STATIC AND TIME-RESOLVED ABSORPTION-SPECTROSCOPY OF THE BACTERIORHODOPSIN MUTANT TYR-185-]PHE - EVIDENCE FOR AN EQUILIBRIUM BETWEEN BR(570) AND AN O-LIKE SPECIES

The light-dark adaptation, photocycle kinetics, and acid-induced blue formation of the bacteriorhodopsin (bR) mutant Tyr-185-->Phe (Y185F) expressed in Halobacterium halobium have been investigated by both static and time-resolved visible absorption spectroscopy. Evidence is presented that a pH-dependent equilibrium exists between a bR570-like form (bR570Y185F) and a red-shifted species in the light-adapted form of Y185F. In two related papers, we show that this species has vibrational features similar to the O intermediate. Key findings are that light adaptation causes formation of a purple species similar to bR570 and a second long-lived red-shifted species with a lambda(max) near 630 nm, well above the pH for the acid-induced blue transition. The concentration of the red-shifted species is pH- and salt-dependent, decreasing reversibly at high pH and high ionic strength. The dark-adapted state of Y185F also contains a small amount of the red-shifted species which is reversibly titratable. Dark adaptation is much slower than wild-type bR and causes a parallel decay of light-adapted bR and the red-shifted species. Time-resolved visible absorption spectroscopy reveals that the purple and the red-shifted species undergo separate photocycles. The purple species exhibits a relatively normal photocycle except for an increased rate of M formation kinetics. The red-shifted species has a photocycle involving a red-shifted K intermediate and a second longer lived intermediate possibly similar to N. The apparent absence of an O intermediate in the late photocycle of Y185F is attributed to cancellation by depletion bands due to the photoreacting red-shifted species. These results are discussed in terms of active-site interactions including the effect of eliminating a Tyr-185/Asp-212 hydrogen bond on the pK(a) of Asp-85 and Asp-212.