Structure of a PL17 Family Alginate Lyase Demonstrates Functional Similarities Among Exotype Depolymerases [Protein Structure and Folding]

January 29th, 2014 by Park, D., Jagtap, S., Nair, S. K.

Brown macroalgae represent an ideal source for complex polysaccharides that can be utilized as precursors for cellulosic biofuels. The lack of recalcitrant lignin components in macroalgae polysaccharide reserves provides for a facile route for depolymerization of constituent polysaccharides into simple monosaccharides. The most abundant sugars in macroalgae are alginate, mannitol and glucan, and while several classes of enzymes that can catabolize the latter two have been characterized, studies of alginate depolymerizing enzymes have lagged. Here, we present several crystal structures of Alg17c from marine bacterium Saccharophagus degredans, along with structure-function characterization of active site residues that are suggested to be involved in the exolytic mechanism of alginate depolymerization. This represents the first structural and biochemical characterization of a PL17 family enzyme. Despite the lack of appreciable sequence conservation, the structure, and β-elimination mechanism for glycolytic bond cleavage by Alg17c is similar to those observed for PL15 and other lyases. This work illuminates the evolutionary relationships amongst enzymes within this unexplored class of polysaccharide lyases, and reinforces the notion of a structure-based hierarchy in the classification of these enzymes.