Structure, function and catalytic mechanism of the carrageenan-sulfatases from the marine bacterium Zobellia galactanivorans Dsij.

Carrageenans are highly diverse sulfated galactans found in red seaweeds. They play various physiological roles within macroalgae, but also serve as carbon sources for heterotrophic marine bacteria living at their surface. Carrageenan sulfatases catalyze the removal of sulfate esters from the glycans to expose the saccharide chain for further enzymatic processing. In the marine flavobacterium Zobellia galactanivorans, three carrageenan sulfatase genes are localized within a carrageenan utilization locus, belonging to three distinct SulfAtlas S1 (formylglycine-dependent sulfatases) subfamilies (S1_19, ZgCgsA; S1_7, ZgCgsB1; and S1_17, ZgCgsC). In this study we combined several techniques to characterize the detailed desulfurylation steps in the catabolic pathway of carrageenan in this model marine bacterium. High resolution UHPLC-MS/MS sequencing of the reaction species provides precise chemical localization of the enzymatic activities for the three carrageenan sulfatases on carrageenan polysaccharides and oligosaccharides. High resolution structures of the S1_19 endo-/exo-lytic carrageenan sulfatase (ZgCgsA) in complex with oligocarrageenan products show substrate plasticity which involve enzyme and glycan conformational rearrangements. A sulfo-enzyme covalent-intermediate sheds light on the catalytic mechanism and highlights the unique chemistry of formylglycine, an essential post-translationally modified catalytic residue in the active site of S1 family sulfatases.