The three-dimensional structure of alginate lyase A1-III (ALYIII) from a Sphingomonas species A1 was determined by X-ray crystallography. The enzyme was crystallized by the hanging-drop vapour-diffusion method in the presence of 49% ammonium sulfate at 20 degrees C. The crystals are monoclinic and belong to the space group C2 with unit cell dimensions of a=49.18 A, b=93.08 A, c=82.10 A and beta=104.12 degrees. There was one molecule of alginate lyase in the asymmetric unit of the crystal. The diffraction data up to 1. 71 A were collected with Rsymof 5.0%. The crystal structure of ALYIII was solved by the multiple isomorphous replacement method and refined at 1.78 A resolution using X-PLOR with a final R -factor of 18.0% for 10.0 to 1.78 A resolution data. The refined model of ALYIII contained 351 amino acid residues, 299 water molecules and two sulfate ions. The three-dimensional structure of ALYIII was abundant in helices and had a deep tunnel-like cleft in a novel (alpha6/alpha5)-barrel structure, which was similar to the (alpha6/alpha6)-barrel found in glucoamylase and cellulase. This structure presented the possibility that alginate molecules might penetrate into the cleft to interact with the catalytic site of ALYIII.
Copyright 1999 Academic Press.