Proton Transport in Halobacterium haloium

Abstract

H. halobium, when cultured in media deficient in organic nutrients, showed light-dependent, transient net fluxes of H+, whose magnitude and initial direction were significantly pH-dependent. However, the extent of H+ translocation ( ¿H+) was almost independent of pH. Also, under these conditions, the steady-state level of bleached (A*max at 415 nm) to unbleached bacteriorhodopsin, and the rate constants to reach it, were pH-dependent. The H+ gradient across the membrane, measured using the compound 5,5-dimethyl-2,4-oxa- zolidinedione, was 0.8 (pH outside, 7.2) or 2.55 (pH outside, 5.5) and was increased c. 0.5 units in the light. The contribution of these gradients to the total driving force for ATP synthesis is discussed. The rigidity of the purple membrane is assumed to be influenced by the external H+ concentration, which also determines the cycling rate of bacteriorhodopsin, a light-dependent H+ carrier.