Membranes as the Energy Source in the Endergonic Transformation of Vitamin A to 11-
            <i>cis</i>
            -Retinol

Deigner, Peter S.; Law, Wing Cheung; Cañada, F. Javier; Rando, Robert R.

doi:10.1126/science.2727688

Cited by 160 publications

(160 citation statements)

References 8 publications

(7 reference statements)

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“…Studies using isotopically labeled substrate demonstrate that the stereochemical configuration of C15 inverts during isomerization (30), and that the hydroxyl oxygen in the reactant is lost during the reaction (31,32). Both observations provide strong evidence that an O-alkyl cleavage event rather than acyl cleavage occurs during the reaction.…”

Section: Resultsmentioning

confidence: 93%

Crystal structure of native RPE65, the retinoid isomerase of the visual cycle

Kiser

Golczak²,

Lodowski³

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

137

227

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Vertebrate vision is maintained by the retinoid (visual) cycle, a complex enzymatic pathway that operates in the retina to regenerate the visual chromophore, 11-cis-retinal. A key enzyme in this pathway is the microsomal membrane protein RPE65. This enzyme catalyzes the conversion of all-trans-retinyl esters to 11-cis-retinol in the retinal pigment epithelium (RPE). Mutations in RPE65 are known to be responsible for a subset of cases of the most common form of childhood blindness, Leber congenital amaurosis (LCA). Although retinoid isomerase activity has been attributed to RPE65, its catalytic mechanism remains a matter of debate. Also, the manner in which RPE65 binds to membranes and extracts retinoid substrates is unclear. To gain insight into these questions, we determined the crystal structure of native bovine RPE65 at 2.14-Å resolution. The structural, biophysical, and biochemical data presented here provide the framework needed for an in-depth understanding of the mechanism of catalytic isomerization and membrane association, in addition to the role mutations that cause LCA have in disrupting protein function.isomerization ͉ metalloprotein ͉ monotopic membrane protein V ision begins when the 11-cis-retinylidene chromophore of rhodopsin is photoisomerized to all-trans-retinylidene, a process resulting in receptor activation and transduction of the light signal (1). After rhodopsin is photoactivated, it is no longer responsive to light, so for vision to continue, a trans-to-cis isomerization mechanism must be present to regenerate lightsensitive visual pigments. In vertebrates, after photoisomerization, all-trans-retinylidene is hydrolyzed from rhodopsin, reduced to all-trans-retinol, and transported to a tissue adjacent to the photoreceptor layer known as the retinal pigment epithelium (RPE), where enzymatic isomerization occurs (2). An RPEspecific, microsomal membrane protein with an apparent molecular mass of 65 kDa, known as RPE65, was determined to be responsible for trans-to-cis retinoid isomerase activity in the RPE (3-5). The importance of this protein in visual function is also evident from the observation that certain RPE65 mutations cause a form of the hereditary childhood blinding disease known as Leber congenital amaurosis (LCA) or the less severe, lateronset disease, retinitis pigmentosa (RP) (6-8).Based on sequence homology, RPE65 belongs to a family of carotenoid cleavage oxygenase (CCO) enzymes that oxidatively cleave ␤-carotene or apocarotenoids (9-11). However, RPE65 is distinct from all other members of this family in that it simultaneously cleaves and isomerizes all-trans-retinyl esters to 11-cisretinol and a fatty acid rather than oxidatively cleaving carotenoids (3-5, 11, 12). Unlike the reactions catalyzed by other CCO family members, there is no obvious role for molecular oxygen in RPE65 enzymology. The only family member with a known crystal structure is an apocarotenoid oxygenase from Synechocystis that is 25% identical and 42% homologous to human RPE65 (13). All members of this fa...

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Section: Resultsmentioning

confidence: 93%

Crystal structure of native RPE65, the retinoid isomerase of the visual cycle

Kiser

Golczak²,

Lodowski³

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

137

227

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“…LRAT transfers a fatty acid from the sn-1 fatty acid of PC to retinol (6, 7), yielding lyso-PC and an all-trans-RE. Rpe65 uses the all-trans-RE as substrate, harnessing the energy of ester hydrolysis to drive all-trans to 11-cis-isomerization (27). Resynthesis of PC from lyso-PC following deacylation by LRAT involves consumption of an activated fatty acid such as palm CoA.…”

Section: Discussionmentioning

confidence: 99%

Identification of the 11- cis- specific retinyl-ester synthase in retinal Müller cells as multifunctional O- acyltransferase (MFAT)

