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At least 1,063 records · Page 59Linked to original sources
PubMed2002

Like night and day: rods and cones have different pigment regeneration pathways.

Sustained vision requires continuous regeneration of visual pigments in rod and cone photoreceptors by the 11-cis-retinal chromophore. In this issue of Neuron, Mata et al. report a novel enzymatic pathway uniquely designed to keep up with the high demand for cone pigment regeneration in bright light and to preclude rods from utilizing chromophore produced in daylight, when rods are not very useful for vision.

Animals↗
PubMed1999

Multiple-quantum relaxation in the magic-angle-spinning NMR of 13C spin pairs.

We determine the decay rate constants of zero-, double- and single-quantum coherence for 13C spin pairs in magic-angle-spinning solid-state NMR. The double-quantum coherence is excited by a C7 pulse sequence and converted into zero-quantum coherence by a frequency-selective pair of pi/2 pulses. The zero-quantum coherence is reconverted into observable magnetization by a second pair of pi/2 pulses followed by a second C7 sequence. In a magnetically dilute system where the 13C-13C distance is 0.296 nm, the relaxation rate constants are consistent with a model of uncorrelated random fields at the two labeled 13C sites. In a fully-labelled system with a short 13C-13C distance of 0.153 nm, the measured rate constants are inconsistent with the uncorrelated random field model.

Carbon Isotopes↗
PubMed1999

Rotational resonance NMR of 13C2-labelled retinal: quantitative internuclear distance determination.

Rotational resonance phenomena are investigated in the solid-state magic-angle spinning NMR of all-E-[11,20-13C2]-retinal at a magnetic field of 4.7 T. We find good agreement between experiments and numerical simulations for the rotational resonance spectral peakshapes and for the rotor-driven magnetization exchange. The internuclear distance between the 13C-labelled C11 and C20 sites is determined to be 0.301 +/- 0.008 nm (from rotational resonance spectra) and 0.300 +/- 0.010 nm (from rotor-driven magnetization exchange), in agreement with the X-ray crystallographic distance of 0.296 nm. We show rotational resonance spectra which display perturbations from intermolecular homonuclear spin-spin interactions.

Carbon Isotopes↗
PubMed1999

Formation and structure of stably dispersed particles composed of retinal with dipalmitoylphosphatidylcholine: coexistence of emulsion particles with bilayer vesicles.

In order to develop an intravenous formulation of all-trans-retinal (vitamin A aldehyde, VAA) for the treatment of night blindness, VAA and dipalmitoylphosphatidylcholine (DPPC) were sonicated and the dispersions in the VAA mole fraction range of 0.1-0.7 were stable at room temperature for 3 days. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VAA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50-70 nm. A limited amount of VAA is incorporated into DPPC bilayer membranes (approximately 5 mole%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VAA/DPPC particles was decreased remarkably with the addition of VAA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO(4). These results indicate that the excess VAA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer. The monolayer-bilayer equilibrium of VAA/DPPC mixtures was estimated by measurement of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of DPPC+core of VAA) with vesicular particles (bilayer) was critically important for the formation of the stably dispersed particles of the lipid mixture.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2001

Bacteriorhodopsin.

High-resolution maps from X-ray diffraction of bacteriorhodopsin and some of its photointermediates have yielded insights into how the isomerization of the bound retinal drives ion transport. Although important mechanistic details are still undecided, the events of the photochemical cycle are now understood to reflect changes in specific hydrogen bonds of protein groups and bound water molecules in response to motions of the retinal chain.

Bacteriorhodopsins↗
PubMed2001

Crystal structure of rhodopsin: implications for vision and beyond.

