Ultraviolet resonance Raman spectra of Trp-182 and Trp-189 in bacteriorhodopsin: novel information on the structure of Trp-182 and its steric interaction with retinal.
Ultraviolet (244 nm) resonance Raman spectra of Trp-182 and Trp-189 in bacteriorhodopsin were obtained by subtracting the spectrum of the mutants, Trp-182-->Phe or Trp-189-->Phe, from that of the wild-type. Analysis of the spectra shows that the chi2,1 torsion angle about the Cbeta-C3 bond is +/-93 degrees for Trp-182 and +/-100 degrees for Trp-189. Both Trp residues are moderately hydrogen bonded to proton acceptors at their indolyl nitrogens in hydrophobic environments. The environmental hydrophobicity is particularly strong for Trp-182, as judged from the splitting of the W7 Raman band to a triplet. The Raman information on the structure and environment of Trp-189 is consistent with the molecular model from electron diffraction [Grigorieff et al. (1996) J. Mol. Biol. 259, 393-421]. On the other hand, the chi2,1 angle and the hydrogen-bonding state of Trp-182 found here differ from those in the model structure. Revision of the model to correspond to the Raman findings would require a 60 degrees rotation of the Trp-182 indole ring about the Cbeta-C3 bond toward the chromophore retinal and the presence of a water molecule that is hydrogen bonded to the indolyl nitrogen. The triplet feature of the W7 band of Trp-182 is attributable to unusually strong steric repulsion between the indole ring and the 9- and 13-methyl groups of the retinal. Resonance Raman spectra in the visible suggest that this steric conflict destabilizes the 13-cis isomeric state of the retinal.
Hashimoto
S
S
Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980-77, Japan.
Obata
K
K
Takeuchi
H
H
Needleman
R
R
Lanyi
J K
JK
D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006217:Halobacterium; D007536:Isomerism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D012172:Retinaldehyde / Q000378:metabolism*; D013054:Spectrophotometry, Atomic; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D014364:Tryptophan / Q000737:chemistry* / Q000378:metabolism
D001436:Bacteriorhodopsins; D014364:Tryptophan; D012172:Retinaldehyde