Asp 46 can substitute Asp 96 as the Schiff base proton donor in bacteriorhodopsin

Bacteriorhodopsin functions as a light-driven proton pump in the purple membrane of Halobacterium salinarium. A variety of studies have established that a proton is transferred over an approximately 10 A distance from Asp 96 to the retinylidene Schiff base during the M --> N transition of the bR photocycle. In order to further explore the mechanism of this Schiff base reprotonation, we compared the properties of the double mutant Thr 46 --> Asp/Asp 96 --> Asn (T46D/D96N), the single mutants Asp 96 --> Asn (D96N) and Thr 46 --> Asp (T46D), and wild-type bR. In contrast to D96N, which exhibits a very slow M decay, T46D/D96N has an M decay close to that of wild-type bR. FTIR difference spectroscopy detects bands in the carboxyl and carboxylate stretch region of T46D/D96N consistent with the deprotonation of Asp 46 during the M --> N transition. In addition, bands associated with structural changes of Asn 96 in the mutant D96N are absent in T46D/D96N. Resonance Raman spectroscopy provides evidence that both T46D/D96N and T46D have a long-lived N-like species in their photocycles. These data demonstrate that Asp 46 can substitute for Asp 96 as the proton donor group in the reprotonation pathway of the Schiff base during the M --> N transition. However, N decay is delayed in comparison to wild-type bR. This may be due to a partial block in the proton pathway leading from the cytoplasmic medium to Asp 46.