Structural And Functional Characterization Of Ulvan Degrading Polysaccharide Lyase Enzymes

Ulvans are the sulfated cell wall polysaccharide present in marine green algae. Ulvan polysaccharides are composed mainly of 3-sulfated rhamnose, glucuronic acid, iduronic acid and xylose. The sulfation pattern in ulvan resemble glycosaminoglycans in vertebrates. The structural features of ulvan make it a good candidate for variety of industrial applications in agriculture, food, pharmaceutical, chemical, and biomaterials industries. Identification of ulvan-degrading microorganism and the corresponding enzymes will increase the potential application of this highly abundant naturally occurring polysaccharide.

Bacterial microbiomes associated with the green algae contain enzymes to degrade ulvan by a lytic β-elimination mechanism. Genome sequencing projects lead to the identification of many such ulvan degrading enzymes from several bacteria Nonlabens ulvanivorans, Pseudoalteromonas sp. strain PLSV, Alteromonas sp. strain LOR and LTR. Using X-ray crystallography and enzyme activity assays, we solved the structure and biochemically characterized three ulvan lyases, PLSV3936, LOR107 and NLR48. All three enzymes share very low sequence identity and act differentially on substrate.
Despite their low sequence identity, PLSV3936 and LOR107 share the same 7-bladed β propeller fold. However, the complex structure with the bound tetrasaccharide substrate reveals the difference in the active site and mode of cleavage. Whereas, NLR48 has a β jelly roll scaffold. Complex structure of NLR48 with tetrasaccharide substrate suggest that NLR48 appears to utilize lysine and tyrosine as catalytic residues and the substrate acidic group is neutralized by a glutamine residue. Our results expand the information about ulvan degrading enzymes to potentiate the use of ulvan polysaccharide.