ORIENTATIONAL CHANGES OF THE ABSORBING DIPOLE OF RETINAL UPON THE CONVERSION OF RHODOPSIN TO BATHORHODOPSIN, LUMIR HODOPSIN, AND ISORHODOPSIN

Retinal dynamics during light activation of rhodopsin revealed by solid-state NMR spectroscopy

Rhodopsin is a canonical member of class A of the G protein-coupled receptors (GPCRs) that are implicated in many of the drug interventions in humans and are of great pharmaceutical interest. The molecular mechanism of rhodopsin activation remains unknown as atomistic structural information for the active metarhodopsin II state is currently lacking. Solid-state (2)H NMR constitutes a powerful approach to study atomic-level dynamics of membrane proteins. In the present application, we describe how information is obtained about interactions of the retinal cofactor with rhodopsin that change with light activation of the photoreceptor. The retinal methyl groups play an important role in rhodopsin function by directing conformational changes upon transition into the active state. Site-specific (2)H labels have been introduced into the methyl groups of retinal and solid-state (2)H NMR methods applied to obtain order parameters and correlation times that quantify the mobility of the cofactor in the inactive dark state, as well as the cryotrapped metarhodopsin I and metarhodopsin II states. Analysis of the angular-dependent (2)H NMR line shapes for selectively deuterated methyl groups of rhodopsin in aligned membranes enables determination of the average ligand conformation within the binding pocket. The relaxation data suggest that the beta-ionone ring is not expelled from its hydrophobic pocket in the transition from the pre-activated metarhodopsin I to the active metarhodopsin II state. Rather, the major structural changes of the retinal cofactor occur already at the metarhodopsin I state in the activation process. The metarhodopsin I to metarhodopsin II transition involves mainly conformational changes of the protein within the membrane lipid bilayer rather than the ligand. The dynamics of the retinylidene methyl groups upon isomerization are explained by an activation mechanism involving cooperative rearrangements of extracellular loop E2 together with transmembrane helices H5 and H6. These activating movements are triggered by steric clashes of the isomerized all-trans retinal with the beta4 strand of the E2 loop and the side chains of Glu(122) and Trp(265) within the binding pocket. The solid-state (2)H NMR data are discussed with regard to the pathway of the energy flow in the receptor activation mechanism.

Structural analysis and dynamics of retinal chromophore in dark and meta I states of rhodopsin from 2H NMR of aligned membranes

Rhodopsin is a prototype for G protein-coupled receptors (GPCRs) that are implicated in many biological responses in humans. A site-directed (2)H NMR approach was used for structural analysis of retinal within its binding cavity in the dark and pre-activated meta I states. Retinal was labeled with (2)H at the C5, C9, or C13 methyl groups by total synthesis, and was used to regenerate the opsin apoprotein. Solid-state (2)H NMR spectra were acquired for aligned membranes in the low-temperature lipid gel phase versus the tilt angle to the magnetic field. Data reduction assumed a static uniaxial distribution, and gave the retinylidene methyl bond orientations plus the alignment disorder (mosaic spread). The dark-state (2)H NMR structure of 11-cis-retinal shows torsional twisting of the polyene chain and the beta-ionone ring. The ligand undergoes restricted motion, as evinced by order parameters of approximately 0.9 for the spinning C-C(2)H(3) groups, with off-axial fluctuations of approximately 15 degrees . Retinal is accommodated within the rhodopsin binding pocket with a negative pre-twist about the C11=C12 double bond that explains its rapid photochemistry and the trajectory of 11-cis to trans isomerization. In the cryo-trapped meta I state, the (2)H NMR structure shows a reduction of the polyene strain, while torsional twisting of the beta-ionone ring is maintained. Distortion of the retinal conformation is interpreted through substituent control of receptor activation. Steric hindrance between trans retinal and Trp265 can trigger formation of the subsequent activated meta II state. Our results are pertinent to quantum and molecular mechanics simulations of ligands bound to GPCRs, and illustrate how (2)H NMR can be applied to study their biological mechanisms of action.

Picosecond dynamics of G-protein coupled receptor activation in rhodopsin from time-resolved UV resonance Raman spectroscopy

The protein response to retinal chromophore isomerization in the visual pigment rhodopsin is studied using picosecond time-resolved UV resonance Raman spectroscopy. High signal-to-noise Raman spectra are obtained using a 1 kHz Ti:Sapphire laser apparatus that provides <3 ps visible (466 nm) pump and UV (233 nm) probe pulses. When there is no time delay between the pump and probe events, tryptophan modes W18, W16, and W3 exhibit decreased Raman scattering intensity. At longer pump-probe time delays of +5 and +20 ps, both tryptophan (W18, W16, W3, and W1) and tyrosine (Y1 + 2xY16a, Y7a, Y8a) peak intensities drop by up to 3%. These intensity changes are attributed to decreased hydrophobicity in the microenvironment near at least one tryptophan and one tyrosine residue that likely arise from weakened interaction with the beta-ionone ring of the chromophore following cis-to-trans isomerization. Examination of the crystal structure suggests that W265 and Y268 are responsible for these signals. These UV Raman spectral changes are nearly identical to those observed for the rhodopsin-to-Meta I transition, implying that impulsively driven protein motion by the isomerizing chromophore during the 200 fs primary transition drives key structural changes that lead to protein activation.

