Bacteriorhodopsin

Target Synonyms and Classification: Bacteriorhodopsin (BR) belongs to the Ion-translocating Microbial Rhodopsin Family (MRF, TCDB: 3.E.1), which catalyzes light-driven ion translocation across microbial cytoplasmic membranes or serve as light receptors.

Function: BR provides a major source of energy: Using light energy it pumps protons across the membrane out of the cell and generates a proton gradient, which in turn is used to generate chemical energy in form of ATP through the action of the ATP-Synthase. Upon light absorption (λ_max = 570 nm), the covalently bound cofactor Retinal undergoes a series of conformational transitions, which are coupled to the translocation of one proton. Proton translocation involves binding, intramolecular transfer, and release of a proton which requires – besides retinal – different amino acids. The whole photocycle takes about 15 ms.

Organism and Localization: Bacteriorhodopsin is used by Archaea, most notably by Haloarchaea like Halobacterium salinarum, an extremophile that occupies hypersaline lakes. BR forms two-dimensional crystalline patches known as the purple membrane, which can occupy up to 50 % of the surface area of the archaeal cell.

Substrates and Inhibitors: Proton translocation by BR is inhibited by blue light, which is known as the “blue light effect”. Blue light enables a shortcut within the photocycle which leads to uncoupling of light adsorption from H⁺ translocation. BR can be inactivated by mutation of the proton donor D96 to asparagine. The mutant can be reactivated using sodium azide, which essentially functions as a mobile proton donor.

Related Transporters: All Rhodopsins share the properties of a covalently bound retinal coupling light adsorption with ion translocation. There are several bacterial sensory Rhodopsins, such as the Cl^- pumping Halorhodopsin, the Na⁺ pumping Rhodopsin KR2, Proteorhodopsin, which acts very similar to BR and Channelrhodopsin, which has a channel-like mechanism. Beside energy conversion they play key roles in phototaxis. Rhodopsins are also found in vertebrates, where they belong to the class of G protein-coupled receptors. Here, they are commonly found in light-sensing organs and responsible for the eyesight in humans.