Structural Insights into Fun174Sb from the GH174 Family Unravel Novel Subsite Specificities of the Endo-1,3-Fucanase

Sulfated fucan has attracted growing attention due to its diverse biological properties. Endo-1,3-fucanases are valuable tools for the degradation of sulfated fucan. This study characterized an endo-1,3-fucanase Fun174Sb from the GH174 family, utilizing a combination of protein crystallography, mutagenesis, computational biology, and nuclear magnetic resonance techniques. The complex structure of Fun174Sb revealed that the enzyme primarily relies on hydrophobic and electrostatic interactions to bind the ligand. Mutations showed that several residues─F145, Y152, W230, and R320─are essential for substrate recognition. Notably, the 2-<i>O</i>-sulfate group of the fucosyl residue at the -1 subsite does not interact with any enzyme residues but instead sterically fits into an open space, enabling the -1 subsite to accommodate both Fuc<i>p</i> and Fuc<i>p</i>2(OSO₃^-). This study enhances the understanding of the subsite specificity of endo-1,3-fucanases, facilitating their application in the research and development of sulfated fucan.