Anoxygenic photosynthesis: Difference between revisions
pared down some details about bacteriochlorophylls to the essence; removed some detail of opsin-type, as that's not photosynth; type I vs type II photosystems should be illustrated. Is that a helio vs the rest comparison? |
type II vs type I rxn centers |
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Bacterial anoxygenic photosynthesis is distinguished from the more familiar terrestrial plant [[photosynthesis|oxygenic photosynthesis]] by the nature of the terminal reductant (e.g. hydrogen sulfide rather than water) and in the byproduct generated (e.g. elemental sulfur instead of molecular oxygen). As its name implies, anoxygenic photosynthesis does not produce oxygen as a byproduct of the reaction. Additionally, all known organisms that carry out anoxygenic photosythesis are obligate anaerobes. Several groups of bacteria can conduct anoxygenic photosynthesis: [[green sulfur bacteria]], |
Bacterial anoxygenic photosynthesis is distinguished from the more familiar terrestrial plant [[photosynthesis|oxygenic photosynthesis]] by the nature of the terminal reductant (e.g. hydrogen sulfide rather than water) and in the byproduct generated (e.g. elemental sulfur instead of molecular oxygen). As its name implies, anoxygenic photosynthesis does not produce oxygen as a byproduct of the reaction. Additionally, all known organisms that carry out anoxygenic photosythesis are obligate anaerobes. Several groups of bacteria can conduct anoxygenic photosynthesis: [[green sulfur bacteria]] (GSB), red and green filamentous phototrophs (FAPs e.g. [[Chloroflexi]]), [[purple bacteria]], [[Acidobacteria]], and [[heliobacteria]].<ref>{{cite journal|author1=Donald A. Bryant |author2=Niels-Ulrik Frigaard |title=Prokaryotic photosynthesis and phototrophy illuminated |journal=Trends in Microbiology |volume=14 |issue=11 |date=November 2006 |pages=488-496 |issn=0966-842X |doi=10.1016/j.tim.2006.09.001 |
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|url=http://www.sciencedirect.com/science/article/pii/S0966842X06002265 |accessdate= 11 March 2016}}</ref><ref>Candidatus Chloracidobacterium thermophilum: An Aerobic Phototrophic Acidobacterium Donald A. Bryant, Amaya M. Garcia Costas, Julia A. Maresca, Aline Gomez Maqueo Chew, Christian G. Klatt, Mary M. Bateson, Luke J. Tallon, Jessica Hostetler, William C. Nelson, John F. Heidelberg, and David M. Ward Science 27 July 2007: 317 (5837), 523-526. {{DOI|10.1126/science.1143236}}</ref> |
|url=http://www.sciencedirect.com/science/article/pii/S0966842X06002265 |accessdate= 11 March 2016}}</ref><ref>''Candidatus'' Chloracidobacterium thermophilum: An Aerobic Phototrophic Acidobacterium Donald A. Bryant, Amaya M. Garcia Costas, Julia A. Maresca, Aline Gomez Maqueo Chew, Christian G. Klatt, Mary M. Bateson, Luke J. Tallon, Jessica Hostetler, William C. Nelson, John F. Heidelberg, and David M. Ward Science 27 July 2007: 317 (5837), 523-526. {{DOI|10.1126/science.1143236}}</ref> |
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The pigments used to carry out anaerobic photosynthesis are similar to [[chlorophyll]] but differ in molecular detail and peak wavelength of light absorbed. [[Bacteriochlorophyll]]s ''a'' through ''g'' absorb electromagnetic photons maximally in the near-[[infrared]] within their natural membrane milieu. This differs from chlorophyll a, the predominant plant pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum). |
The pigments used to carry out anaerobic photosynthesis are similar to [[chlorophyll]] but differ in molecular detail and peak wavelength of light absorbed. [[Bacteriochlorophyll]]s ''a'' through ''g'' absorb electromagnetic photons maximally in the near-[[infrared]] within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and [[cyanobacterial|cyanobacteria]] pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum). |
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Some archaea (e.g. ''Halobacterium'') capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin |
Some [[archaea]] (e.g. ''[[Halobacterium]]'') capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin capture light energy with the aid of [[diterpene]]s to move ions against the gradient and produce [[ATP]] via [[chemiosmosis]] in the manner of mitochondria. |
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There are two main types of anaerobic photosynthetic electron transport chains in bacteria |
There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers found in GSB, Chloracidobacterium, and Heliobacteria and the type II reaction centers found in [[Chloroflexi|FAPs]] and Purple Bacteria |
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'''Type I Reaction Centers''' |
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'''Purple non-sulfur bacteria''' |