A heptahelical transmembrane bundle is a common structural feature of G-protein-coupled receptors (GPCRs) and bacterial retinal-binding proteins, two functionally distinct groups of membrane proteins. Rhodopsin, a photoreceptor protein involved in photopic (rod) vision, is a prototypical GPCR that contains 11-cis-retinal as its intrinsic chromophore ligand. Therefore, uniquely, rhodopsin is a GPCR and also a retinal-binding protein, but is not found in bacteria. Rhodopsin functions as a typical GPCR in processes that are triggered by light and photoisomerization of its ligand. Bacteriorhodopsin is a light-driven proton pump with an all-trans-retinal chromophore that photoisomerizes to 13-cis-retinal. The recent crystal structure determination of bovine rhodopsin revealed a structure that is not similar to previously established bacteriorhodopsin structures. Both groups of proteins have a heptahelical transmembrane bundle structure, but the helices are arranged differently. The activation of rhodopsin involves rapid cis-trans photoisomerization of the chromophore, followed by slower and incompletely defined structural rearrangements. For rhodopsin and related receptors, a common mechanism is predicted for the formation of an active state intermediate that is capable of interacting with G proteins.

Animals↗
PubMed2002

Electronic excitations of biomolecules studied by quantum chemistry.

Significant methodological advances have been made over the past ten years in developing reliable quantum chemical methods for the treatment of electronically excited states. These methods can nowadays be used routinely by the experienced researcher to accurately compute excitation spectra of medium-sized organic molecules; results have been reported for several popular photobiological systems, including green fluorescent protein. First steps are currently being taken to account for the solvochromic shifts of chromophore excitations caused by particular protein environments and to dynamically simulate photochemical reactions in the excited states.

Bacterial Proteins↗
PubMed1998

The structure and mechanism of the family of retinal proteins from halophilic archaea.

Retinal proteins from halophilic archaea provide a unique opportunity to analyze vectorial ion translocation. Studies on its structure, conformational changes, proton conduction and electrogenic steps have helped to elucidate the catalytic cycle of bacteriorhodopsin in increasing detail. Experimental modulation of the vectoriality and ion specificity by altering the substrate availability, point mutations and light conditions for the different retinal proteins allows the proposal of a general model of ion transport for this protein family.

Archaeal Proteins↗
PubMed1999

Opsin shift in an aldolase antibody.

An antibody-retinal assembly that mimics the opsin shift (OS) of the naturally occurring visual pigments is reported. Both experiments and calculations show that the aldolase antibody 33F12 covalently binds all-trans retinal via a protonated Schiff base with a lysine residue. This chromophore, which exhibits a remarkable opsin red shift (140 nm), represents a useful model system for studying the factors that contribute to the OS.

Antibodies↗
PubMed1995

Colour vision. Dalton's eyes and monkey genes.

Recent molecular genetic studies show how changes in the protein component of a visual pigment alters its absorbance; they also explain the abnormal colour vision of one of the great pioneers of visual science.

Animals↗
PubMed1998

Sheep liver cytosolic aldehyde dehydrogenase: the structure reveals the basis for the retinal specificity of class 1 aldehyde dehydrogenases.

BACKGROUND: . Enzymes of the aldehyde dehydrogenase family are required for the clearance of potentially toxic aldehydes, and are essential for the production of key metabolic regulators. The cytosolic, or class 1, aldehyde dehydrogenase (ALDH1) of higher vertebrates has an enhanced specificity for all-trans retinal, oxidising it to the powerful differentiation factor all-trans retinoic acid. Thus, ALDH1 is very likely to have a key role in vertebrate development. RESULTS: . The three-dimensional structure of sheep ALDH1 has been determined by X-ray crystallography to 2.35 A resolution. The overall tertiary and quaternary structures are very similar to those of bovine mitochondrial ALDH (ALDH2), but there are important differences in the entrance tunnel for the substrate. In the ALDH1 structure, the sidechain of the general base Glu268 is disordered and the NAD+ cofactor binds in two distinct modes. CONCLUSIONS: . The submicromolar Km of ALDH1 for all-trans retinal, and its 600-fold enhanced affinity for retinal compared to acetaldehyde, are explained by the size and shape of the substrate entrance tunnel in ALDH1. All-trans retinal fits into the active-site pocket of ALDH1, but not into the pocket of ALDH2. Two helices and one surface loop that line the tunnel are likely to have a key role in defining substrate specificity in the wider ALDH family. The relative sizes of the tunnels also suggest why the bulky alcohol aversive drug disulfiram reacts more rapidly with ALDH1 than ALDH2. The disorder of Glu268 and the observation that NAD+ binds in two distinct modes indicate that flexibility is a key facet of the enzyme reaction mechanism.

