Retinalophototroph
From Wikipedia, the free encyclopedia
A retinalophototroph is one of two different types of phototrophs, and are named for retinal-binding proteins (microbial rhodopsins) they utilize for cell signaling and converting light into energy.[1][2][3][4] Like all phototrophs, retinalophototrophs absorb photons to initiate their cellular processes.[2][3][4] In contrast with chlorophototrophs, retinalophototrophs do not use chlorophyll or an electron transport chain to power their chemical reactions.[5][2][3] This means retinalophototrophs are incapable of traditional carbon fixation, a fundamental photosynthetic process that transforms inorganic carbon (carbon contained in molecular compounds like carbon dioxide) into organic compounds.[5][4] For this reason, experts consider them to be less efficient than their chlorophyll-using counterparts, chlorophototrophs.[6]
Retinalophototrophs achieve adequate energy conversion via a proton-motive force.[3][4] In retinalophototrophs, proton-motive force is generated from rhodopsin-like proteins, primarily bacteriorhodopsin and proteorhodopsin, acting as proton pumps along a cellular membrane.[1][4]
To capture photons needed for activating a protein pump, retinalophototrophs employ organic pigments known as carotenoids, namely beta-carotenoids.[7][3][4] Beta-carotenoids present in retinalophototrophs are unusual candidates for energy conversion, but they possess high Vitamin-A activity necessary for retinaldehyde, or retinal, formation.[7][3][4] Retinal, a chromophore molecule configured from Vitamin A, is formed when bonds between carotenoids are disrupted in a process called cleavage.[7][3][4] Due to its acute light sensitivity, retinal is ideal for activation of proton-motive force and imparts a unique purple coloration to retinalophototrophs.[1][4] Once retinal absorbs enough light, it isomerizes, thereby forcing a conformational (i.e., structural) change among the covalent bonds of the rhodopsin-like proteins.[1][3][4] Upon activation, these proteins mimic a gateway, allowing passage of ions to create an electrochemical gradient between the interior and exterior of the cellular membrane.[1][4] Ions diffusing outwards across the gradient through proton pumps are then bound to ATP synthase proteins on the cell's surface.[1][4] As they diffuse back into the cell, their protons catalyze the creation of ATP (from ADP and a phosphorus ion), providing energy for retinalophototrophic self-sustenance and proliferation.[1][4]
Interaction with carbon
Many, if not all, retinalophototrophs are photoheterotrophs: although sufficient ATP is produced by light, they cannot subsist on light and inorganic substances alone because they cannot produce needed organic materials from only CO2. This category includes retinalophototrophs that perform anaplerotic fixation, such as a flavobacterium that can use pyruvate and CO2 to make malate. This ability does, however, help "stretch" limited supplies of carbon.[8]