Crystal structure of alginate lyase A1-III complexed with trisaccharide product at 2.0 A resolution

The structure of A1-III from a Sphingomonas species A1 complexed with a trisaccharide product (4-deoxy-l-erythro-hex-4-enepyranosyluronate-mannuronate-mannuronic acid) was determined by X-ray crystallography at 2.0 A with an R-factor of 0.16. The final model of the complex form comprising 351 amino acid residues, 245 water molecules, one sulfate ion and one trisaccharide product exhibited a C(alpha) r.m.s.d. value of 0.154 A with the reported apo form of the enzyme. The trisaccharide was bound in the active cleft at subsites -3 approximately -1 from the non-reducing end by forming several hydrogen bonds and van der Waals interactions with protein atoms. The catalytic residue was estimated to be Tyr246, which existed between subsites -1 and +1 based on a mannuronic acid model oriented at subsite +1.