Evidence that aspartate-85 has a higher pK(a) in all-trans than in 13-cisbacteriorhodopsin

Three experimental observations indicate that the pK(a) of the purple-to-blue transition (the pK(a) of Asp-85) is higher for all-trans-bR(1) than for 13-cis-bR. First, light adaptation of bacteriorhodopsin (bR) at pHs near the pK(a) of Asp-85 causes an increase in the fraction of the blue membrane present. This transformation is reversible in the dark. Second, the pK(a) of the purple-to-blue transition in the dark is lower than that in the light-adapted bR (pK(a)(DA) = 3.5, pK(a)(LA) = 3.8 in 10 microM K(2)SO(4)). Third, the equilibrium fractions of 13-cis and all-trans isomers are pH dependent; the fraction of all-trans-bR increases upon formation of the blue membrane. Based on the conclusion that thermal all-trans <=> 13-cis isomerization occurs in the blue membrane rather than in the purple, we have developed a simple model that accounts for all three observations. From the fit of experimental data we estimate that the pK(a) of Asp-85 in 13-cis-bR is 0.5 +/- 0.1 pK(a) unit less than the pK(a) of all-trans-bR. Thus in 10 microM K(2)SO(4), pK(a)(c) = 3.3, whereas pK(a)(t) = 3.8.