Halide control of color of the chicken cone pigment iodopsin

We tested the effect of anions on the absorbance spectrum of native visual pigments as measured by microspectrophotometry in individual cone outer segments of four species of fish and one species of amphibian. In all species tested, the long-wavelength-absorbing cone pigments were anion sensitive, and their A.. could be tuned over a range of 55 nm depending on the identity of the anion present. Cl-and Br-were the only anions that produced native pigment spectra by red shifting A.. from its value under anion-free conditions. Lyotropic anions such as NO-, SCN-, BF4, and ClIO caused substantial and graded blue shifts of A... The apparent Kd of binding sites on the pigment for Cl-and for CIO4 was -2 mM. Taken together with previous findings on three visual pigments from the reptilian, avian, and amphibian classes, our results support the hypothesis that all long-wavelength-absorbing vertebrate visual pigments are spectrally tuned in part through the binding of a chloride ion. We propose that the site of anion tuning is near the protonated Schiff base of the chromophore, whose counterion may be complex and include Cl-as an exchangeable anion. This counterion configuration may resemble the one present in the light-driven Cl-pump halorhodopsin.Color vision in vertebrates is based on sets of two, three, or four different photopigments that reside in separate classes of cone photoreceptors. The wavelength of peak absorbance (Amax) of these pigments ranges from the UV (360 nm) to the far red (635 nm). All vertebrate visual pigments are integral membrane proteins and contain 11-cis-retinal or 11-cisdehydroretinal as the chromophore that is covalently bound through a Schiffbase linkage to a lysine residue on the protein (opsin) moiety ofthe pigment (1). Although great progress has been made in recent years in unraveling the structure of visual pigments, the molecular basis of spectral tuning is still only incompletely understood.A variety of mechanisms have been invoked to account for photopigment tuning (for reviews, see refs. 2 and 3). For example, protonation of the retinal Schiff base shifts Amax from 360 to 430 nm (4). Blue-absorbing pigments may contain an unperturbed chromophore behaving much like a protonated retinal Schiff base in a nonpolar solvent (5). Closer interaction of the chromophore with the protein perturbs the electronic structure of the chromophore and results in an additional red shift of Am,,,. This shift is commonly called the "opsin shift" and very likely comprises multiple components. For example, Amax is extremely sensitive to the charge environment provided by the counterion, which is paired with the protonated Schiff base (3). A decreased interaction between protonated Schiff base and counterion, perhaps caused by an increased distance between the two, may be responsible for the opsin shift in green-absorbing rhodopsins (Amax, "500 nm). Amino acid side chains, which form the hydrophobic binding pocket for the chromophore, also can interact with and perturb the chromophore. In fact, much ...

Section: Discussionsupporting

confidence: 84%

Section: Introductionsupporting

confidence: 73%

mentioning

confidence: 99%

See 1 more Smart Citation

Anion sensitivity and spectral tuning of cone visual pigments in situ.

Kleinschmidt

Hárosi

1992

“…Iodopsin has a chloride binding site and chloride binding affects the absorption maximum of iodopsin (18)(19)(20). Our low-temperature experiments (12) revealed that bathoiodopsin produced from chlotOur experiments (17) on the determination of the photosensitivity of iodopsin using a steady light source at room temperature showed that the photosensitivity ratio of iodopsin (CHAPS/PC) to cattle rhodopsin (2% digitonin) is 1.08.…”

Section: Discussionmentioning

confidence: 91%

Bathoiodopsin, a primary intermediate of iodopsin at physiological temperature.

