Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness.
Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.
Retinal is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
Proteorhodopsin is a family of transmembrane proteins that use retinal as a chromophore for light-mediated functionality, in this case, a proton pump. pRhodopsin is found in marine planktonic bacteria, archaea and eukaryotes (protae), but was first discovered in bacteria.
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.
Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed in cells of other organisms, they enable light to control electrical excitability, intracellular acidity, calcium influx, and other cellular processes. Channelrhodopsin-1 (ChR1) and Channelrhodopsin-2 (ChR2) from the model organism Chlamydomonas reinhardtii are the first discovered channelrhodopsins. Variants that are sensitive to different colors of light or selective for specific ions have been cloned from other species of algae and protists.
Photoheterotrophs are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements; these compounds include carbohydrates, fatty acids, and alcohols. Examples of photoheterotrophic organisms include purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria. These microorganisms are ubiquitous in aquatic habitats, occupy unique niche-spaces, and contribute to global biogeochemical cycling. Recent research has also indicated that the oriental hornet and some aphids may be able to use light to supplement their energy supply.
Halorhodopsin is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is a chloride-specific light-activated ion pump found in archaea known as halobacteria. It is activated by green light wavelengths of approximately 578 nm. Halorhodopsin also shares sequence similarity to channelrhodopsin, a light-gated ion channel.
Rhodopsin kinase is a serine/threonine-specific protein kinase involved in phototransduction. This enzyme catalyses the following chemical reaction:
Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin. While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, rhodopsin in the broad sense refers to any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle.
A retinalophototroph is one of two different types of phototrophs, and are named for retinal-binding proteins they utilize for cell signaling and converting light into energy. Like all phototrophs, retinalophototrophs absorb photons to initiate their cellular processes. In contrast with chlorophototrophs, retinalophototrophs do not use chlorophyll or an electron transport chain to power their chemical reactions. This means retinalophototrophs are incapable of traditional carbon fixation, a fundamental photosynthetic process that transforms inorganic carbon into organic compounds. For this reason, experts consider them to be less efficient than their chlorophyll-using counterparts, chlorophototrophs.
Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group, also known as Marine Benthic Group B. Lokiarchaeota is part of the superphylum Asgard containing the phyla: Lokiarchaeota, Thorarchaeota, Odinarchaeota, Heimdallarchaeota, and Helarchaeota. A phylogenetic analysis disclosed a monophyletic grouping of the Lokiarchaeota with the eukaryotes. The analysis revealed several genes with cell membrane-related functions. The presence of such genes support the hypothesis of an archaeal host for the emergence of the eukaryotes; the eocyte-like scenarios.
Microbial rhodopsins, also known as bacterial rhodopsins, are retinal-binding proteins that provide light-dependent ion transport and sensory functions in halophilic and other bacteria. They are integral membrane proteins with seven transmembrane helices, the last of which contains the attachment point for retinal. Most microbial rhodopsins pump inwards, however "mirror rhodopsins" which function outwards. have been discovered.
Anion-conducting channelrhodopsins are light-gated ion channels that open in response to light and let negatively charged ions enter a cell. All channelrhodopsins use retinal as light-sensitive pigment, but they differ in their ion selectivity. Anion-conducting channelrhodopsins are used as tools to manipulate brain activity in mice, fruit flies and other model organisms (Optogenetics). Neurons expressing anion-conducting channelrhodopsins are silenced when illuminated with light, an effect that has been used to investigate information processing in the brain. For example, suppressing dendritic calcium spikes in specific neurons with light reduced the ability of mice to perceive a light touch to a whisker. Studying how the behavior of an animal changes when specific neurons are silenced allows scientists to determine the role of these neurons in the complex circuits controlling behavior.
Oded Béjà is a professor in the Technion-Israel Institute of Technology, in the field of marine microbiology and metagenomics.
Archaerhodopsin proteins are a family of retinal-containing photoreceptors found in the archaea genera Halobacterium and Halorubrum. Like the homologous bacteriorhodopsin (bR) protein, archaerhodopsins harvest energy from sunlight to pump H+ ions out of the cell, establishing a proton motive force that is used for ATP synthesis. They have some structural similarities to the mammalian G protein-coupled receptor protein rhodopsin, but are not true homologs.
Paul Hargrave is an American biochemist whose laboratory work established key features of the structure of rhodopsin.
Photoactivated adenylyl cyclase (PAC) is a protein consisting of an adenylyl cyclase enzyme domain directly linked to a BLUF type light sensor domain. When illuminated with blue light, the enzyme domain becomes active and converts ATP to cAMP, an important second messenger in many cells. In the unicellular flagellate Euglena gracilis, PACα and PACβ (euPACs) serve as a photoreceptor complex that senses light for photophobic responses and phototaxis. Small but potent PACs were identified in the genome of the bacteria Beggiatoa (bPAC) and Oscillatoria acuminata (OaPAC). While natural bPAC has some enzymatic activity in the absence of light, variants with no dark activity have been engineered (PACmn).
William Archer Hagins was an American medical researcher. He was chief of the Section of Membrane Biophysics in National Institute of Diabetes and Digestive and Kidney Diseases's Laboratory of Chemical Physics upon his retirement in 2007. Hagins and colleagues made the seminal discovery of the dark current in photoreceptor cells. This finding became central to understanding how the visual cells worked and led to knowledge of the importance of reattaching a detached retina as soon as possible for continued use. As a fellow of Fulbright Program, he'd also served in the United States Navy as a Research Medical Officer. He joined NIDDK's Laboratory of Physical Biology in 1958, doing independent research in the Section of Photobiology, headed by Frederick Sumner Brackett. Hagins was a mentor to many, particularly through his work with the Brackett Foundation.
Jan Steyaert is a Belgian bioengineer and molecular biologist. He started his career as an enzymologist but the Steyaertlab is best known for pioneering work on (engineered) nanobodies for applications in structural biology, omics and drug design. He is full professor and teaches biochemistry at the Vrije Universiteit Brussel and Director of the VIB-VUB Center for Structural Biology, one of the Research Centers of the Vlaams Instituut voor Biotechnologie (VIB). He was involved in the foundation of three spin-off companies: Ablynx, Biotalys, and Confo Therapeutics.
