Generation of Proton‐Motive Force by an archaeal terminal quinol oxidase from Sulfolobus acidocaldarius

The terminal quinol oxidase of the cytochrome aa₃ type was isolated from the extreme thermo‐acidophilic archaeon Sulfolobus acidocaldarius. In micellar solution, the enzyme oxidized various quinols and exerted the highest activity with the physiological substrate caldariella quinol. The enzyme was functionally reconstituted into monolayer liposomes composed of archaeal tetraether lipids also derived from S. acidocaldarius. With the electron donor system ascorbate and N,N,N′,N′‐tetramethyl‐p‐phenylenediamine, the reconstituted enzyme was more active in the archaeal lipids as compared to lipids derived from Escherichia coli at temperatures above 50°C. Due to the low proton permeability of the tetraether lipids, it was possible to generate a steady‐state transmembrane electrical potential (ΔΨ, interior negative), and transmembrane pH gradient (ΔpH, interior alkaline) at temperatures up to 70°C. The successful functional reconstitution of the cytochrome aa₃‐type quinol oxidase from Sulfolobus identifies it as the key energy converter in the respiratory system of this hyperthermophilic archaeon.