Researchers bring new information about photosy

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Photosynthesis is the most important basis of life on Earth. In it, single-celled plants and algae use energy from the sun and convert that energy into sugar and biomass. In this process, oxygen is released. Plant biotechnologists and structural biologists from the universities of Münster (Germany) and Stockholm (Sweden) have clarified the structure of a new protein complex that catalyzes energy conversion processes in photosynthesis. This protein complex is photosystem I, known as the single (monomer) protein complex in plants. The team of researchers led by Prof. Michael Hippler from the University of Münster and Prof. Alexey Amunts from the University of Stockholm have now shown, for the first time, that two photosystem I monomers in plants can join in a dimer, and they describe the molecular structure of this new type of molecular machine. The results, which have been published in the journal ‘Nature Plants’, provide molecular information about the process of photosynthesis with a degree of precision never before seen. They could help to use the reducing force (i.e. the ability to give up electrons) of photosystem I more efficiently in the future, for example to produce hydrogen as an energy source.

The background: There are two photosynthesis complexes, called photosystems I and II, which work best with light of different wavelengths. The absorption of light energy in photosystems I and II allows electrons to be transported within the molecular “photosynthetic machine”, thus causing the conversion of light energy into chemical energy. In the process, electrons from photosystem I are passed to the protein ferredoxin. In green algae, ferredoxin can pass electrons from photosynthesis to an enzyme called hydrogenase, which then produces molecular hydrogen. This molecular hydrogen is thus produced by the supply of light energy, that is to say that it is produced in a renewable way and could be a future source of energy. The researchers asked themselves the question: “How is the production of photosynthetic hydrogen related to the structural dynamics of the monomer and dimer photosystem I?

The results in detail

The homodimeric photosystem I of green algae Chlamydomonas reinhardtii consists of 40 protein subunits with 118 transmembrane helices providing structure for 568 photosynthesis pigments. Using cryogenic electron microscopy, the researchers showed that the absence of subunits with the designations PsaH and Lhca2 leads to head-to-head orientation of monomer photosystem I (PSI) and its associated light-harvesting proteins. (LHCI). The light-harvesting protein Lhca9 is the key element ensuring this dimerization.

In the study, researchers define the most accurate PSI-LHCI model available at 2.3 Ångström resolution (one Ångström is one ten-millionth of a millimeter), including the flexible bond electron emitter plastocyanin , and they assign the correct identity and orientation to all pigments, as well as to 621 water molecules that influence energy transmission pathways. Related to the loss of a second gene (pgr5), the genetically induced downregulation of the Lhca2 subunit results in the highly efficient production of hydrogen in the double mutant. As Michael Hippler puts it, “Lhca2 depletion promotes PSI dimer formation, and we therefore suggest that hydrogenase may promote photosynthetic electron targeting of the PSI dimer, as we have proposed in our previous work. structure of the PSI dimer allows us to make targeted genetic modifications in order to test the hypothesis of a better production of hydrogen thanks to the PSI dimer.


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