Critical Role of Transmembrane Segment Zinc Binding in the Structure and Function of Rhodopsin

Abstract

Zinc deficiency and retinitis pigmentosa are both important factors resulting in retinal dysfunction and night blindness. In this study, we address the critical biochemical and structural relevance of zinc ions in rhodopsin and examine whether zinc deficiency can lead to rhodopsin dysfunction. We report the identification of a high-affinity zinc coordination site within the transmembrane domain of rhodopsin, coordinated by the side chains of two highly conserved residues, Glu 122 in transmembrane helix III and His 211 in transmembrane helix V. We also demonstrate that this zinc coordination is critical for rhodopsin folding, 11-cis-retinal binding, and the stability of the chromophore-receptor interaction, defects of which are observed in retinitis pigmentosa. Furthermore, a cluster of retinitis pigmentosa mutations is localized within and around this zinc binding site. Based on these studies, we believe that improvement in zinc binding to rhodopsin at this site within the transmembrane domain may be a pharmacological approach for the treatment of select retinitis pigmentosa mutations. Transmembrane coordination of zinc may also be an important common principle across G-protein-coupled receptors.Several neurodegenerative disorders, Alzheimer disease, Parkinson disease, familial amyotrophic lateral sclerosis, and the transmissible spongiform encephalopathies, share a common pathogenesis involving the misfolding and aggregation of specific proteins. Mounting evidence has demonstrated the direct binding of zinc (Zn 2ϩ ) to the ␤-amyloid (Alzheimer disease), ␣-synuclein (Parkinson disease), superoxide dismutase (amyotrophic lateral sclerosis), and prion (transmissible spongiform encephalopathies) proteins, linking either the gain or loss of Zn 2ϩ binding to the progression of these severe protein misfolding disorders. Zn 2ϩ promotes aggregation of the highly fibrillogenic prion peptide, PrP106 -126 (1), and of the ␤-amyloid protein (2-4) into amyloidogenic aggregates. Clioquinol, a metal chelating agent, is currently being investigated as a potential therapeutic solution to inhibit ␤-amyloid neurotoxicity (5). In contrast, in mutant superoxide dismutase protein, loss of affinity for Zn 2ϩ results in diminished protein activity (6), reduced protein stability (7), and formation of amyloid-like filaments (8). Thus, both zinc deficiencies and excesses can lead to neurodegenerative diseases.In the G protein-coupled photoreceptor rhodopsin, there have been over 100 distinct, heritable mutations identified, many of which induce an altered protein conformation, misfolding, aggregation, and cell death, followed by the clinical manifestation of the retinal neurodegenerative disorder retinitis pigmentosa (9). Interestingly, Zn 2ϩ deficiency is also known to cause retinal neurodegeneration and night blindness (10), symptoms reminiscent of retinitis pigmentosa. Furthermore, Zn 2ϩ has been shown to directly bind rhodopsin (11,12) and to reduce rhodopsin thermal stability and regeneration with 11-cis-retinal at higher Zn 2ϩ concen...

Odour character differences for enantiomers correlate with molecular flexibility

“…The Leffingwell database contains many odorants that are structurally similar to 11-cis-retinal, such as the strongsmelling irones, ionones and damascones. It is reported that the b-ionone ring of 11-cis-retinal exists 8 Å from a zinc coordination site within the binding site in rhodopsin (Stojanovic et al 2004). With zinc deficiency, the receptor rhodopsin malfunctions; similarly, a lack of zinc causes anosmia.…”

Stabilizing Effect of Zn2+ in Native Bovine Rhodopsin

“…Rhodopsin can directly associate with this bivalent metal ion (5,6), and Zn 2ϩ increases the level of rhodopsin phosphorylation (7). Furthermore, changes are observed in the pattern of thermal bleaching and regeneration of rhodopsin by 11-cis-retinal in the presence and absence of Zn 2ϩ (6,8). Therefore, Zn 2ϩ appears to affect the structure and function of rhodopsin.…”

“…The role of Zn 2ϩ in vision is likely related, in part, to rhodopsin, the light receptor that initiates phototransduction in the rod outer segments (ROS) 5 of retinal photoreceptor cells (4). Rhodopsin can directly associate with this bivalent metal ion (5,6), and Zn 2ϩ increases the level of rhodopsin phosphorylation (7). Furthermore, changes are observed in the pattern of thermal bleaching and regeneration of rhodopsin by 11-cis-retinal in the presence and absence of Zn 2ϩ (6,8).…”

Zinc Deficiency Mediates Alcohol-Induced Alveolar Epithelial and Macrophage Dysfunction in Rats

“…Zinc levels in the bronchoalveolar lavage fluid and in the isolated alveolar macrophages and epithelial cells were measured using a zincspecific dye FluoZin-3 (Invitrogen, Carlsbad, CA), a recently developed fluorochrome with a high affinity for zinc (K d 15 nM) and little affinity for calcium or magnesium (19,20). This free acid form is not membrane permeable and is mainly used to detect zinc release in extracellular spaces such as lavage fluid.…”