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. 1993 Nov 1;90(21):10206–10210. doi: 10.1073/pnas.90.21.10206

Protonation states of membrane-embedded carboxylic acid groups in rhodopsin and metarhodopsin II: a Fourier-transform infrared spectroscopy study of site-directed mutants.

K Fahmy 1, F Jäger 1, M Beck 1, T A Zvyaga 1, T P Sakmar 1, F Siebert 1
PMCID: PMC47743  PMID: 7901852

Abstract

A method was developed to measure Fourier-transform infrared (FTIR) difference spectra of detergent-solubilized rhodopsin expressed in COS cells. Experiments were performed on native bovine rhodopsin, rhodopsin expressed in COS cells, and three expressed rhodopsin mutants with amino acid replacements of membrane-embedded carboxylic acid groups: Asp-83-->Asn (D83N), Glu-122-->Gln (E122Q), and the double mutant D83N/E122Q. Each of the mutant opsins bound 11-cis-retinal to yield a visible light-absorbing pigment. Upon illumination, each of the mutant pigments formed a metarhodopsin II-like species with maximal absorption at 380 nm that was able to activate guanine nucleotide exchange by transducin. Rhodopsin versus metarhodopsin II-like photoproduct FTIR-difference spectra were recorded for each sample. The COS-cell rhodopsin and mutant difference spectra showed close correspondence to that of rhodopsin from disc membranes. Difference bands (rhodopsin/metarhodopsin II) at 1767/1750 cm-1 and at 1734/1745 cm-1 were absent from the spectra of mutants D83N and E122Q, respectively. Both bands were absent from the spectrum of the double mutant D83N/E122Q. These results show that Asp-83 and Glu-122 are protonated both in rhodopsin and in metarhodopsin II, in agreement with the isotope effects observed in spectra measured in 2H2O. A photoproduct band at 1712 cm-1 was not affected by either single or double replacements at positions 83 and 122. We deduce that the 1712 cm-1 band arises from the protonation of Glu-113 in metarhodopsin II.

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Selected References

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