Structure and function of photosynthesis protein explained in detail
Photosynthetic complex is a key element in photosynthetic electron transport, but little has been known about it so far.
An international team of researchers has solved the structure and elucidated the function of photosynthetic complex I. This membrane protein complex plays a major role in dynamically rewiring photosynthesis. The team of 14 researchers, including Dr. James Birrell and Prof. Wolfgang Lubitz from MPI CEC in Mülheim/Ruhr and Dr. Nicholas Cox from the Australian National University (ANU) in Canberra, report the work in the journal Science, published online on 20 December 2018.
Biology’s electrical circuits
Complex I, is found in most living organisms. In plant cells it is used in two places: one is in mitochondria, the cell’s power plants, the other is in chloroplasts, where photosynthesis occurs. In both instances, it forms part of an electron transport chain, which can be thought of as biology’s electrical circuit. These are used to drive the cells molecular machines responsible for energy production and storage. The structure and function of mitochondrial complex I as part of cellular respiration has been well investigated, whereas photosynthetic complex I has been little studied so far.
Using cryoelectron microscopy, an method for imaging minute cell structures, the researchers were able to solve for the first time the molecular structure of photosynthetic complex I. They showed that it differs considerably from its respiratory relative. In particular, the part responsible for electron transport has a different structure, since it is optimised for cyclic electron transport in photosynthesis. Additional insight was provided by the researchers in MPI CEC and ANU by studying the parts of complex I that transport electrons, the so-called iron-sulfur-clusters. "Iron-sulfur-clusters are nature’s tools for building electrical circuits" says Professor Wolfgang Lubitz, head of the team in MPI CEC. "In complex I they transport electrons from one side of the enzyme to the other. In photosynthetic complex I, this wire is significantly shorter than in mitochondrial complex I, which is crucial for its function."
Cyclic electron transport represents a molecular short circuit in which electrons are reinjected into the photosynthetic electron transport chain instead of being stored. The research team simulated the process in a test tube and showed that the iron- and sulphur-containing protein ferredoxin plays a major role. Using spectroscopic methods, the scientists also demonstrated that the electron transport between ferredoxin and complex I is highly efficient.
Molecular fishing rod
In the next step, the group analysed at the molecular level which structural elements are responsible for the efficient interaction of complex I and ferredoxin. Further spectroscopic measurements showed that complex I has a particularly flexible part in its structure, which captures the protein ferredoxin like a fishing rod. This allows ferredoxin to reach the optimal binding position for electron transfer.
"This enabled us to bring the structure together with the function of the photosynthetic complex I and gain a detailed insight into the molecular basis of electron transport processes," summarises Professor Marc Nowaczyk, the lead author of the study. "In the future, we plan to use this knowledge to create artificial electron transport chains that will enable new applications in the field of synthetic biology.”
Max Planck Institute for Biochemistry, Max Planck Institute for Chemical Energy Conversion, Yokohama City University, Australian National University, Ludwig-Maximilians-Universität München, Université Paris-Saclay, Osaka University, Ruhr-Universität Bochum
The German Research Foundation supported the study within the framework of the Cluster of Excellence Resolv (EXC 1069), the research group FOR2092 (EN 1194/1-1 and NO 836/3-2), the priority programme 2002 (NO 836/4-1) and the project NO 836/1-1. Further funding came from the Max Planck Society, the Australian Research Council (FT140100834, N.C.), JST-CREST (JPMJCR13M4, G.K.), MEXT-KAKENHI (16H06560, G.K.), the French Infrastructure for Integrated Structural Biology (FRISBI ANR-10-INSB-05) and the International Joint Research Promotion Program of Osaka University.