Vibrational spectroscopy of bacteriorhodopsin mutants. Evidence for the interaction of aspartic acid 212 with tyrosine 185 and possible role in the proton pump mechanism.
The role of Asp-212 in the proton pumping mechanism of bacteriorhodopsin (bR) has been studied by a combination of site-directed mutagenesis and Fourier transform infrared difference spectroscopy. Difference spectra were recorded at low temperature for the bR----K and bR----M photoreactions of the mutants Asp-212----Glu, Asp-212----Asn, and Asp-212----Ala. Despite an increased proportion of the 13-cis form of bR (normally associated with dark adaptation), all of the mutants exhibited a light-adapted form containing as a principal component the normal all-trans retinal chromophore. The absence of a shift in the retinal C = C stretching frequency in these mutants indicates that Asp-212 is not a major determinant of the visible absorption wavelength maximum in light-adapted bR. It is unlikely that Asp-212 is the acceptor group for the Schiff base proton since both the Asp-212----Glu and Asp-212----Ala mutants formed an M intermediate. All of the Asp-212 mutants were missing a Fourier transform infrared difference band that had been assigned previously to protonation changes of Tyr-185. These results are discussed in terms of a model in which Tyr-185 and Asp-212 form a polarizable hydrogen bond and are positioned near the C13-Schiff base portion of the chromophore. These 2 residues may be involved in stabilizing the relative orientation of the F and G helices and isomerizing the retinal in a regioselective manner about the C13 = C14 double bond.
Rothschild
K J
KJ
Department of Physics, Boston University, Massachusetts 02215.
Braiman
M S
MS
He
Y W
YW
Marti
T
T
Khorana
H G
HG
D000595:Amino Acid Sequence; D001224:Aspartic Acid; D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D005583:Fourier Analysis; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D012545:Schiff Bases; D013055:Spectrophotometry, Infrared; D014732:Vibration
D012545:Schiff Bases; D001224:Aspartic Acid; D001436:Bacteriorhodopsins