# Bacteriorhodopsin
**Bacteriorhodopsin** (bR) is a light-driven proton pump found in the membrane of the halophilic archaeon *Halobacterium salinarum*, in which a buried all-trans retinal chromophore is photoisomerized to a 13-cis conformation to transport protons across the membrane against a proton concentration gradient.[1](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327) It was isolated in 1971 by [Dieter Oesterhelt](https://www.edgepedia.org/dieter-oesterhelt) and [Walther Stoeckenius](https://www.edgepedia.org/walther-stoeckenius) from the purple membrane of *H. salinarum* (then classified as *Halobacterium halobium*), and it is the first recognized and most widely studied microbial rhodopsin.[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327)
| Key fact | Detail |
|---|---|
| Function | Outward-directed light-driven proton pump; the resulting proton gradient drives ATP synthesis and transport reactions in the archaeon[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) • [3](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/bacteriorhodopsin) |
| Discovery | Isolated from *Halobacterium salinarum* in 1971 by Oesterhelt and Stoeckenius[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) |
| Structure | Single amino-acid chain with seven transmembrane α-helices binding all-trans retinal[4](https://academic.oup.com/femsre/article/35/6/1082/523392) • [5](https://pubmed.ncbi.nlm.nih.gov/10212980/) |
| Purple membrane | bR is the only protein in this specialized membrane and is responsible for its purple color[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) |
| Absorption | Broad light-absorption maximum at about 570 nm (reported as 568 nm in retinal-pigment studies)[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) • [7](https://arxiv.org/abs/1810.05150) |
| Photocycle | Photoisomerization of retinal from all-trans to 13-cis initiates directional proton transport[1](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327) |
| Structural methods | Electron microscopy (Henderson and Unwin, 1975), X-ray crystallography, and time-resolved serial femtosecond crystallography at X-ray free electron lasers[4](https://academic.oup.com/femsre/article/35/6/1082/523392) • [1](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327) |
## Structure and photochemistry
Bacteriorhodopsin is a seven-transmembrane helical protein containing all-trans retinal, the same light-sensitive molecule used in vertebrate vision, attached to the protein as a protonated [Schiff base](https://www.edgepedia.org/schiff-base).[5](https://pubmed.ncbi.nlm.nih.gov/10212980/) Microbial rhodopsins of this kind form a single amino-acid chain with a characteristic fold of seven transmembrane α-helices, and they act either as light-dependent ion transporters or as photoreceptors for signal transduction.[4](https://academic.oup.com/femsre/article/35/6/1082/523392) The protein absorbs light broadly around 570 nm, giving the [purple membrane](https://www.edgepedia.org/purple-membrane) its color.[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570)
When retinal absorbs a photon, it changes conformation from all-trans to 13-cis, and this change propagates to the surrounding protein to drive the release of protons from the cytoplasm to the extracellular space against an electrochemical gradient.[1](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327) • [6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) The full sequence of structural states between absorption and return to the ground state is called the photocycle. [Fourier transform](https://www.edgepedia.org/fourier-transform) infrared and [Raman spectroscopy](https://www.edgepedia.org/raman-spectroscopy), solid-state NMR, and [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography) have all contributed detailed information about the structural changes in this proton transport cycle.[8](https://www.annualreviews.org/content/journals/10.1146/annurev.physiol.66.032102.150049)
## Role in the cell
*H. salinarum* lives in environments with extreme salt concentrations, and bacteriorhodopsin provides it a light-based energy source.[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) The pump creates an electrochemical gradient of ions, which ATP synthase, a second protein embedded in the membrane, then turns into chemical energy.[3](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/bacteriorhodopsin) Soon after the protein's discovery, it was proposed that the proton gradient built by bR drives ATP synthesis and transport reactions in the archaeon.[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327)
## History of study
The discovery grew out of electron microscopy. In the late 1960s Walther Stoeckenius studied the membranes of *Halobacterium* to test a proposed "subunit membrane" structure, and in 1971 he and Oesterhelt identified the retinylidene proton pump in the purple membrane and named it bacteriorhodopsin.[9](https://www.sciencedirect.com/science/article/pii/S0005272813001436) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC5747503/) In 1973 the purple membrane was shown to function as a light-dependent proton exporter.[4](https://academic.oup.com/femsre/article/35/6/1082/523392)
**A model membrane protein.** Richard Henderson and [Nigel Unwin](https://www.edgepedia.org/nigel-unwin), at the Medical Research Council Laboratory of Molecular Biology in Cambridge, established a three-dimensional structural model of the protein by electron microscopy in 1975, depicting its α-helices as columns perpendicular to the membrane plane; their paper became the most cited bacteriorhodopsin work of the 1970s.[4](https://academic.oup.com/femsre/article/35/6/1082/523392) The purple membrane itself made the protein unusually tractable: it could be prepared easily in large amounts, contained only one protein in a crystalline arrangement, and resisted electron radiation, detergents, and removal from its native environment better than many other membrane proteins.[4](https://academic.oup.com/femsre/article/35/6/1082/523392) Notably, the field developed for its first decade without genetic techniques, working from phenotypic traits of carefully bred strains; genetic methods entered only in the 1980s, when [H. Gobind Khorana](https://www.edgepedia.org/h-gobind-khorana)'s group at MIT identified the gene coding for bacteriorhodopsin.