Aspartic acids 96 and 85 play a central role in the function of bacteriorhodopsin as a proton pump

A spectroscopic and functional analysis of two point-mutated bacteriorhodopsins (BRs) from phototrophic negative halobacterial strains is reported. Bacteriorhodopsin from strain 384 contains a glutamic acid instead of an aspartic acid at position 85 and BR from strain 326 contains asparagine instead of aspartic acid at position 96. Compared to wild-type BR, the M formation in BR Asp85---Glu is accwelerated approximately 10-fold, whereas the M decay in BR Asp96---Asn is slowed down approximately 50-fold at pH6. Purple membrane sheets containing the mutated BRs were oriented and immobilized in polyacrylamide gels or adsorbed to planar lipid films. The measured kinetics of the photocurrents under various conditions agree with the observed photocycle kinetics. The ineffectivity of BR Asp85---Glu resides in the dominance of an inactive species absorbing maximally at approximately 610 nm, while BR Asp96---Asn is ineffective due to its slow photocycle. These experimental results suggest that aspartic acid 96 plays a crucial role for the reprotonation of the Schiff base. Both residues are essential for an effective proton pump.