A new pathway in the photoreaction cycle of trans-bacteriorhodopsin and the absorption spectra of its intermediates

Abstract

The intermediates of trans-bacteriorhodopsin (trans-bR) in the photoreaction cycle were investigated under two different conditions. In a low salt and neutral pH medium (10 mM phosphate buffer, pH 6.6), trans-bR was irradiated with 500 nm light at −190‡ C, resulting in formation of batho-trans-bR (batho-bRt). On warming in the dark, batho-bRt converted to lumi-trans-bR (lumi-bRt), meta-trans-bR (meta-bRt) and finally to trans-bR. The intermediates N and O, which had been detected by others by flash photolysis, were not observed. The thermal decay of lumi-bRt in a high salt and high pH medium (10 mM borate buffer with l M NaCl, pH 10.0) proceeded simultaneously through two pathways; one to meta-bRt and another to trans-bR. About 72% of lumi-bRt converted to trans-bR directly and the residue converted to meta-bRt. By use of this value, the absorption spectra of batho-bRt (λmax: 626 nm), lumi-bRt (λmax: 543 nm) and meta-bRt (λmax: 418 nm) were calculated. A photoreaction cycle of bacteriorhodopsin was proposed on the basis of the above findings.