[4](https://academic.oup.com/femsre/article/35/6/1082/523392)
**Structural resolution.** By around 2000, models at up to 2.5 Å resolution had been published, and bacteriorhodopsin became the first integral membrane protein to have its catalytic mechanism structurally characterized in detail.[11](https://www.annualreviews.org/content/journals/10.1146/annurev.biophys.28.1.367) • [12](https://www.sciencedirect.com/science/article/pii/S0005273602005667) More recently, X-ray free electron lasers (XFELs) have allowed structural dynamics to be probed at room temperature on timescales from femtoseconds to milliseconds using [time-resolved serial femtosecond crystallography](https://www.edgepedia.org/time-resolved-serial-femtosecond-crystallography) (TR-SFX), revealing new details of retinal photoisomerization and proton pumping.[1](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327)
## The microbial rhodopsin family
Bacteriorhodopsin was the first of the microbial (type 1) rhodopsins to be identified, and it remains by far the most studied.[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) The inward-directed chloride pump halorhodopsin and the signaling proteins sensory rhodopsin I and II were later identified in the same organism, and microbial rhodopsins have since been detected in bacteria, archaea, and eukaryotic microorganisms.[2](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327) • [4](https://academic.oup.com/femsre/article/35/6/1082/523392) Microbial rhodopsins and animal rhodopsins share structural and mechanistic similarities but show no sequence homology, and they are designated type 1 and type 2 rhodopsins respectively.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC5747503/)
The family's practical importance extends to optogenetics, in which genetically targeted microbial rhodopsins allow precise light control of transmembrane ion currents and neuronal activity; the bacteriorhodopsins, halorhodopsins, and channelrhodopsins are the three major branches used for this purpose.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC5747503/) • [13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4790845/)
## Applications
The protein's stability, which is attributed largely to its two-dimensional crystalline arrangement in the purple membrane and to the extreme salt and temperature conditions of its native environment, has made it attractive for biotechnology.[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) It can be formed into thin films, polymers, and gels, and its large absorption cross-section, high quantum efficiency, and optical nonlinearities support a range of light-based uses.[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) Genetic variants with enhanced photochromic and photoelectric properties have been applied in solar-energy harvesting, security inks, thin-film memories, reversible holographic storage, photovoltaic converters, artificial retinas, picosecond photodetectors, and optical computing.[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) Device performance depends on the photocycle: the wild-type proton-pumping cycle occurs on a millisecond timescale in the device context reviewed, which limits the efficiency of photovoltaic cells built from it.[6](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570) • [14](https://pmc.ncbi.nlm.nih.gov/articles/PMC8664958/)
## References
1. [Wickstrand C, Nogly P, Nango E, Iwata S, Standfuss J, Neutze R (2019). Bacteriorhodopsin: Structural Insights Revealed Using X-Ray Lasers and Synchrotron Radiation. Annual Review of Biochemistry 88:59-83.](https://www.annualreviews.org/content/journals/10.1146/annurev-biochem-013118-111327)
2. [Bacteriorhodopsin: Structural Insights (Annual Reviews PDF excerpt on discovery and purple membrane)](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-013118-111327)
3. [Bacteriorhodopsin - ScienceDirect Topics overview](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/bacteriorhodopsin)
4. [Enlightening the life sciences: the history of halobacterial and microbial rhodopsin research. FEMS Microbiology Reviews.](https://academic.oup.com/femsre/article/35/6/1082/523392)
5. [Bacteriorhodopsin (review abstract), PubMed 10212980.](https://pubmed.ncbi.nlm.nih.gov/10212980/)
6. [Recent advances in bacteriorhodopsin-based energy harvesters and sensing devices. Nano Energy.](https://www.sciencedirect.com/science/article/abs/pii/S2211285520310570)
7. [Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures. arXiv:1810.05150.](https://arxiv.org/abs/1810.05150)
8. [Bacteriorhodopsin (Annual Review of Physiology 2004).](https://www.annualreviews.org/content/journals/10.1146/annurev.physiol.66.032102.150049)
9. [Of ion pumps, sensors and channels - Perspectives on microbial rhodopsins between science and history. Biochimica et Biophysica Acta.](https://www.sciencedirect.com/science/article/pii/S0005272813001436)
10. [Govorunova EG, Sineshchekov OA, Li H, Spudich JL (2017). Microbial Rhodopsins: Diversity, Mechanisms, and Optogenetic Applications. Annual Review of Biochemistry 86:845-872 (PMC version).](https://pmc.ncbi.nlm.nih.gov/articles/PMC5747503/)
11. [Haupts U, Tittor J, Oesterhelt D (1999). Closing in on bacteriorhodopsin: Progress in Understanding the Molecule. Annual Review of Biophysics 28:367-99.](https://www.annualreviews.org/content/journals/10.1146/annurev.biophys.28.1.367)
12. [Neutze R et al. (2002). Bacteriorhodopsin: a high-resolution structural view of vectorial proton transport. Biochimica et Biophysica Acta.](https://www.sciencedirect.com/science/article/pii/S0005273602005667)
13. [Optogenetics: 10 years of microbial opsins in neuroscience (PMC).](https://pmc.ncbi.nlm.nih.gov/articles/PMC4790845/)
14. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC8664958/)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Archaea › Extremophilic archaea › Extremozymes and archaeal biotechnology › Industrial and environmental applications › Haloarchaeal commercial products*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license