EXPERIMENTAL-EVIDENCE FOR HYDROGEN-BONDED NETWORK PROTON-TRANSFER IN BACTERIORHODOPSIN SHOWN BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY USING AZIDE AS CATALYST

Experimental evidence for proton transfer via a hydrogen-bonded network in a membrane protein is presented. Bacteriorhodopsin's proton transfer mechanism on the proton uptake pathway between Asp-96 and the Schiff base in the M-to-N transition was determined. The slowdown of this transfer by removal of the proton donor in the Asp-96 --> Asn mutant can be accelerated again by addition of small weak acid anions such as azide. Fourier-transform infrared experiments show in the Asp-96 --> Asn mutant a transient protonation of azide bound to the protein in the M-to-N transition and, due to the addition of azide, restoration of the IR continuum band changes as seen in wild-type bR during proton pumping, The continuum band changes indicate fast proton transfer on the uptake pathway in a hydrogen-bonded network for wild-type bR and the Asp-96 --> Asn mutant with azide, Since azide is able to catalyze proton transfer steps also in several kinetically defective bR mutants and in other membrane proteins, our finding might point to a general element of proton transfer mechanisms in proteins.