Fig. 3: Substrate recognition mechanism of subfamily Ⅰ.

a Cleavage site of subfamily Ⅰ towards different sulfated fucan. The incomplete circles represent the cleavage sites of the enzymes, with the solid circles indicating the preferred cleavage sites, while the dashed circles denote sites where the enzyme can only cleave weakly. b The conformation of the ligand associated with WfGH168_Ⅰ. The number of subsites are labeled. c Critical residues within the negative subsite region of WfGH168_Ⅰ. The conserved key residues of the family and the characteristic residues of subfamily Ⅰ are represented by white sticks and yellow sticks, respectively. d Key residues within the positive subsite region of WfGH168_Ⅰ. Subsites +2 and +3 of WfGH168_Ⅰ interact with the sulfate group via K114, R205, and Q227; however, these residues exhibit high variability and are not a conserved feature of this subfamily. e The +1 subsite of WfGH168_Ⅰ accommodates an open space for the 4-O-sulfate group. f The enzymatic product profile of Ht-FUC by PkGH168_Ⅰ. g MS/MS spectrum of the dominant oligosaccharide in the product of Ht-FUC by PkGH168_Ⅰ.