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J. Biol. Chem., Vol. 265, Issue 28, 16985-16991, Oct, 1990

Vibrational spectroscopy of bacteriorhodopsin mutants. Evidence for the interaction of aspartic acid 212 with tyrosine 185 and possible role in the proton pump mechanism

KJ Rothschild, MS Braiman, YW He, T Marti and HG Khorana
Department of Physics, Boston University, Massachusetts 02215.

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.
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