The proton-driven rotor of ATP synthase: Ohmic conductance (10 fS), and absence of voltage gating

Abstract

The membrane portion of F0F1-ATP synthase, F 0, 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 ApH alone, without voltage. The reported voltage gating in F0F 1 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.