Proteorhodopsin
Template:Short description Template:Cs1 config Template:Infobox protein family
Proteorhodopsin (PR or pRhodopsin) belongs to the family of bacterial transmembrane rhodopsins (retinylidene proteins).<ref name="Bamann_2014">Template:Cite journal</ref> In 1971, the first microbial transmembrane rhodopsin - Bacteriorhodopsin was discovered in archea domain by Dieter Oesterhelt and Walther Stoeckenius.<ref>Template:Cite journal</ref> Later in 2000, the first bacterial transmembrane rhodopsins was discovered by Oded Béjà and Edward DeLong.<ref>Template:Cite journal</ref> The Proteorhodopsin is widely expressed in various type of aquatic habitats.<ref name="Bamann_2014" /> It functions as light-driven proton pumps with the help of retinal chromophore at the active site.<ref name="Bamann_2014" /><ref name="Beja_2013">Template:Cite encyclopedia</ref> The light-driven proton pump gives bacteria energy in the form of adenosine triphosphate (ATP).<ref name="Bamann_2014" /><ref name="Beja_2013" />
Discovery
Efforts by Oded Béjà from Edward DeLong research group in pioneering bacterial artificial chromosome metagenomics analysis led the discovery of pRhodopsin in bacteria domain.<ref name="Beja_2013" /> It was first detected in uncultured gammaproteobacteria ribotype group SAR86 at Monterey Bay water column in 2000.<ref name="Beja_2013" /> Oded Béjà observed the sequence similarity between SAR86 pRhodopsin and bacteriorodopsin (a light driven proton pump in haloarchea) open reading frame.<ref name="Beja_2013" /> To further established pRhodopsin function as retinal-based light-driven proton pump, he expressed pRhodopsin open reading frame in Escherichia coli system.<ref name="Beja_2013" /> Before the discovery of bacterial Proteorhodpsin, it was understood that light driven active transport only evolved in extreme halophilic archaea domain (bacteriorodopsin, halorhodopsin, and sensory rhodopsin) and animal kingdom (as a visual rhodopsin).<ref name="Bamann_2014" /><ref name="Beja_2013" />
Species distribution
pRhodopsin is not confined to a single species and single habitat.<ref name="Bamann_2014" /> It is distributed in many microorganisms from all over the world.<ref name="Bamann_2014" /> pRhodopsin containing microorganisms is distributed in Gammaproteobacteria, Alphaproteobacteria, Betaproteobacteria, Flavobacteria, Planctomycetes, Cyanobacteria, Actinobacteria, marine Archaea, and different eukaryotic groups, including fungi and dinoflagellates.<ref name="Bamann_2014" /><ref name="Beja_2013" /> pRhodopsin containing microorganisms are habited in marine environments, sea ice, brackish environments, fresh water lakes and on high mountains.<ref name="Bamann_2014" /><ref name="Beja_2013" /> In the marine environment, pRhodopsin containing microorganisms is primarily found in photic zone.<ref name="Bamann_2014" /><ref name="Beja_2013" />
Structure
The topology and active site residues for proton transporting retinylidene proteins was first characterized in bacteriorhodopsin.<ref name="Bamann_2014" /> The pRhodopsin topology and active site residues are conserved to Bacteriorhodopsin.<ref name="Bamann_2014" /> pRhodopsin is a seven transmembrane α-helices that form a pocket in which retinal (vitamin A aldehyde) is covalently linked to ligand binding domain, as a protonated schiff base, to a lysine in the seventh transmembrane α-helix.<ref name="Bamann_2014" /> At ground state the retinal chromophore is all-trans configuration.<ref name="Bamann_2014" /> When visible light illuminates on pRhodopsin, the all-trans retinal molecule absorbs light energy and uses it to isomerize into13-cis configuration.<ref name="Bamann_2014" /> This triggers a sequence of protein conformational changes including several proton transfer reactions against concentration gradient, generating a proton motive force.<ref name="Bamann_2014" />
Function
Light-activated proteorhodopsin pumps protons outwardly, increasing the proton motive force across the microbial cell membrane.<ref name="Bamann_2014" /><ref name="Beja_2013" /> Protons can then reenter the cell through the ATP synthase complex, powering the synthesis of ATP. Proteorhodopsin thus allows microbial cells to harvest light energy and convert it into usable chemical energy without the involvement of chlorophyll-based photosystems.<ref name="Bamann_2014" /><ref name="Beja_2013" />
Microbes containing proteorhodopsin are considered phototrophs due to its functionality as a light-sensitive proton pump.<ref name="Bamann_2014" /><ref name="Beja_2013" /> Different variants of proteorhodopsin are spectrally tuned to absorb specific wavelengths of light, such as green or blue.<ref name="Bamann_2014" /> These adaptations allow organisms to occupy distinct ecological niches based on light availability at different water column depths.<ref name="Bamann_2014" /> These functional advantages make proteorhodopsin a key component in the marine microbial energy budget.<ref name="Beja_2013" />
Genetic engineering
If the gene for proteorhodopsin is inserted into E. coli and retinal is given to these modified bacteria, then they will incorporate the pigment into their cell membrane and will pump H+ in the presence of light energy.<ref name="Harder_2016">Template:Cite journal</ref> This functionality can be used to acidify a vesicle type organelle.<ref name="Harder_2016" />
See also
- Bacteriorhodopsin
- Rhodopsin
Gallery
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Holoenzyme (Green) with helices A-G labeled (purple) as well as Retinal ligand (orange)
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Surface visualization of Proteorhodopsin showing terminals
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Visualization of the retinal bound active site of the 2L6X protein structure of pRhodopsin, residues color coded and labeled by activity, ligand is orange.
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2L6x In-Active-Site Cartoon Color Coded and Labeled Visualization, D and E Helices hidden for vantage, Retinal ligand binding site
References
<references /> Template:Optogenetics