maria A. Kozlova ^*, Dmitry A. Knorre ^* , Dmitry A. Cherepanov ^* , Armen Y. Mulkidjanian ^* and Wolfgang Junge ^*

^* Division of Biophysics, Faculty of Biology/Chemistry, University of Osnabrück, Osnabrück, Germany; A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, Russia; and Institute of Electrochemistry, Russian Academy of Sciences, Moscow, Russia

Correspondence: Address reprint requests to Wolfgang Junge, Abt. Biophysik, FB Biologie/Chemie, Universität Osnabrück, D-49069, Osnabrück, Germany. Tel.: 49-541-969-2872; Fax: 49-541-969-2262; E-mail: junge{at}uos.de.

The membrane portion of F₀F₁-ATP synthase, F₀, translocates protons by a rotary mechanism. Proton conduction by F₀ was studied in chromatophores of the photosynthetic bacterium Rhodobacter capsulatus. The discharge of a light-induced voltage jump was monitored by electrochromic absorption transients to yield the unitary conductance of F₀. The current-voltage relationship of F₀ was linear from 7 to 70 mV. The current was extremely proton-specific (>10⁷) and varied only slightly (threefold) from pH 6 to 10. The maximum conductance was 10 fS at pH 8, equivalent to 6240 H⁺ s^–1 at 100-mV driving force, which is an order-of-magnitude greater than of coupled F₀F₁. There was no voltage-gating of F₀ even at low voltage, and proton translocation could be driven by pH alone, without voltage. The reported voltage gating in F₀F₁ is thus attributable to the interaction of F₀ with F₁ but not to F₀ proper. We simulated proton conduction by a minimal rotary model including the rotating c-ring and two relay groups mediating proton exchange between the ring and the respective membrane surface. The data fit attributed pK values of 6 and 10 to these relays, and placed them close to the membrane/electrolyte interface.

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