During photosynthesis at high light intensities dangerous oxygen radicals can form inside cells. Dinoflagellates have a unique light-harvesting complex (antenna) which can divert superfluous energy extremely efficiently to avoid this cell damage. In cooperation with colleagues in the USA and the Czech Republic, a team of biophysicists from the Ruhr-University Bochum around Prof. Eckhard Hofmann and Tim Schulte has now been able to determine which molecules in the antenna are of significance.
In the complex four carotenoid molecules cluster around a central chlorophyll molecule. The researchers were able to identify one specific carotenoid as a type of integrated lightning rod. It interacts with a “short-living” (nanosecond range, one millionth of a millisecond), energetically activated state of the chlorophyll and diverts the superfluous energy as soon as the chlorophyll passes into a “long-living” (microsecond range, a thousandth of a millisecond) energy state dangerous for the cell.
The scientists have published their findings in the current edition of the Proceedings of the National Academy of Science (PNAS).
During photosynthesis, plants and algae use biophysical and biochemical processes to convert light-energy into chemically stable forms of energy storage. Pigments bound in protein complexes are essential for light-harvesting.
Different pigments absorb different wavelengths of the natural light spectrum, leading to the different colours visible to the human eye.
Plants primarily use green chlorophyll for light absorption, but also contain carotinoids (yellow, orange or red) responsible for the wide spectrum of colouring of autumn foliage, or fruits such as the red tomato. In plants, carotinoids cannot only harvest light, but are also capable of quenching of superfluous light-energy that cannot be used during photosynthesis. They thus primarily have a protective function, preventing the organism from building toxic oxygen radicals when solar radiation is too high.
Dinoflagellates use the carotinoid peridinin as light-harvesting pigment
Dinoflagellates are an important part of marine plankton and live at a depth of about ten metres below sea level. A special attribute of dinoflagellates is that they use a carotinoid, namely peridinin, as light-harvesting pigment. These algae make use of the fact that peridinin absorbs light of exactly the wavelength that predominates at this specific level below the surface of the sea. The dinoflagellates produce a unique light-harvesting complex, the peridinin-chlorophyll-protein (PCP) for this purpose. This complex consists of one chlorophyll molecule per four peridinin molecules. The peridinins harvest the incoming light and transfer the energy extremely efficiently onto the internal chlorophyll molecule. Presumably the energy is then transferred from this chlorophyll molecule to other light-harvesting proteins and ultimately to the central photosystems where energy transformation and oxygen production take place.
Carotinoid notices excited state of chlorophyll
By targeted modification of the peridinin-chlorophyll-protein, scientists in Bochum -- working at an international level -- have now managed to identify a single peridinin molecule within the peridinin quartet, which interacts more strongly with the central chlorophyll. This peridinin molecule is located so close to the chlorophyll that it is aware of its excited energy state. This proximity also appears to be the prerequisite for preventing the development of a long-living excited state of the chlorophyll. The formation of such a long-living so-called triplet state leads to the development of toxic oxygen radicals that damage the cells.
International cooperation to clarify the relationship between the structure and function
The structural biological research performed in Bochum had to be combined with femtosecond (10 to the power ‑5 seconds) resolution absorption spectroscopy to be able to investigate this carotinoid-chlorophyll interaction. This was done in Connecticut, USA. Eckhard Hofmann praised this project as an outstanding example of international interdisciplinary cooperation. Based on the results of preparatory research work performed by Roger Hiller and his team from Australia, the biophysicists in Bochum were able to analyze the structure of the proteins investigated. The ultrafast spectroscopic measurements were carried out in extremely close cooperation with Harry Frank and Dariusz Niedzwiedzki's research team in Connecticut. The results were interpreted with the support of Robert Birge (Connecticut) and Tomás Polivka (University of South Bohemia, Czech Republic).
Biophysics and Protein Research Department
The infrastructure for the protein crystallography used in this project was developed by Prof. Eckhard Hofmann during the past few years, originally within the frameworks of the Protein Centre and currently in the Protein Research Department (Contact: Prof. Klaus Gerwert) which is part of the Department of Biophysics, Faculty of Biology and Biotechnology. The work is funded within the collaborative research center SFB480 "Molecular Biology of complex functions in botanical systems" (Contact: Prof. Ulrich Kück).
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