On the Mechanism of Wavelength Regulation in Visual Pigments

Abstract

Abstract— The contributions of different factors that might be responsible for the 500 nm absorption maximum of bovine rhodopsin are evaluated in detail. These include: (1) electrostatic interactions between the chromophore and a charged amino acid on the apoprotein; (2) exciton interactions with aromatic amino acids; (3) twisting about single bonds which have considerable double bond character; (4) weak interactions between the Schiff base and a putative counter‐ion. Analysis of these mechanisms in terms of theoretical and experimental results suggests that(2–4) are all capable of contributing to the protein induced spectral shifts. However, the “external point charge” model proposed previously, i.e. mechanism (1), appears to provide the crucial interaction. In this paper, the theoretical basis for this model is discussed in detail. The model is briefly evaluated in light of the amino‐acid sequence of bovine rhodopsin and possible implications for other visual pigments are considered.

Asp83, Glu113 and Glu134 are not specifically involved in Schiff base protonation or wavelength regulation in bovine rhodopsin

“…This invalidates previous postulations that this single residue protonates the Schiff base [15,17,18]. Glutamic acid (134) is also strongly conserved during visual pigment evolution [ 151, but the substitution of this residue by aspartic acid had no influence at all on the position of the absorbance band ( fig.2C, table 2).…”

Molecular evolution of color vision in vertebrates

“…The strong synergistic interactions in the SWS1 pigments may occur because of the highly limited access of water molecules to the Schiff base pocket of the UV pigments Shi et al, 2001). Thus, UV pigments may have an unprotonated Schiff baselinked chromophore (Kakitani et al, 1985;Fahmy and Sakmar, 1993;Vought et al, 1999;Shi et al, 2001;Babu et al, 2001). To elucidate the mechanisms of UV vision, this possibility needs to be explored.…”

Evidence for Proton Transfer from Glu-46 to the Chromophore during the Photocycle of Photoactive Yellow Protein

“…Therefore, elucidation of the chromophore structure of PYP M is essential to understand the photocycle of PYP. For the spectral blue-shift of PYP M , the following explanations would be possible: (i) the phenolic oxygen of the chromophore is protonated; (ii) the conjugated double bond system is broken by extreme distortion of the chromophore (25); and (iii) the electrons of the conjugated double bond system are localized by a nearby positive charge (e.g. Arg-52) (26).…”