Structure-function relationship of the GH168 fucanase reveals an unusual enzyme recognition mechanism for sulfated polysaccharide.

Sulfated fucan is one of the most recalcitrant polysaccharides. The molecular mechanism underlying the endo-1,3-fucanase, which plays a critical role in the breakdown of sulfated fucan, remains unexplained. Here, we conduct a comprehensive structure-function relationship investigation on the endo-1,3-fucanases within a family space-GH168. The family can be divided into four subfamilies according to phylogenetic relationship and functional similarities. Subfamily I, Ⅱ and Ⅳ preferentially recognize Fucp2(OSO3-), Fucp2,4(OSO3-) and Fucp units at the +1 subsite, respectively, while consistently recognizing the Fucp2(OSO3-) unit at the -1 subsite. Remarkably, two-thirds of the interacting residues are dedicated to the recognition of sulfate groups along the glycoside chains. This mechanism is distinct from the direct recognition of the sugar backbone employed by neutral polysaccharide hydrolases. These findings unveil a critical enzyme recognition mechanism for sulfate polysaccharides and promote the application of endo-1,3-fucanases in the structural analysis and oligosaccharide production of sulfated fucan.