Aldehyde Dehydrogenase↗
PubMed2001

Kinetics of photo-active bacteriorhodopsin analog 3,4-didehydroretinal.

Kinetics of the photo-induced processes of the transient states of the 3,4-didehydroretinal (3,4-dhr) modified bacteriorhodopsin (bR) was studied by a flash photolysis method in a water suspension at room temperature. The excitation initiated a photocycle with several transient intermediates similar to the trans photocycle of native bR. The main observation of the study was that although major part (80%) of the population of the M state relaxed via the O intermediate as in natural bR, 20% relaxed directly to the bR ground state in 200 ms.

Bacteriorhodopsins↗
PubMed2001

A semiempirical study of the optimized ground and excited state potential energy surfaces of retinal and its protonated Schiff base.

The electronic ground and first excited states of retinal and its Schiff base are optimized for the first time using the semiempirical AM1 Hamiltonian. The barrier for rotation about the C(11)-C(12) double bond is characterized by variation of both the twist angle delta(C(10)-C(11)-C(12)-C(13)) and the bond length d(C(11)-C(12)). The potential energy surface is obtained by varying these two parameters. The calculated ground state rotational barrier is equal to 15.6 kcal/mol for retinal and 20.5 kcal/mol for its Schiff base. The all-trans conformation is more stable by 3.7 kcal/mol than the 11-cis geometry. For the first excited state, S(1,) the 90 degrees twisted geometry represents a saddle point for retinal with the rotational barrier of 14.6 kcal/mol. In contrast, this conformation is an energy minimum for the Schiff base. It can be easily reached at room temperature from the planar minima since it is separated from them by a barrier of only 0.6 kcal/mol. The 90 degrees minimum conformation is more stable than the all-trans by 8.6 kcal/mol. We are thus able to present a reaction path on the S(1) surface of the retinal Schiff base with an almost barrier-less geometrical relaxation into a twisted minimum geometry, as observed experimentally. The character of the ground and first excited singlet states underscores the need for the inclusion of double excitations in the calculations.

Energy Transfer↗
PubMed2002

All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH.

The flash photolysis kinetic spectra of the intermediate M(412) of bacteriorhodopsin were monitored during the process of acid titration. In the light-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin decreased (pK(a)=3.40+/-0.05) as the pH decreased from 7.3 to 1.9. In the dark-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin increased as the pH decreased from 6.9 to 4.1, and then decreased (pK(a)=2.85+/-0.05) as the pH dropped to 2.1. These different trends in the change in the maximum peak amplitude suggested that not only the transition of purple membrane to blue membrane had taken place in both light and dark-adapted states, but also the fraction of all-trans-bR had changed during the acid titration. The pH-dependent absorption changes at 640 nm of bacteriorhodopsin in both light- and dark-adapted states were also observed. The pK(a)-values of the purple-to-blue transition were 3.80+/-0.05 in light-adapted state and 3.40+/-0.05 in dark-adapted state, respectively. According to Balashov's method, the fraction of all-trans-bR was assayed as the pH decreased. All these results indicated that the purple-to-blue transition of light-adapted bacteriorhodopsin was accompanied by an all-trans to 13-cis retinal isomerization at acidic pH.

Bacteriorhodopsins↗
PubMed1996

Polyenes and vision.

Polyenes are important for vision in all sighted species. The visual pigments (the rhodopsins) all use 11-cis-retinal as the chromophore; some possible reasons for the importance of this isomer are now emerging. New results on the involvement of xanthophils in the maintenance of the retina are also discussed.

Humans↗