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  • Title: First crystal structure of an endo-levanase - the BT1760 from a human gut commensal Bacteroides thetaiotaomicron.
    Author: Ernits K, Eek P, Lukk T, Visnapuu T, Alamäe T.
    Journal: Sci Rep; 2019 Jun 11; 9(1):8443. PubMed ID: 31186460.
    Abstract:
    The endo-levanase BT1760 of a human gut commensal Bacteroides thetaiotaomicron randomly cuts a β-2,6-linked fructan, levan, into fructo-oligosaccharides providing a prebiotic substrate for gut microbiota. Here we introduce the crystal structure of BT1760 at resolution of 1.65 Å. The fold of the enzyme is typical for GH32 family proteins: a catalytic N-terminal five-bladed β-propeller connected with a C-terminal β-sandwich domain. The levantetraose-bound structure of catalytically inactive mutant E221A at 1.90-Å resolution reveals differences in substrate binding between the endo-acting fructanases. A shallow substrate-binding pocket of the endo-inulinase INU2 of Aspergillus ficuum binds at least three fructose residues at its flat bottom. In the levantetraose-soaked crystal of the endo-levanase E221A mutant the ligand was bent into the pond-like substrate pocket with its fructose residues making contacts at -3, -2, -1 and + 1 subsites residing at several pocket depths. Binding of levantetraose to the β-sandwich domain was not detected. The N- and C-terminal modules of BT1760 did not bind levan if expressed separately, the catalytic domain lost its activity and both modules tended to precipitate. We gather that endo-levanase BT1760 requires both domains for correct folding, solubility and stability of the protein.
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