title: A defective proton pump, point-mutated bacteriorhodopsin Asp96----Asn is fully reactivated by azide.
abstract: Addition of azide fully restored the proton pump activity of defective bacteriorhodopsin (BR) mutant protein Asp96----Asn. The decay time of M of BR Asp96----Asn, the longest living intermediate, was decreased from 500 ms at pH 7.0 to approximately 1 ms under conditions of saturating azide concentrations. This decay was faster than the decay of M in the wild-type, where no such azide effect was detectable. Stationary photocurrents, measured with purple membranes immobilized and oriented in a polyacrylamide gel, increased upon addition of azide up to the level of the wild-type. Different small anions of weak acids restored the pump activity with decreasing affinity in the order: cyanate greater than azide greater than nitrite greater than formiate greater than acetate. The activation energy of the M decay in the mutant was higher in the presence (48 kJ/mol) than in the absence (27 kJ/mol) of 100 mM azide even though the absolute rate was dramatically increased by azide. This effect of azide is due to the substitution of a carboxamido group for a carboxylic group at position 96 which removes the internal proton donor and causes an increase in the entropy change of activation for proton transfer which is reversed by azide.
authors.affiliation: Max-Planck-Institut für Biochemie, Martinsried, FRG.
authors.forename: J
authors.initials: J
authors.lastname: Tittor
authors.forename: C
authors.initials: C
authors.lastname: Soell
authors.forename: D
authors.initials: D
authors.lastname: Oesterhelt
authors.forename: H J
authors.initials: HJ
authors.lastname: Butt
authors.forename: E
authors.initials: E
authors.lastname: Bamberg
mesh_terms: D000838:Anions; D001216:Asparagine; D001224:Aspartic Acid; D001386:Azides / Q000494:pharmacology*; D001436:Bacteriorhodopsins / Q000235:genetics* / Q000378:metabolism; D001692:Biological Transport; D006863:Hydrogen-Ion Concentration; D009154:Mutation; D011522:Protons; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000838:Anions; D001386:Azides; D011522:Protons; D001224:Aspartic Acid; D001436:Bacteriorhodopsins; D001216:Asparagine