Kandori

Mizukami

Okada

et al. 1990

Measurement of the primary photochemical reaction of iodopsin, a chicken red-sensitive cone visual pigment, was carried out at room temperature by using picosecond (ps) laser photolysis. Excitation of iodopsin with a ps green pulse (pulse width, 21 ps) caused the instantaneous formation of a bathochromic product, which was stable on a ps time scale. This product may correspond to "bathoiodopsin," which was detected by low-temperature spectrophotometry. Although bathoiodopsin produced at the temperature of liquid nitrogen or helium reverted to the original pigment (iodopsin) on warming (above -170'C), the bathoiodopsin produced at physiological temperature decayed to all-trans-retinal and R-photopsin (the protein moiety of iodopsin) presumably through several intermediates. The absorption maximum of bathoiodopsin at room temperature was at 625 nm, a wavelength slightly shorter than that measured at low temperature (A4., 640 nm). The extinction coefficient of bathoiodopsin at room temperature was lower than that at low temperature and close to that of the original iodopsin at room temperature.The vertebrate retina contains two kinds of photoreceptor cells, rods and cones. A rod is a photoreceptor cell that has evolutionally acquired an extremely high photosensitivity so as to act under twilight conditions. The visual pigment in rods is called rhodopsin. Cones, however, act under daylight conditions and have a photoresponse with a wider dynamic range and a faster latency (1). Since there are at least three types of cones, each with its own distinct visual pigment possessing characteristic absorption maximum (2), integration of the photoresponses from cones generates the sensation of color in the visual cortex.The primary photoreceptive mechanism in visual transduction has been extensively investigated in rods, which contain rhodopsin as the visual pigment. Rhodopsin has an 11-cis-retinylidene chromophore attached by a protonated Schiff base linkage to a specific lysine residue of the apoprotein opsin. The primary photochemical event in rhodopsin is an isomerization around the C11=C12 bond of the 11-cisretinylidene chromophore to form photorhodopsin, a highly twisted all-trans photoproduct (3,4 (6, 7).On absorption of light, iodopsin bleaches to all-transretinal and R-photopsin (the protein moiety of iodopsin) by way of lumiiodopsin and then metaiodopsin, according to low-temperature spectrophotometry (8). Yoshizawa and Wald (9) observed that irradiation of iodopsin at the temperature of liquid nitrogen produced bathoiodopsin. Prolonged irradiation of this sample formed a steady-state photo mixture composed of iodopsin, isoiodopsin, and bathoiodopsin. This photochemical behavior of iodopsin is very similar to that of rhodopsin (Fig. 1). On warming, however, an unexpected difference between iodopsin and rhodopsin was observed; unlike bathorhodopsin, bathoiodopsin was not thermally converted to lumiiodopsin but reverted back to iodopsin above -170°C. Thus the question arises whether bathoiodopsin is a physiolo...

“…In keeping with these models, sequence comparisons and spectroscopic studies of site-directed mutants have identified a glutamate counterion that is present in all vertebrate photoreceptor pigments sequenced to date and is required for protonation of the retinylidene Schiff base (16)(17)(18), and three positions where increases in side-chain polarity lead to red shifts of 5-14 nm that together account for most of the variation in absorption maxima among previously characterized members of the longwave subfamily (19)(20)(21)(22). A third red-shifting mechanism involves an interaction between chloride and a histidine in the second putative extracellular loop in members of the longwave pigment subfamily (23)(24)(25)(26)(27)(28); the mechanistic basis of the chloride effect is unclear.…”

mentioning

confidence: 99%

Mechanisms of spectral tuning in the mouse green cone pigment

Sun

Macke

Nathans

1997

182

153

Diversification of cone pigment spectral sensitivities during evolution is a prerequisite for the development of color vision. Previous studies have identified two naturally occurring mechanisms that produce variation among vertebrate pigments by red-shifting visual pigment absorbance: addition of hydroxyl groups to the putative chromophore binding pocket and binding of chloride to a putative extracellular loop. In this paper we describe the use of two blue-shifting mechanisms during the evolution of rodent long-wave cone pigments. The mouse green pigment belongs to the long-wave subfamily of cone pigments, but its absorption maximum is 508 nm, similar to that of the rhodopsin subfamily of visual pigments, but blue-shifted 44 nm relative to the human red pigment, its closest homologue. We show that acquisition of a hydroxyl group near the retinylidene Schiff base and loss of the chloride binding site mentioned above fully account for the observed blue shift. These data indicate that the chloride binding site is not a universal attribute of long-wave cone pigments as generally supposed, and that, depending upon location, hydroxyl groups can alter the environment of the chromophore to produce either red or blue shifts.Visual pigments differ between species and between different photoreceptor cells within the same species in their wavelengths of maximal absorption. This diversity of visual pigment absorption spectra is determined by interactions between the 11-cis-retinal chromophore and the particular opsin to which it binds, or in rare instances by the alternate use of an 11-cis-dehydroretinal chromophore (1). Among vertebrate visual pigments that use 11-cis-retinal, absorption maxima have been identified throughout the wavelength interval from 360 nm (2, 3) to 570 nm (4). Sequencing of visual pigments from diverse species shows a limited range of wavelengths of maximal absorption among members of each of the three major branches of the visual pigment family: the short-wave subfamily absorbs maximally at wavelengths less than 500 nm, the rhodopsin subfamily absorbs maximally at wavelengths near 500 nm, and the long-wave subfamily absorbs at wavelengths greater than 500 nm (5-7).Experiments with retinal and its derivatives in solution have led to a number of plausible mechanisms for spectral tuning in visual pigments (8-11). The two photochemical properties of retinal that are most relevant are the shift in absorption maximum from 360 nm to 440 nm upon formation of a protonated Schiff base (12, 13) and the increase in -electron delocalization and the concomitant change in dipole moment that accompanies photoexcitation (14,15). Therefore, amino acids from the apoprotein that alter the polarity, polarizability, or charge density around either the conjugated -electron system or the retinylidene Schiff base would be expected to shift the absorption spectrum of the pigment. In keeping with these models, sequence comparisons and spectroscopic studies of site-directed mutants have identified a glutamate counterio...