The steric trigger in rhodopsin activation

Rhodopsin is the seven transmembrane helix receptor responsible for dim light vision in vertebrate rod cells. The protein has structural homology with the other G protein-coupled receptors, which suggests that the tertiary structures and activation mechanisms are likely to be similar. However, rhodopsin is unique in several respects. The most striking is the fact that the receptor "ligand", 11-cis retinal, is covalently bound to the protein and is converted from an "antagonist" to an "agonist" upon absorption of light. NMR studies of rhodopsin and its primary photoproduct, bathorhodopsin, have generated structural constraints that enabled docking of the 11-cis and all-trans retinal chromophores into a low-resolution model of the protein proposed by Baldwin. These studies also suggest a mechanism for how retinal isomerization leads to rhodopsin activation. More recently, mutagenesis studies have extended these results by showing how the selectivity of the retinal-binding site can be modified to favor the all-trans over the 11-cis isomer. The structural constraints produced from these studies, when placed in the context of a high-resolution model of the protein, provide a coherent picture of the activation mechanism, which we show involves a direct steric interaction between the retinal chromophore and transmembrane helix 3 in the region of Gly121.

Magnetic field effects on biomolecules, cells, and living organisms

This article surveys three major areas of biomagnetic research: (a) the magneto-orientation effect; (b) the role of the geomagnetic field in bird orientation and navigation; and (c) the biological effects of extremely low-frequency magnetic fields. The magneto-orientation effect is caused by diamagnetic anisotropy of highly ordered biological structures, such as visual photoreceptor and chloroplast membranes, in a homogeneous magnetic field of about 10 kG. While it is not possible to orient the individual constituent molecules with such a field because of thermal fluctuation, these ordered structures can be oriented as a whole by virtue of summing the anisotropy over a large number of mutually oriented molecules. While the magneto-orientation effect seems to require the use of unphysiologically strong magnetic fields, certain birds apparently have highly sensitive sensors to detect the geomagnetic field for the purpose of orientation and navigation. However, the advances in this latter field were made mainly in the behavioral studies; the magneto-sensors the the neural mechanisms remain elusive. A number of candidates of the sensors are evaluated. We suggest that pecten oculi, which is unique to avian eyes, should not be overlooked for its possible role as a magneto-sensor based on the magneto-orientation effect. Birds primarily use a static (DC) magnetic field for orientation, but recent investigations indicate that weak alternating (AC) magnetic fields with extremely low frequency (ELF) may have hazardous health effects. Such reports are often received with skepticism, because the effects usually involve magnetic energies that are less than the kT energy. However, some of the in vitro studies yield experimental results that are too significant to be ignored. Here, we propose an argument to explain why low-level magnetic fields can be detected without being overshadowed by thermal noises. Relevance of biomagnetic research to the development of biosensors and novel computational paradigms is also discussed.

Photointermediates of visual pigments

Much progress has been made in recent years toward understanding the interactions between various proteins responsible for visual transduction which are initiated by an activated state of visual pigments. However, the changes which take place in the visual pigments themselves to convert them to the activated state are more poorly understood. Many spectroscopic techniques have been applied to this problem in recent years and considerable progress has been made. A major goal of these efforts is to understand at which stages protein change occurs and to characterize its structural features. In the visual system evidence is accumulating, for example, that chromophore independent protein change begins immediately prior to lumirhodopsin formation. Considerable insight has been gained recently into the early intermediates of visual transduction and the stage is set to achieve similar understanding of the later intermediates leading to rhodopsin's activated state.

Evidence for a common BATHO-intermediate in the bleaching of rhodopsin and isorhodopsin

Nanosecond transient spectroscopic measurements of the BATHO products formed from photolysis of bovine rhodopsin (RHO) and isorhodopsin (ISO) are discussed. BATHO absorption spectra of RHO and ISO differ slightly, but both pigments exhibit wavelength maxima near 560 nm. An additional transient absorption at 440 nm is observed immediately following excitation of RHO but not ISO. The decay times and Arrhenius activation energies of the two 560 nm absorbing transients are the same. In addition to spectral differences in the photolysis of RHO and ISO, the bleaching yield as a function of 532 nm laser power is different, with the yield in RHO saturating at lower laser power than ISO. The bleaching yield of the two pigments has been modeled using the known extinction coefficients and quantum yields for the interconversion of RHO, ISO, and a single BATHO species. Agreement between experiment and the model is found if the effects of the laser polarization are considered. The data are consistent with a common BATHO in the photolysis of RHO and ISO.