Structural and Mutational Characterization of the Catalytic A-module of the Mannuronan C-5-epimerase AlgE4 from

Alginate is a family of linear copolymers of (134)-linked-D-mannuronic acid and its C-5 epimer -L-guluronic acid. The polymer is first produced as polymannuronic acid and the gulu-ronic acid residues are then introduced at the polymer level by mannuronan C-5-epimerases. The structure of the catalytic A-module of the Azotobacter vinelandii mannuronan C-5-epi-merase AlgE4 has been determined by x-ray crystallography at 2.1-A ̊ resolution. AlgE4A folds into a right-handed parallel -helix structure originally found in pectate lyase C and subse-quently in several polysaccharide lyases and hydrolases. The -helix is composed of four parallel -sheets, comprising 12 complete turns, and has an amphipathic -helix near the N ter-minus. The catalytic site is positioned in a positively charged cleft formed by loops extending from the surface encompassing Asp152, an amino acid previously shown to be important for th