Structure and mechanism of l-fucose isomerase from Escherichia coli

Abstract

The three-dimensional structure of l-fucose isomerase from Escherichia coli has been determined by X-ray crystallography at 2.5 Å resolution. This ketol isomerase converts the aldose l-fucose into the corresponding ketose l-fuculose using Mn 2+ as a cofactor. Being a hexamer with 64,976 Da per subunit, l-fucose isomerase is the largest structurally known ketol isomerase. The enzyme shows neither sequence nor structural similarity with other ketol isomerases. The hexamer obeys D 3 symmetry and forms the crystallographic asymmetric unit. The strict and favorably oriented local symmetry allowed for a computational phase extension from 7.3 Å to 2.5 Å resolution. The structure was solved with an l-fucitol molecule bound to the catalytic center such that the hydroxyl groups at positions 1 and 2 are ligands of the manganese ion. Most likely, l-fucitol mimics a bound l-fucose molecule in its open chain form. The protein environment suggests strongly that the reaction belongs to the ene-diol type.