Driven by the energy of a photon, the visual pigments in rod and cone photoreceptor cells isomerize 11-cis-retinal to the all-trans configuration. This photochemical reaction initiates the signal ...transduction pathway that eventually leads to the transmission of a visual signal to the brain and leaves the opsins insensitive to further light stimulation. For the eye to restore light sensitivity, opsins require recharging with 11-cis-retinal. This trans-cis back conversion is achieved through a series of enzymatic reactions composing the retinoid (visual) cycle. Although it is evident that the classical retinoid cycle is critical for vision, the existence of an adjunct pathway for 11-cis-retinal regeneration has been debated for many years. Retinal pigment epithelium (RPE)-retinal G protein-coupled receptor (RGR) has been identified previously as a mammalian retinaldehyde photoisomerase homologous to retinochrome found in invertebrates. Using pharmacological, genetic, and biochemical approaches, researchers have now established the physiological relevance of the RGR in 11-cis-retinal regeneration. The photoisomerase activity of RGR in the RPE and Müller glia explains how the eye can remain responsive in daylight. In this review, we will focus on retinoid metabolism in the eye and visual chromophore regeneration mediated by RGR.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Carotenoid cleavage dioxygenases (CCDs) comprise a superfamily of mononuclear non-heme iron proteins that catalyze the oxygenolytic fission of alkene bonds in carotenoids to generate apocarotenoid ...products. Some of these enzymes exhibit additional activities such as carbon skeleton rearrangement and trans-cis isomerization. The group also includes a subfamily of enzymes that split the interphenyl alkene bond in molecules such as resveratrol and lignostilbene. CCDs are involved in numerous biological processes ranging from production of light-sensing chromophores to degradation of lignin derivatives in pulping waste sludge. These enzymes exhibit unique features that distinguish them from other families of non-heme iron enzymes. The distinctive properties and biological importance of CCDs have stimulated interest in their modes of catalysis. Recent structural, spectroscopic, and computational studies have helped clarify mechanistic aspects of CCD catalysis. Here, we review these findings emphasizing common and unique properties of CCDs that enable their variable substrate specificity and regioselectivity.
This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.
•CCDs are iron-dependent, alkene-cleaving enzymes broadly found in nature.•Crystal structures for six distinct alkene-cleaving CCDs have been solved to date.•Universal and unique structural features of CCDs are described.•CCD catalysis is discussed in light of recent experimental and computational studies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Carotenoid cleavage dioxygenases (CCDs) constitute a superfamily of enzymes that are found in all domains of life where they play key roles in the metabolism of carotenoids and apocarotenoids as well ...as certain phenylpropanoids such as resveratrol. Interest in these enzymes stems not only from their biological importance but also from their remarkable catalytic properties including their regioselectivity, their ability to accommodate diverse substrates, and the additional activities (e.g., isomerase) that some of these enzyme possess. X-ray crystallography is a key experimental approach that has allowed detailed investigation into the structural basis behind the interesting biochemical features of these enzymes. Here, we describe approaches used by our lab that have proven successful in generating single crystals of these enzymes in resting or ligand-bound states for high-resolution X-ray diffraction analysis.
The vitamin A derivative 11-
retinaldehyde plays a pivotal role in vertebrate vision by serving as the chromophore of rod and cone visual pigments. In the initial step of vision, a photon is absorbed ...by this chromophore resulting in its isomerization to an all-
state and consequent activation of the visual pigment and phototransduction cascade. Spent chromophore is released from the pigments through hydrolysis. Subsequent photon detection requires the delivery of regenerated 11-
retinaldehyde to the visual pigment. This
conversion is achieved through a process known as the visual cycle. In this review, we will discuss the enzymes, binding proteins and transporters that enable the visual pigment renewal process with a focus on advances made during the past decade in our understanding of their structural biology.
