title: Weakened coupling of conserved arginine to the proteorhodopsin chromophore and its counterion implies structural differences from bacteriorhodopsin.
abstract: In wild-type proteorhodopsin (pR), titration of the chromophore's counterion Asp(97) occurs with a pK(a) of 8.2+/-0.1. R94C mutation reduces this slightly to 7.0+/-0.2, irrespective of treatment with ethylguanidinium. This contrasts with the homologous archaeal protein bacteriorhodopsin (bR), where R82C mutation was previously shown to elevate the pK(a) of Asp(85) by approximately 5 units, while reconstitution with ethylguanidinium restores it nearly to the wild-type value of 2.5. We conclude there is much weaker electrostatic coupling between Arg(94) and Asp(97) in the unphotolyzed state of pR, in comparison to Arg(82) and Asp(85) in bR. Therefore, while fast light-driven H(+) release may depend on these two residues in pR as in bR, no tightly conserved pre-photolysis configuration of them is required.
authors.affiliation: Syracuse University Chemistry Department, Syracuse, NY 13244-4100, United States.
authors.forename: Ranga
authors.initials: R
authors.lastname: Partha
authors.forename: Richard
authors.initials: R
authors.lastname: Krebs
authors.forename: Tamara L
authors.initials: TL
authors.lastname: Caterino
authors.forename: Mark S
authors.initials: MS
authors.lastname: Braiman
mesh_terms: D001120:Arginine / Q000737:chemistry*; D001224:Aspartic Acid / Q000737:chemistry; D001436:Bacteriorhodopsins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics; D025604:Rhodopsins, Microbial; D012545:Schiff Bases; D013056:Spectrophotometry, Ultraviolet
chemical_list: D025604:Rhodopsins, Microbial; D012545:Schiff Bases; C413548:proteorhodopsin; D001224:Aspartic Acid; D001436:Bacteriorhodopsins; D012243:Rhodopsin; D001120:Arginine