Kaylor¹,

Cook²,

Makshanoff³

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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Absorption of a photon by a rhodopsin or cone-opsin pigment isomerizes its 11-cis-retinaldehyde (11-cis-RAL) chromophore to all-trans-retinaldehyde (all-trans-RAL), which dissociates after a brief period of activation. Light sensitivity is restored to the resulting apo-opsin when it recombines with another 11-cis-RAL. Conversion of all-trans-RAL to 11-cis-RAL is carried out by an enzyme pathway called the visual cycle in cells of the retinal pigment epithelium. A second visual cycle is present in Müller cells of the retina. The retinol isomerase for this noncanonical pathway is dihydroceramide desaturase (DES1), which catalyzes equilibrium isomerization of retinol. Because 11-cis-retinol (11-cis-ROL) constitutes only a small fraction of total retinols in an equilibrium mixture, a subsequent step involving selective removal of 11-cis-ROL is required to drive synthesis of 11-cis-retinoids for production of visual chromophore. Selective esterification of 11-cis-ROL is one possibility. Crude homogenates of chicken retinas rapidly convert all-trans-ROL to 11-cis-retinyl esters (11-cis-REs) with minimal formation of other retinyl-ester isomers. This enzymatic activity implies the existence of an 11-cis-specific retinyl-ester synthase in Müller cells. Here, we evaluated multifunctional O-acyltransferase (MFAT) as a candidate for this 11-cis-RE-synthase. MFAT exhibited much higher catalytic efficiency as a synthase of 11-cis-REs versus other retinyl-ester isomers. Further, we show that MFAT is expressed in Müller cells. Finally, homogenates of cells coexpressing DES1 and MFAT catalyzed the conversion of all-trans-ROL to 11-cis-RP, similar to what we observed with chicken-retina homogenates. MFAT is therefore an excellent candidate for the retinyl-ester synthase that cooperates with DES1 to drive synthesis of 11-cis-retinoids by mass action.L ight perception begins with the absorption of a photon by an opsin pigment in the membranous outer segment (OS) of a rod or cone photoreceptor cell. The light-absorbing chromophore in most vertebrate opsins is 11-cis-retinaldehyde (11-cis-RAL). Photon capture isomerizes the 11-cis-RAL to all-trans-retinaldehyde (all-trans-RAL), inducing conformational changes in the protein that lead to its active meta-II state. After a brief period of signaling through the transduction cascade, meta II decays to yield apo-opsin and free all-trans-RAL. Light sensitivity is restored to the apo-opsin when it combines with 11-cis-RAL to regenerate the pigment. Conversion of all-trans-RAL to 11-cis-RAL is carried out by a multistep enzyme pathway called the visual cycle, located in cells of the retinal pigment epithelium (RPE) (1, 2). The retinoid isomerase in this pathway is Rpe65, which converts an all-transretinyl ester (all-trans-RE), such as all-trans-retinyl palmitate (alltrans-RP), to 11-cis-retinol (11-cis-ROL) and a free fatty acid (3-5). Retinyl esters are synthesized in RPE cells by lecithin:retinol acyl transferase (LRAT), which transfers a fatty acid from phosphatidylcholine to retinol (6, ...

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“…One model concerning the isomerization step in the retinoid cycle assumes that the hydrolysis of retinyl esters provide the excess energy necessary for the isomerization process (42,43). It has been hypothesized that potent inhibition by cis-retinols prevents isomerization of all-trans-retinyl esters (44).…”

Section: Accumulation Of 11-cis-retinyl Esters In 11-cis-rdhϫ/ϫmentioning

confidence: 99%

Characterization of a Dehydrogenase Activity Responsible for Oxidation of 11-cis-Retinol in the Retinal Pigment Epithelium of Mice with a Disrupted RDH5 Gene

Jang

Hooser

Kuksa

et al. 2001

Journal of Biological Chemistry

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In the vertebrate retina, the final step of visual chromophore production is the oxidation of 11-cis-retinol to 11-cis-retinal. This reaction is catalyzed by 11-cis-retinol dehydrogenases (11-cis-RDHs), prior to the chromophore rejoining with the visual pigment apo-proteins. The RDH5 gene encodes a dehydrogenase that is responsible for the majority of RDH activity. In humans, mutations in this gene are associated with fundus albipunctatus, a disease expressed by delayed dark adaptation of both cones and rods. In this report, an animal model for this disease, 11-cis-rdh؊/؊ mice, was used to investigate the flow of retinoids after a bleach, and microsomal membranes from the retinal pigment epithelium of these mice were employed to characterize remaining enzymatic activities oxidizing 11-cis-retinol. Lack of 11-cis-RDH leads to an accumulation of cis-retinoids, particularly 13-cis-isomers. The analysis of 11-cisrdh؊/؊ mice showed that the RDH(s) responsible for the production of 11-cis-retinal displays NADP-dependent specificity toward 9-cis-and 11-cis-retinal but not 13-cisretinal. The lack of 13-cis-RDH activity could be a reason why 13-cis-isomers accumulate in the retinal pigment epithelium of 11-cis-rdh؊/؊ mice. Furthermore, our results provide detailed characterization of a mouse model for the human disease fundus albipunctatus and emphasize the importance of 11-cis-RDH in keeping the balance between different components of the retinoid cycle.

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Membranes as the Energy Source in the Endergonic Transformation of Vitamin A to 11- cis -Retinol

Cited by 160 publications

References 8 publications

Crystal structure of native RPE65, the retinoid isomerase of the visual cycle

Crystal structure of native RPE65, the retinoid isomerase of the visual cycle

Identification of the 11- cis- specific retinyl-ester synthase in retinal Müller cells as multifunctional O- acyltransferase (MFAT)

Characterization of a Dehydrogenase Activity Responsible for Oxidation of 11-cis-Retinol in the Retinal Pigment Epithelium of Mice with a Disrupted RDH5 Gene

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