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Advances in structural basic research of cyanobacteria photosynthetic ring electron transfer
[ Instrument Network R & D ] On January 30th, the journal Nature Communications published an article in the form of Chang Wenrui / Li Mei Research Group, Zhang Xinzheng Research Group, and the Chinese Academy of Sciences Molecular Plant Science in the form of Article. Collaborative research result by Mi Hualing's research group of the Center for Excellence and Innovation / Institute of Plant Physiology and Ecology, entitled Structural basis for electron transport mechanism of complex I-like photosynthetic NAD (P) H dehydrogenase (DOI: 10.1038 / s41467-020-14456- 0). In this work, a single particle cryo-electron microscopy method was used to analyze a multi-subunit membrane protein complex NDH-1L derived from thermophilic cyanobacteria T. elongatus BP-1 and involved in photosynthetic cyclic electron transfer, and its combined electron donor. The three-dimensional structure of ferredoxin (Fd).
In the redox reaction, there are aerobic transfer, hydrogen transfer, and electron transfer. The same applies to the redox reaction of living organisms. In the case of oxygenase, it is the transfer of oxygen, but the transfer of hydrogen is considered to be the transfer of electrons and hydrogen ions, which is not fundamentally different from the transfer of electrons.
The electron transfer of photosynthetic organisms can be divided into two types: linear electron transfer and ring electron transfer. Linear electron transfer produces NADPH and ATP for use in the photosynthetic Calvin cycle and other cellular metabolic responses. However, the ratio of ATP and NADPH generated in the process of linear electron transfer is not sufficient to meet the ratio of ATP and NADPH consumed by the Calvin cycle, so there is still ring-shaped electron transfer in photosynthetic organisms. During this process, electrons circulate between PSI and the cytochrome b6f (Cytb6f) complex through plastoquinone (PQ), producing only ATP and not NADPH. Ring electron transfer has very important physiological significance. It can increase the ATP / NADPH ratio, thereby improving the efficiency of the Calvin cycle. In addition, ring electron transfer also plays an important role in light protection and helps photosynthetic organisms respond to the environment quickly. Variety.
For dehydrogenases that use pyridine nucleotides as coenzymes, there is only one hydrogen atom moving from the substrate, and the others are passed to pyridine coenzymes as electrons + H +. In the respiration, molecular oxygen is transferred through the cytochrome system and combined with hydrogen to generate water. The redox between cytochromes undergoes electron transfer with the divalent and trivalent changes of heme. Generally, the oxidation of the substrate is carried out according to the polyvalent acid electron transfer from the substrate to oxygen, forming a complete respiratory chain.
A sequence of redox factors accompanied by energy conversion. Certain reduction products of metabolism (such as NADH2, succinic acid formed in the tricarboxylic acid cycle) can serve as electron donors, passing this chain to transfer electrons to molecular oxygen (forming water). Electron transfer along the chain is thought to form an energy-giving state and entity (unclear in nature) that can be used in many energy-demanding processes. In eukaryotic systems, the electron transport chain exists on the inner membrane of the mitochondria, and is basically composed of cytochromes, ubiquinone, sulfur-containing non-heme ferritin, two flavin proteins (NADH2-dehydrogenase and succinate dehydrogenase). Catalase) composition. In the bacterial system, the composition of the electron transfer chain of different types of bacteria is very different, even the same species growing under different conditions are not the same. The transfer chain is limited to the cell membrane, including a series of carriers, such as cytochrome a , B, c and d, ubiquinone and / or naphthoquinone type quinones. In some cell electron transport chains, there are a large number of dehydrogenases that are bound to the membrane and directly supply electrons to the transport chains.
In cyanobacteria, a type I NAD (P) H dehydrogenase complex (NDH-1L) is essential for cyclic electron transfer. Studies have shown that ferredoxin (Fd) accepts PSI-transmitted electrons and further transfers them to the NDH-1L complex. PQ molecules bound to NDH-1L transfer electrons to Cytb6f. At the same time, NDH-1L Protons are pumped from the cytoplasmic side to the thylakoid cavity side, forming a transmembrane proton gradient to drive ATP generation. The NDH-1L complex of cyanobacteria contains 19 subunits, the structure of which has been reported by two research groups in the United States and the United Kingdom in 2019, but the reported structure lacks an important subunit, NdhV, which is closely related to its function. In addition, the specific positions and interaction information of PQ molecules and electron donor Fd binding to NDH-1L are unclear. The study of the three-dimensional structure of NDH-1L and its Fd / PQ complexes will help reveal the mechanism of ring electron transfer in photosynthesis.
The joint team of the Institute of Biophysics and the Center for Molecular Plant Excellence, through close cooperation, analyzed the two NDH-1L structures derived from the thermophilic cyanobacterium T. elongatus BP-1 using single-particle cryo-electron microscopy, which were combined with endogenous PQ The molecular (NDH-PQ) and electron donor Fd protein (NDH-Fd) have a resolution of 3 angstroms and 3.2 angstroms, respectively. The NDH-PQ structure reveals the binding sites of PQ molecules, an important component of the cyclic electron transport chain; the NDH-Fd structure contains all 19 protein subunits of the NDH-1L complex, revealing the electron donor Fd and NDH-1L Complex interaction details. The results provided for the first time a complete structural model of NDH-1L, revealed the binding position of PQ molecules, constructed the electron transfer pathway from Fd to PQ, and measured the activity of NDH-1L under different pH conditions and its relationship with Fd Affinity. This work provides an important molecular basis for revealing the ring-shaped electron transfer and proton pump coupling mechanism involved in NDH-1L.
Source: Encyclopedia, Institute of Biophysics