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At least 1,261 records · Page 70Linked to original sources
PubMed1999

Isomerization of all-trans-9- and 13-desmethylretinol by retinal pigment epithelial cells.

Photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototransduction in the retinal photoreceptor cells and causes ultimately the sensation of vision. 11-cis-Retinal is enzymatically regenerated through a complex set of reactions in adjacent retinal pigment epithelial cells (RPE). In this study using all-trans-9-desmethylretinol (lacking the C(19) methyl group) and all-trans-13-desmethylretinol (lacking the C(20) methyl group), we explored the effects of C(19) and C(20) methyl group removals on isomerization of these retinols in RPE microsomes. The C(19) methyl group may be involved in the substrate activation, whereas the C(20) methyl group causes steric hindrance with a proton in position C(10) of 11-cis-retinol; thus, removal of this group could accelerate isomerization. We found that all-trans-9-desmethylretinol and all-trans-13-desmethylretinol are isomerized to their corresponding 11-cis-alcohols, although with lower efficiencies than isomerization of all-trans-retinol to 11-cis-retinol. These findings make the mechanism of isomerization through the C(19) methyl group unlikely, because in the case of 9-desmethylretinol, the isomerization would have to progress by proton abstraction from electron-rich olefinic C(9). The differences between all-trans-retinol, all-trans-9-desmethylretinol, and all-trans-13-desmethylretinol appear to be a consequence of the enzymatic properties, and binding affinities of the isomerization system, rather than differences in the chemical or thermodynamic properties of these compounds. This observation is also supported by quantum chemical calculations. It appears that both methyl groups are not essential for the isomerization reaction and are not likely involved in formation of a transition stage during the isomerization process.

Alcohol Oxidoreductases↗
PubMed2000

FTIR analysis of the SII540 intermediate of sensory rhodopsin II: Asp73 is the Schiff base proton acceptor.

Sensory rhodopsin II (SRII), a repellent phototaxis receptor found in Halobacterium salinarum, has several homologous residues which have been found to be important for the proper functioning of bacteriorhodopsin (BR), a light-driven proton pump. These include Asp73, which in the case of bacteriorhodopsin (Asp85) functions as the Schiff base counterion and proton acceptor. We analyzed the photocycles of both wild-type SRII and the mutant D73E, both reconstituted in Halobacterium salinarum lipids, using FTIR difference spectroscopy under conditions that favor accumulation of the O-like, photocycle intermediate, SII540. At both room temperature and -20 degrees C, the difference spectrum of SRII is similar to the BR-->O640 difference spectrum of BR, especially in the configurationally sensitive retinal fingerprint region. This indicates that SII540 has an all-trans chromophore similar to the O640 intermediate in BR. A positive band at 1761 cm-1 downshifts 40 cm-1 in the mutant D73E, confirming that Asp73 undergoes a protonation reaction and functions in analogy to Asp85 in BR as a Schiff base proton acceptor. Several other bands in the C=O stretching regions are identified which reflect protonation or hydrogen bonding changes of additional Asp and/or Glu residues. Intense bands in the amide I region indicate that a protein conformational change occurs in the late SRII photocycle which may be similar to the conformational changes that occur in the late BR photocycle. However, unlike BR, this conformational change does not reverse during formation of the O-like intermediate, and the peptide groups giving rise to these bands are partially accessible for hydrogen/deuterium exchange. Implications of these findings for the mechanism of SRII signal transduction are discussed.

Archaeal Proteins↗
PubMed2001

Steric barrier to bathorhodopsin decay in 5-demethyl and mesityl analogues of rhodopsin.

Absorbance difference spectra were recorded from 20 ns to 1 micros after 20 degrees C photoexcitation of artificial visual pigments derived either from 5-demethylretinal or from a mesityl analogue of retinal. Both pigments produced an early photointermediate similar to bovine bathorhodopsin (Batho). In both cases the Batho analogue decayed to a lumirhodopsin (Lumi) analogue via a blue-shifted intermediate, BSI, which formed an equilibrium with the Batho analogue. The stability of 5-demethyl Batho, even though the C8-hydrogen of the polyene chain cannot interact with a ring C5-methyl group to provide a barrier to Batho decay, raises the possibility that the 5-demethylretinal ring binds oppositely from normal to form a pigment with a 6-s-trans ring-chain conformation. If 6-s-trans binding occurred, the ring C1-methyls could replace the C5-methyl in its interaction with the chain C8-hydrogen to preserve the steric barrier to Batho decay, consistent with the kinetic results. The possibility of 6-s-trans binding for 5-demethylretinal also could account for the unexpected blue shift of 5-demethyl visual pigments and could explain why 5-demethyl artificial pigments regenerate so slowly. Although the mesityl analogue BSI's absorption spectrum was blue-shifted relative to its pigment spectrum, the blue shift was much smaller than for rhodopsin's or 5-demethylisorhodopsin's BSI. This suggests that increased C6-C7 torsion may be responsible for some of BSI's blue shift, which is not the case for mesityl analogue BSI either because of reduced spectral sensitivity to C6-C7 torsion or because the symmetry of the mesityl retinal analogue precludes having 6-s-cis and 6-s-trans conformers. The similarity of the mesityl analogue BSI and native BSI lambda(max) values supports the idea that BSI has a 6-s angle near 90 degrees, a condition which could disconnect the chain (and BSI's spectrum) from the double bond specifics of the ring.

Animals↗