Apocarotenoids are important signaling molecules generated from carotenoids through the action of carotenoid cleavage dioxygenases (CCDs). These enzymes have a remarkable ability to cleave ...carotenoids at specific alkene bonds while leaving chemically similar sites within the polyene intact. Although several bacterial and eukaryotic CCDs have been characterized, the long-standing goal of experimentally visualizing a CCD–carotenoid complex at high resolution to explain this exquisite regioselectivity remains unfulfilled. CCD genes are also present in some archaeal genomes, but the encoded enzymes remain uninvestigated. Here, we address this knowledge gap through analysis of a metazoan-like archaeal CCD from Candidatus Nitrosotalea devanaterra (NdCCD). NdCCD was active toward β-apocarotenoids but did not cleave bicyclic carotenoids. It exhibited an unusual regiospecificity, cleaving apocarotenoids solely at the C14′–C13′ alkene bond to produce β-apo-14′-carotenals. The structure of NdCCD revealed a tapered active site cavity markedly different from the broad active site observed for the retinal-forming Synechocystis apocarotenoid oxygenase (SynACO) but similar to the vertebrate retinoid isomerase RPE65. The structure of NdCCD in complex with its apocarotenoid product demonstrated that the site of cleavage is defined by interactions along the substrate binding cleft as well as selective stabilization of reaction intermediates at the scissile alkene. These data on the molecular basis of CCD catalysis shed light on the origins of the varied catalytic activities found in metazoan CCDs, opening the possibility of modifying their activity through rational chemical or genetic approaches.
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Carotenoid cleaving dioxygenases (CCD) are a group of proteins involved in oxidative cleavage of carotenoids and non-carotenoids using a non-heme iron center. Previous studies on structure and ...function of CCD in plants, cyanobacteria as well animals show these are typically regio- and stereo-specific for substrate cleavage. Some are also known to show double bond isomerization similar to the visual cycle isomerase RPE65. However, it is still unclear as to how certain members of this family evolved to function distinctly. More so, dearth of knowledge on CCDs in archaea makes it implausible to link their evolutionary divergence. Recent phylogenetic studies propose that eukaryotes have emerged from archaea, favoring a two-domain tree of life model with Archaea and Bacteria. Probing into the cleavage mechanism of CCDs in archaea would provide valuable insight into the evolution of these proteins. In an attempt to bridge this gap, we characterized a putative oxygenase from an archaeon, Candidatus Nitrosotalea devanaterra (NdCCD), with a close evolutionary relationship to metazoan CCDs. The structure of NdCCCD in complex with its apocarotenoid product displays similarity to RPE65 and helps discern the active site determinants for cleavage.RDH5, an 11-cis-retinol dehydrogenase present in the retinal pigment epithelium (RPE) of the eye retina catalyzes oxidation of 11-cis-retinol to 11- retinaldehyde, thus regenerating the visual chromophore. In humans, mutations in RDH5 are predominantly associated with fundus albipunctatus, characterized by presence of white flecks in the retina and delayed rod adaptation impairing night vision. However, a subset of patients also develops macular atrophy or cone dystrophy. Rdh5-/- mice do not recapitulate human phenotype and exhibit delayed dark adaptation only after prolonged bleaching. We characterized a new domestic cat model with a loss of function missense mutation in RDH5 that results in atrophy of its area centralis region (equivalent to the human macula). As in human patients, not all cats developed atrophy in their central retina, displaying phenotypic variability. This novel cat model provides opportunities to improve molecular understanding of macular degeneration and test therapeutic interventions for RDH5-associated retinopathies seen in humans.
