| β-(1→3)-glucans widely exsists in nature and has been studied in rencent years because of their unique physicochemical properties and biological activities.β-(1→3)-glucans can be captured as a pathogen-related pattern by pattern recognition receptors on the surface of immune cells,activating signaling pathways in the immune system,and can achive the function of immunomodulatory,anti-tumor,anti-coagulant,anti-inflammatory,and infection-resisting activities.Dectin-1,as the main pattern recognition receptor,plays a crucial role in the specific recognition ofβ-(1→3)-glucans.However,due to the chain flexibility of β-(1→3)-glucans as well as the presence of active hydroxyl groups and hydrophobic pyranose rings on the chains,β-(1→3)-glucans have complex conformations in aqueous solution,where multiple conformations exists.This complex conformational state makes it difficult to characterize the conformational properties in detail using traditional experimental methods,thus hindering the usnderstanding of conformantional details ofβ-(1→3)-glucans.It leads to the lack of specific structural information for the recognition of β-(1→3)-glucans and Dectin-1,and making it impossible for people to explain the microscopic mechanisms of the biological activities.With the development of computer and computational science,molecular dynamic simulation method is very suitable for the study of the structure and dynamics of glycans and sugar complexes.However,in terms of studying the conformational properties ofβ-(1→3)-glucan in aqueous solution and the recognition mechanism betweenβ-(1→3)-glucan and Dectin-1,there has been no efficient simulation method platform.Therefore,in this dissertation,we take the linear β-(1→3)-glucans as our research object and use molecular dynamic simulation method to systematically study the conformational properties of the linear β-(1→3)-glucans and the recognition mechanism between the linear β-(1→3)-glucans and Dectin-1.The specific research contents are as follows:1.The conformational properties of the single-stranded linear β-(1→3)-glucans in aqueous solution:In order to study the effect of aqueous solvent on the chain conformations,this work constructed and compared results of explicit and implicit solvent molecular dynamic simulations,taking the single-stranded state of linearβ-(1→3)-glucans as the research object.The results show that right-handed 6/1 helix structure is thermodynamically the most stable conformation in solution.The formation of single-stranded helical conformation is not only related to the glycosidic bonds and hydrogen bonding interaction of the chain itself,but also strongly depends on the interaction with surrounding water molecules.This work also explored the effects of temperature and chain length,and found that temperature increasing would reduce the proportion of single-stranded helical conformation,which is rooted in the fact that the increased temperature can increase the mobility of water molecules,thereby reducing the hydrogen bond between single-stranded linear β-(1→3)-glucan and water molecules.When the chain length increases,the the proportion of single-stranded helical conformation is also decreased because of the change in chain flexibility.This study helps people to further clarify the conformational behavior of single-stranded β-(1→3)-glucan,and reveals the important role of aqueous solvent in the conformation formation.2.The conformational properties of the multi-stranded linear β-(1→3)-glucans in aqueous solution:Current researches focus more on the stable triple helix,while the kinetic pathway of triple helix formation is not well described.Therefore,in this work,a variety of multi-stranded initial models were constructed,and the kinetic path of β-(1→3)-glucans triple helix formation was found and proposed:linearβ-(1→3)-glucans preferentially form a double-helix conformation in solution,and then the free single-chain binds to the double-helix,finally forming a right-handed triple-helix thermodynamically stable conformation.It is found that the right-handed triple helix is well maintained and stabilized by an inter-strand hydrogen bonding network of the C2 hydroxyls.The inter-strand hydrogen bonds exhibit a left-handed double helix pattern with preferred global orientations.This work also explored the effects of different temperatures and chain lengths,and discovered that the formation of triple helix is temperature sensitive,but the already formed triple helix is not.Linear β-(1→3)-glucans with 6 sugar units may be the critical chain length for the formation of triple-helical conformation.This study helps people to deeply understand the higher-order conformational properties of linear β-(1→3)-glucans in aqueous solution,and provides a solid theoretical basis for the study of its structure-function relationship.3.The recognition mechanism of the linear β-(1→3)-glucans and Dectin-1:Current experimental studies have found that the pattern recognition receptor Dectin-1 can capture linear β-(1→3)-glucans to trigger the corresponding immune function,but it is difficult to analyze the specific interaction and structural information of the recognition process and the dependence on the conformation and chain length of the ligand.In this work,the recognition model of Dectin-1 and the linearβ-(1→3)-glucans with varius conformations and different chain lengths was carefully designed using a combination of molecular dynamic simulation and docking technology.This work clarified the recognition mechanism between the linearβ-(1→3)-glucans and Dectin-1:the binding structure exhibits apparent endo-type recognition between the C-type lectin-like domain(CTLD)groove formed by Trp221,His223,Tyr228,as well as other residues around them,and the conformational patterns of triple-helix bglc.This recognition mode shows a clear preference on the relative direction of the triple-helix bglc with respect to the CTLD groove.Trp221,His223,and Tyr228 play an important role in stabilizing the recognition complex through forming a simple but fixed hydrogen bond network with the C6 and C4 hydroxyls.Moreover,this recognition mode is not influenced by chain length,except when reaching the lower limit that may destabilize triple-helix formation.It was further found and proposed that the biological activity of single-helix and double-helix structure is inferior to triple-helix structure because the increase of chain flexibility leads to the instability of the surface-ordered docking recognition structure.This work provides a theoretical basis for in-depth exploration of the biological function of β-(1→3)-glucan.Taking the structure and function of linear β-(1→3)-glucan as the basic framework,this dissertation summarizes and develops a set of computer simulation research methods for the structure and function of glycans.In this paper,a complete physical image of the conformation of linear β-(1→3)-glucan in aqueous solution within a certain chain length range is displayed,and the recognition pattern and structural information between linear β-(1→3)-glucan and Dectin-1 are clarified.This study helps us to deeply understand the microscopic mechanism of the functions and the structure-function relationship of the linear β-(1→3)-glucans,and provides a theoretical basis for exploring their potential biological application value. |
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