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Which structures form the protein Rhodopsin?
Rhodopsin is 348 amino acids long.(1) Amino acids bond together to form secondary structures, which include alpha helices, beta pleated sheets or random coils. As seen in figure 3, Rhodopsin contains 7 vertical helices, 1 horizontal helix and 4 sheets found at the end of the protein.(2-4) Random coils connect the helices and sheets together, forming a functional protein.(4)
Figure 3: 3-Dimensional Structure of Rhodopsin, at two different angles, with alpha helices in red, beta sheets in yellow and random coils in green.(4)
Watch the video below about the structure of Rhodopsin.(1-4)
How does the 3-Dimensional Structure of Rhodopsin correlate to its function?
Rhodopsin is a barrel-like structure containing a transmembrane region, a region facing the outer segment of the rod and a region facing the inner segment of the rod, as seen in figure 4.(2) The portions of rhodopsin that sit outside the membrane are important for maintaining the structural integrity of the helices.
Figure 4: Structure of a Rod Cell and the location of Rhodopsin. Rhodopsin protrudes through membranes in the outer segment and parts of the protein are found in the areas between these membranes.(2)
Rhodopsin is comprised of the protein opsin bound to a molecule called retinal.(2) Retinal has two forms called cis and trans. Cis-retinal binds to opsin within the helices, as seen in figure 5.(4) When retinal absorbs light, this causes it to change shape from cis-retinal to trans-retinal, seen in figure 6. Because of its relationship with opsin, the change in shape from cis-retinal to trans-retinal changes the shape of the whole rhodopsin protein.(2) Change in shape activates rhodopsin, causing a cascade of signals that results in vision.
Figure 5: Location of Retinal (blue) within the alpha helices of the protein Opsin (magenta). Together opsin and retinal form rhodopsin.(4)
Figure 6: Pathway of Rhodopsin Activation. When light is present, this causes a change in shape from cis-retinal to trans-retinal. As retinal is bound to rhodopsin, this causes rhodopsin to change shape, resulting in its activation and ultimately vision.
References
NCBI [Internet]. Bethesda, MD: National Library of Medicine, 1994. Homo sapiens chromosome 3, GRCh38.p14 Primary Assembly; April 6 2022 [cited 2022 Sept 30]; [about 12 screens]. Available from:
https://www.ncbi.nlm.nih.gov/nuccore/NC_000003.12?report=genbank&from=129527632&to=129536349
Palczewki K, Kumasaka T, Yamamoto M, Miyano M, Hori T, Behnke CA, Motoshima H, Fox BA, Le Trong I, Teller DC, Okada T, Stenkamp RE. Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor. Science (AAAS) [Internet]. 2000 [cited 2022 Sept 30];289(5480):739-745. Available from: https://go.gale.com/ps/i.do?p=AONE&u=monash&id=GALE%7CA64697932&v=2.1&it=r
UniProt [Internet]. Geneva, Switzerland:UniProt consortium;2022. P08100 OPSD_HUMAN [cited 2022 Sept 30]; [about 30 screens]. Available from: https://www.uniprot.org/uniprotkb/P08100/entry
Schrödinger L, DeLano W. PyMOL [Internet]. 2020. Available from: http://www.pymol.org/pymo
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