bicarbonate is acting as an acceptor for the protonsというのである。光合成中心IIでは確か高酸化状態のマンガン４クラスターが水分子のスプリットに寄与していましたよね。
Dual role of carbon dioxide in photosynthesis: Pioneering findings
Date:April 13, 2014
Source:Umeå universitet
Source:Umeå universitet
Summary:
Carbon dioxide, in its ionic form bicarbonate, has a regulating function in the splitting of water in photosynthesis, researchers have found.
This means that carbon dioxide has an additional role to being reduced to sugar.
The pioneering work opens up a new research field where researchers can investigate possible biological and ecological consequences of the dual role of carbon dioxide.
（sciencedaily.com/releases/2014/04/140413154053）
Researchers at Umeå University in Sweden have found that carbon dioxide, in its ionic form bicarbonate, has a regulating function in the splitting of water in photosynthesis.
This means that carbon dioxide has an additional role to being reduced to sugar.
The pioneering work is published in the latest issue of the Proceedings of the National Academy of Sciences.
It is well known that inorganic carbon in the form of carbon dioxide, CO2, is reduced in a light driven process known as photosynthesis to organic compounds in the chloroplasts.
Less well known is that inorganic carbon also affects the rate of the photosynthetic electron transport and thus the rate of photosynthetic oxygen production.
This result was first published by the Nobel Prize winner Otto Warburg and his collaborator in the late 50s.
Their explanation for the stimulating effect was logical at that time since they proposed that carbon dioxide was the source of the oxygen that plants produce. Their idea was proven to be incorrect many years later and instead we now know that water, H2O, is the source of oxygen in the atmosphere.
The observed stimulating effect by inorganic carbon on photosynthetic electron transport, has continued to cause an inflammatory debate among photosynthetic researchers around the world and resulted in hundreds of papers published on this issue.
"Our results will now put an end to this debate,"
says Johannes Messinger, Professor at the Department of Chemistry.
His research group has developed a very sensitive techniques based on "Membrane Inlet Mass Spectroscopy" that can be used to measure the production of gases in photosynthetic samples under analytically controlled conditions.
With this sensitive method they were able to test an earlier hypothesis that bicarbonate is acting as an acceptor for the protons that are produced when water is split in photosystem II.
If so, a light driven production of carbon dioxide in addition to oxygen should be detected.
To their delight, the scientists could detect relatively large amounts of carbon dioxide in the mass spectrometric experiments.
"Therefore, it seems as if two different carbon species, both derived from the carbonic acid cycle, have got the optimal chemical properties to be used as terminal electron acceptor (CO2) in the very end of the photosynthetic reaction and at the same time as proton acceptor (HCO3-) in the very beginning of the photosynthetic reaction,"
says Johannes Messinger.
The results open up a new research field where researchers can investigate possible biological and ecological consequences of the dual role of carbon dioxide.
The water-splitting reaction centre
John F. Allen & William Martin
Nature 445, 610-612(8 February 2007)
doi:10.1038/445610a
光合成４
（chm.bris.ac.uk/motm/oec/motm）
（chm.bris.ac.uk/motm/oec/motm）
The image (left), is a typical representation of the Joliot-Kok Cycle. This particular schematic is from an article published by Dr. James Barber, Imperial College of London, International Journal of Photoenergy 6:43-51 (2004). (Take the time to visit Dr. Barber's web site to see his Photosystem II images.
The 4-step Kok Cycle is represented schematically in the image at left. Four incoming photons are required to complete the process.
The OEC starts at State So, and then with each incoming photon moves on to Sate S1, then S2, S3 and finally S4. There are a total of five oxidation states of the OEC during this process.
As the OEC cycles through this process four electrons are removed from bound water (see mechanism below). The deposition of protons during this process varies but it is believed 2 protons are released during the final S3S4 transition.
The schematic at least depicts one proton released per photon absorbed. Yz is a nearby Tyrosine residue, and P680 the reaction center special chlorophyll dimer (as discussed).
Red, Orange, Yellow, Green, Blue, Indigo, and Violet
アメリカでは虹の7色を覚える時に Roy G. Bivロイジービブと言って覚えるという。
もちろん日本では赤橙黄緑青藍紫セキトウオウリョクセイランシですね。
アメリカでは虹の7色を覚える時に Roy G. Bivロイジービブと言って覚えるという。
もちろん日本では赤橙黄緑青藍紫セキトウオウリョクセイランシですね。