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The electron transport chain of |
The electron transport chain of green sulfur bacteria — such as is present in model organism ''Chlorobaculum tepidum'' — uses the [[photosynthetic reaction centre|reaction centre]] bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD<sup>+</sup>. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by [[cytochrome]] c<sub>555</sub>{{Citation needed|date=November 2012}}. |
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'''Type II Reaction Centers''' |
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Although the type II reaction centers are structurally and sequentially analogous to [[Photosystem_II]] (PSII) in plant chloroplasts and cyanobacteria, known organisms that exhibit anoygenic photosynthesis do not have a region analogous to the [[oxygen-evolving complex]] of PSII. |
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The electron transport chain of green sulfur bacteria — such as is present in model organism ''Chlorobaculum tepidum'' — uses the [[photosynthetic reaction centre|reaction centre]] bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD<sup>+</sup>. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by [[cytochrome]] c<sub>555</sub>{{Citation needed|date=November 2012}}. |
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The electron transport chain of purple non-sulfur bacteria begins when the [[photosynthetic reaction centre|reaction centre]] bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to [[bacteriopheophytin]], which then passes it on to a series of [[electron carrier]]s down the [[Electron transport chain|electron chain]]. In the process, it will generate an electro-chemical gradient which can then be used to synthesize ATP by [[chemiosmosis]]. P870 has to to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecular hydrogen in the bacterial environment is the usual electron donor. |
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==References== |
==References== |
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Revision as of 18:33, 10 August 2016
Bacterial anoxygenic photosynthesis is distinguished from the more familiar terrestrial plant oxygenic photosynthesis by the nature of the terminal reductant (e.g. hydrogen sulfide rather than water) and in the byproduct generated (e.g. elemental sulfur instead of molecular oxygen). As its name implies, anoxygenic photosynthesis does not produce oxygen as a byproduct of the reaction. Additionally, all known organisms that carry out anoxygenic photosythesis are obligate anaerobes. Several groups of bacteria can conduct anoxygenic photosynthesis: green sulfur bacteria (GSB), red and green filamentous phototrophs (FAPs e.g. Chloroflexi), purple bacteria, Acidobacteria, and heliobacteria.[1][2]
The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls a through g absorb electromagnetic photons maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum).
Some archaea (e.g. Halobacterium) capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin capture light energy with the aid of diterpenes to move ions against the gradient and produce ATP via chemiosmosis in the manner of mitochondria.
There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers found in GSB, Chloracidobacterium, and Heliobacteria and the type II reaction centers found in FAPs and Purple Bacteria
Type I Reaction Centers
The electron transport chain of green sulfur bacteria — such as is present in model organism Chlorobaculum tepidum — uses the reaction centre bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD+. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by cytochrome c555[citation needed].
Type II Reaction Centers
Although the type II reaction centers are structurally and sequentially analogous to Photosystem_II (PSII) in plant chloroplasts and cyanobacteria, known organisms that exhibit anoygenic photosynthesis do not have a region analogous to the oxygen-evolving complex of PSII.
The electron transport chain of purple non-sulfur bacteria begins when the reaction centre bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate an electro-chemical gradient which can then be used to synthesize ATP by chemiosmosis. P870 has to to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecular hydrogen in the bacterial environment is the usual electron donor.
References
- ^ Donald A. Bryant; Niels-Ulrik Frigaard (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. ISSN 0966-842X. Retrieved 11 March 2016.
- ^ Candidatus Chloracidobacterium thermophilum: An Aerobic Phototrophic Acidobacterium Donald A. Bryant, Amaya M. Garcia Costas, Julia A. Maresca, Aline Gomez Maqueo Chew, Christian G. Klatt, Mary M. Bateson, Luke J. Tallon, Jessica Hostetler, William C. Nelson, John F. Heidelberg, and David M. Ward Science 27 July 2007: 317 (5837), 523-526. doi:10.1126/science.1143236