Apocarotenoids are important signaling molecules generated from carotenoids through the action of carotenoid cleavage dioxygenases (CCDs). These enzymes have a remarkable ability to cleave ...carotenoids at specific alkene bonds while leaving chemically similar sites within the polyene intact. Although several bacterial and eukaryotic CCDs have been characterized, the long-standing goal of experimentally visualizing a CCD–carotenoid complex at high resolution to explain this exquisite regioselectivity remains unfulfilled. CCD genes are also present in some archaeal genomes, but the encoded enzymes remain uninvestigated. Here, we address this knowledge gap through analysis of a metazoan-like archaeal CCD from Candidatus Nitrosotalea devanaterra (NdCCD). Nd CCD was active toward β-apocarotenoids but did not cleave bicyclic carotenoids. It exhibited an unusual regiospecificity, cleaving apocarotenoids solely at the C14'–C13' alkene bond to produce β-apo-14'-carotenals. The structure of Nd CCD revealed a tapered active site cavity markedly different from the broad active site observed for the retinal-forming Synechocystis apocarotenoid oxygenase ( Syn ACO) but similar to the vertebrate retinoid isomerase RPE65. The structure of Nd CCD in complex with its apocarotenoid product demonstrated that the site of cleavage is defined by interactions along the substrate binding cleft as well as selective stabilization of reaction intermediates at the scissile alkene. These data on the molecular basis of CCD catalysis shed light on the origins of the varied catalytic activities found in metazoan CCDs, opening the possibility of modifying their activity through rational chemical or genetic approaches.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Abstract
Pathogenic variants in retinol dehydrogenase 5 (RDH5) attenuate supply of 11-cis-retinal to photoreceptors leading to a range of clinical phenotypes including night blindness because of ...markedly slowed rod dark adaptation and in some patients, macular atrophy. Current animal models (such as Rdh5−/− mice) fail to recapitulate the functional or degenerative phenotype. Addressing this need for a relevant animal model we present a new domestic cat model with a loss-of-function missense mutation in RDH5 (c.542G > T; p.Gly181Val). As with patients, affected cats have a marked delay in recovery of dark adaptation. In addition, the cats develop a degeneration of the area centralis (equivalent to the human macula). This recapitulates the development of macular atrophy that is reported in a subset of patients with RDH5 mutations and is shown in this paper in seven patients with biallelic RDH5 mutations. There is notable variability in the age at onset of the area centralis changes in the cat, with most developing changes as juveniles but some not showing changes over the first few years of age. There is similar variability in development of macular atrophy in patients and while age is a risk factor, it is hypothesized that genetic modifying loci influence disease severity, and we suspect the same is true in the cat model. This novel cat model provides opportunities to improve molecular understanding of macular atrophy and test therapeutic interventions for RDH5-associated retinopathies.
Abstract
Protein-based agonists of apoptotic death receptors have shown remarkable preclinical efficacy but limited clinical response. The short circulating half-life of recombinant human TRAIL and ...the necessity of Fc-mediated clustering for potentiating agonistic antibodies against DR4 and DR5 have been proposed to be major impediments to the clinical success of this class. To address these limitations we have created Fc-scTRAIL, a single fusion polypeptide consisting of an IgG1 Fc region followed by three successive TRAIL monomers connected by two fifteen-amino acid linkers. While Fc-scTRAIL showed potent activity in vitro, we observed a low TM (48 °C) and rapid inactivation in serum indicating protein instability. Subsequently, we applied a directed evolution approach using yeast surface display to identify mutations that would stabilize the TRAIL trimer. When individual mutations were transferred to the Fc-scTRAIL format, we observed a dramatic increase in the TM (66-70 °C) while the combination of three mutations improved serum stability by ten-fold. Stabilized Fc-scTRAIL shows greater pro-apoptotic activity across a panel of cancer cell lines when compared to mapatumumab (anti-DR4) and drozitumab (anti-DR5), or the combination of antibodies even in the presence of anti-Fc cross-linking. Moreover, anti-Fc did not improve Fc-scTRAIL activity suggesting that the hexavalent design of the molecule maximizes death receptor activation. Currently, in vivo evaluation of Fc-scTRAIL for pharmacokinetic properties and activity is underway. We believe this format, when combined with an appropriate patient selection strategy, will result in improved clinical outcomes.
Citation Format: Eric M. Tam, Hannah Hudson, Tamara Dake, Sara Ghassemifar, Andreas Raue, Yasmin Hashambhoy-Ramsay, Stephen L. Sazinsky, Anahita Daruwalla, Neeraj Kohli, Lihui Xu, Charlotte F. Mc Donagh, Birgit Schoeberl, Diana H. Chai. Design and engineering of TRAIL fusion proteins for cancer therapy. abstract. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3842.
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