Proteorhodopsin Photocycle Kinetics Between pH 5 and pH 9.

The retinal protein proteorhodopsin is a homolog of the well-characterized light-driven proton pump bacteriorhodopsin. Basic mechanisms of proton transport seem to be conserved, but there are noticeable differences in the pH ranges of proton transport. Proton transport and protonation state of a carboxylic acid side chain, the primary proton acceptor, are correlated. In case of proteorhodopsin, the pKa of the primary proton acceptor Asp-97 (pKa  ≈ 7.5) is unexpectedly close to environmental pH (pH ≈ 8). A significant fraction of proteorhodopsin is possibly inactive at natural pH, in contrast to bacteriorhodopsin. We investigated photoinduced kinetics of proteorhodopsin between pH 5 and pH 9 by time resolved UV/vis absorption spectroscopy. Kinetics is inhomogeneous within that pH region and can be considered as a superposition of two fractions. These fractions are correlated with the Asp-97 titration curve. Beside Asp-97, protonation equilibria of other groups influence kinetics, but the observations do not point toward major differences of primary proton acceptor function in proteorhodopsin and bacteriorhodopsin. The pKa of proteorhodopsin and some of its variants is suspected to be an example of molecular adaptation to the physiology of the original organisms.