Induced-fit motion of a lid loop involved in catalysis in alginate lyase A1-III

The structures of two mutants (H192A and Y246F) of a mannuronate-specific alginate lyase, A1-III, from Sphingomonas species A1 complexed with a tetrasaccharide substrate [4-deoxy-L-erythro-hex-4-ene-pyranosyluronate-(mannuronate)₂-mannuronic acid] were determined by X-ray crystallo­graphy at around 2.2 Å resolution together with the apo form of the H192A mutant. The final models of the complex forms, which comprised two monomers (of 353 amino-acid residues each), 268–287 water molecules and two tetrasaccharide substrates, had R factors of around 0.17. A large conformational change occurred in the position of the lid loop (residues 64–85) in holo H192A and Y246F compared with that in apo H192A. The lid loop migrated about 14 Å from an open form to a closed form to interact with the bound tetrasaccharide and a catalytic residue. The tetrasaccharide was bound in the active cleft at subsites −3 to +1 as a substrate form in which the glycosidic linkage to be cleaved existed between subsites −1 and +1. In particular, the O^η atom of Tyr68 in the closed lid loop forms a hydrogen bond to the side chain of a presumed catalytic residue, O^η of Tyr246, which acts both as an acid and a base catalyst in a syn mechanism.