Analogies between halorhodopsin and bacteriorhodopsin

The light-activated proton-pumping bacteriorhodopsin and chloride ion-pumpi ng halorhodopsin are compared. They belong to the family of retinal protein s, with 25% amino acid sequence homology. Both proteins have seven cl helic es across the membrane, surrounding the retinal binding pocket. Photoexcita tion of all-trans retinal leads to ion transporting photocycles, which exhi bit great similarities in the two proteins, despite the differences in the ion transported. The spectra of the K, L, N and O intermediates, calculated using time-resolved spectroscopic measurements, are very similar in both p roteins. The absorption kinetic measurements reveal that the chloride ion t ransporting photocycle of halorhodopsin does not have intermediate M charac teristic for deprotonated Schiff base, and intermediate L dominates the pro cess. Energetically the photocycle of bacteriorhodopsin is driven mostly by the decrease of the entropic energy, while the photocycle of halorhodopsin is enthalpy-driven. The ion transporting steps were characterized by the e lectrogenicity of the intermediates, calculated from the photoinduced trans ient electric signal measurements. The function of both proteins could be d escribed with the 'local access' model developed for bacteriorhodopsin. In the framework of this model it is easy to understand how bacteriorhodopsin can be converted into a chloride pump, and halorhodopsin into a proton pump , by changing the ion specificity with added ions or site-directed mutagene sis. (C) 2000 Elsevier Science B.V. All rights reserved.