FTIR difference spectroscopy of the bacteriorhodopsin mutant Tyr-185-->Phe: detection of a stable O-like species and characterization of its photocycle at low temperature.
Fourier transform infrared difference spectroscopy has been used to study the photocycle of the mutant Tyr-185-->Phe expressed in native Halobacterium halobium and isolated as intact purple membrane fragments. We find several changes in the low-temperature bR-->K, bR-->L, and bR-->M FTIR difference spectra of Y185F relative to wild-type bR which are not directly related to the absorption bands associated with Tyr-185. We show that these features arise from the photoreaction of a stable red-shifted species (OY185F) with a vibrational spectrum similar to the O intermediate. By using photoselection and FTIR spectroscopy, we have been able to characterize the photoproducts of this OY185F species. A K-like photoproduct is formed at 80 K which has a 13-cis structure. However, it differs from K630, exhibiting an intense band at 990 cm-1 most likely due to a hydrogen-out-of-plane vibrational mode of the chromophore. At 170 and 250 K, photoexcitation of OY185F produces an intermediate with vibrational features similar to the N intermediate in the wild-type bR photocycle. However, no evidence for an M-like intermediate is found. Although Asp-96 undergoes a change in its environment/protonation state during the OY185F photocycle, no protonation changes involving Asp-85 and Asp-212 were detected. These results provide strong evidence that light adaptation of Y185F produces two species similar to bR570 and the O intermediate. Differences in their respective photocycles can be explained on the basis of differences in the protonation states of the residues Asp-85 and Asp-212 which are ionized in bR570 and undergo net protonation upon OY185F formation.
He
Y
Y
Physics Department, Boston University, Massachusetts 02215.
Krebs
M P
MP
Fischer
W B
WB
Khorana
H G
HG
Rothschild
K J
KJ
D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000235:genetics; D003080:Cold Temperature; D005583:Fourier Analysis; D019617:Halobacterium salinarum / Q000737:chemistry; D009154:Mutation; D010649:Phenylalanine / Q000737:chemistry*; D010777:Photochemistry; D013055:Spectrophotometry, Infrared; D014443:Tyrosine / Q000737:chemistry*
D014443:Tyrosine; D010649:Phenylalanine; D001436:Bacteriorhodopsins