When the resolution is raised from 3 Å to 2.3 Å, a magical world emerges: an unprecedented 5',5'-phosphodiester bond is directly observed, weaving adjacent sugar chains into a sugar network! You don't know until you "see", the world is really amazing! However, what has been reported so far is only a superficial observation, and a large number of biological problems behind it are waiting to be "peeled" bit by bit.
Carbohydrates are the basic substances of life, and Glycans, including polysaccharides, are essential for cellular activities. Although glycans have important basic biological significance, their three-dimensional structural information is largely missing. So far, the high-resolution structure of glycans is limited to relatively simple sugar ligands bound to proteins or a few sugar rings connected to amino acids on protein modifications. This lack of structural information has seriously hindered our understanding of the mechanisms of a wide range of physiological and pathological functions in which glycans participate. Considering that glycans are the most abundant life molecules on Earth, the analysis of their advanced structures is crucial to deciphering the folding principles of glycans and Glycoconjugates, and may help multiple research fields including materials science.
Previously, Nieng Yan and her collaborators reported the high-resolution structure of the mastigoneme complex of Chlamydomonas reinhardtii in the journal Cell. The researchers discovered and identified the previously unknown core component of mastigoneme and named it "Mastigoneme-specific axial protein (Mstax)", revealing for the first time the key role of Arabinan in the advanced assembly of biological structures, thus providing important clues for understanding the role of structural polysaccharide molecules in life processes, highlighting the role transformation of modern structural biology from a relatively downstream mechanistic analysis tool for biological research to a source of innovative discovery. This study also suggests that choosing a suitable biological system can achieve unexpected breakthroughs in the structural biology of glycans.
The "Lotus Sugar Moonlight" project, which was carried out at the same time as this work, further used cryo-EM as an observation tool to discover completely unknown biomacromolecules through the research strategy of "CryoSeek". Combining cryo-EM analysis, AI-assisted automatic modeling and bioinformatics analysis, Nieng Yan's team reported the structures and potential functions of a variety of new fibrin and Glycoproteins from the lotus pond of Tsinghua University this year. Revealed the mechanism of a new highly reproducible and highly ordered assembly of glycans.
On December 25, 2024, Nieng Yan and her team of collaborators published their latest results on the preprint website BioRxiv. The title of the paper is "High-resolution mastigoneme structure reveals 5',5'-phosphodiesters stabilized glycan folding". The resolution of up to 2.3 Å (0.23 nm) not only allows researchers to accurately determine the type and stereochemical information of glycans, but also reveals a 5', 5'-phosphodiester bonds that has never been found in nature before, which covalently fixes adjacent sugar chains, thereby weaving sugar chains modified on different amino acids into a "sugar network". Structural Analysis also revealed the secondary structural unit of the glycosylation complex and named it poly-hydroxyproline (pHP) glycohelix.
Fig. 1 Stabilization of the glycan arrays in Mstax through unprecedented 5’,5’-phosphodiester bonds. (Huang, et al., 2024)
The wider species distribution of 5',5'-phosphodiester bonds and their physiological functions beyond structural effects remain to be studied. More importantly, the enzyme that catalyzes the formation of this chemical bond has not yet been identified. The unprecedented 5',5'-phosphodiester bonds discovered based on structural information suggests that the complexity of the advanced assembly mode of glycans may be beyond our imagination.
This study defines a new basic secondary structural unit of carbohydrates, marking an important step in the systematic study of the structures of carbohydrates and glycoconjugates, and laying an important foundation for deciphering the carbohydrate folding code.
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