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Color tuning in short wavelength-sensitive human and mouse visual pigments: ab initio quantum mechanics/molecular mechanics studies.

ABSTRACT: We have investigated the protonation state and photoabsorption spectrum of Schiff-base (SB) nitrogen bound 11-cis-retinal in human blue and mouse UV cone visual pigments as well as in bovine rhodopsin by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. We have employed both multireference (MRCISD+Q, MR-SORCI+Q, and MR-DDCI2+Q) and single reference (TD-B3LYP and RI-CC2) QM methods. The calculated ground-state and vertical excitation energies show that UV-sensitive pigments have deprotonated SB nitrogen, while violet-sensitive pigments have protonated SB nitrogen, in agreement with some indirect experimental evidence. A significant blue shift of the absorption maxima of violet-sensitive pigments relative to rhodopsins arises from the increase in bond length alternation of the polyene chain of 11-cis-retinal induced by polarizing fields of these pigments. The main counterion is Glu113 in both violet-sensitive vertebrate pigments and bovine rhodopsin. Neither Glu113 nor the remaining pigment has a significant influence on the first excitation energy of 11-cis-retinal in the UV-sensitive pigments that have deprotonated SB nitrogen. There is no charge transfer between the SB and beta-ionone terminals of 11-cis-retinal in the ground and first excited states.

SUBMITTER: Altun A

PROVIDER: S-EPMC2766436 | biostudies-literature | 2009 Oct

REPOSITORIES: biostudies-literature

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Color tuning in short wavelength-sensitive human and mouse visual pigments: ab initio quantum mechanics/molecular mechanics studies.

Altun Ahmet A Yokoyama Shozo S Morokuma Keiji K

The journal of physical chemistry. A 20091001 43

We have investigated the protonation state and photoabsorption spectrum of Schiff-base (SB) nitrogen bound 11-cis-retinal in human blue and mouse UV cone visual pigments as well as in bovine rhodopsin by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. We have employed both multireference (MRCISD+Q, MR-SORCI+Q, and MR-DDCI2+Q) and single reference (TD-B3LYP and RI-CC2) QM methods. The calculated ground-state and vertical excitation energies show that UV-sensitive pigments hav ...[more]

PMID: 19630373

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Project description:Reliable studies of enzymatic reactions by combined quantum mechanics/molecular mechanics (QM/MM) approaches, with an ab initio description of the quantum region, presents a major challenge to computational chemists. The main problem is the need for a very large computer time for the evaluation of the QM energy, which in turn makes it extremely challenging to perform proper configurational sampling. A seemingly reasonable alternative is to perform energy minimization studies of the type used in gas-phase ab initio studies. However, it is hard to see why such an approach should give reliable results in protein active sites. To examine the problems with energy minimization QM/MM approaches, we chose the hypothetical reaction of a metaphosphate ion with water in the Ras.GAP complex. This hypothetical reaction served as a simple benchmark reaction. The possible problems with the QM/MM minimization were explored by generating several protein configurations from long MD simulations and using energy minimization and scanning of the reaction coordinates to evaluate the corresponding potential energy surfaces of the reaction for each of these different protein configurations. Comparing these potential energy surfaces, we found major variations of the corresponding minima. Furthermore, the reaction energies and activation energies also varied significantly even for similar protein configurations. The specific coordination of a magnesium ion, present in the active center of the protein complex, turned out to influence the energetics of the reaction in a major way, where a direct coordination to the reactant leads to an increase of the activation energy by 17 kcal/mol. Apparently, using energy minimization to generate potential surfaces for an enzymatic reaction, while starting from a single protein structure, could lead to major errors in calculations of activation free energies and binding free energies. Thus we believe that extensive samplings of the configurational space of the protein are essential for meaningful determination of the energetics of enzymatic reactions. The possible relevance of our conclusion with regard to a recent study of the RasGAP reaction is discussed.

Project description:BACKGROUND:The diversity of visual systems in fish has long been of interest for evolutionary biologists and neurophysiologists, and has recently begun to attract the attention of molecular evolutionary geneticists. Several recent studies on the copy number and genomic organization of visual pigment proteins, the opsins, have revealed an increased opsin diversity in fish relative to most vertebrates, brought about through recent instances of opsin duplication and divergence. However, for the subfamily of opsin genes that mediate vision at the long-wavelength end of the spectrum, the LWS opsins, it appears that most fishes possess only one or two loci, a value comparable to most other vertebrates. Here, we characterize the LWS opsins from cDNA of an individual guppy, Poecilia reticulata, a fish that is known exhibit variation in its long-wavelength sensitive visual system, mate preferences and colour patterns. RESULTS:We identified six LWS opsins expressed within a single individual. Phylogenetic analysis revealed that these opsins descend from duplication events both pre-dating and following the divergence of the guppy lineage from that of the bluefin killifish, Lucania goodei, the closest species for which comparable data exists. Numerous amino acid substitutions exist among these different LWS opsins, many at sites known to be important for visual pigment function, including spectral sensitivity and G-protein activation. Likelihood analyses using codon-based models of evolution reveal significant changes in selective constraint along two of the guppy LWS opsin lineages. CONCLUSION:The guppy displays an unusually high number of LWS opsins compared to other fish, and to vertebrates in general. Observing both substitutions at functionally important sites and the persistence of lineages across species boundaries suggests that these opsins might have functionally different roles, especially with regard to G-protein activation. The reasons why are currently unknown, but may relate to aspects of the guppy's behavioural ecology, in which both male colour patterns and the female mate preferences for these colour patterns experience strong, highly variable selection pressures.

Dataset Information

Color tuning in short wavelength-sensitive human and mouse visual pigments: ab initio quantum mechanics/molecular mechanics studies.

Publications

Color tuning in short wavelength-sensitive human and mouse visual pigments: ab initio quantum mechanics/molecular mechanics studies.

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