CRYSTAL-STRUCTURE OF L-2-HALOACID DEHALOGENASE FROM PSEUDOMONAS SP YL- AN ALPHA BETA HYDROLASE STRUCTURE THAT IS DIFFERENT FROM THE ALPHA/BETA HYDROLASE FOLD/

L-2-Haloacid dehalogenase catalyzes the hydrolytic dehalogenation of L -2-haloalkanoic acids to yield the corresponding D-2-hydroxyalkanoic a cids. The crystal structure of the homodimeric enzyme from Pseudomonas sp. YL has been determined by a multiple isomorphous replacement meth od and refined at 2.5 Angstrom resolution to a crystallographic R-fact or of 19.5%. The subunit consists of two structurally distinct domains : the core domain and the subdomain. The core domain has an alpha/beta structure formed by a six-stranded parallel beta-sheet flanked by fiv e alpha-helices. The subdomain inserted into the core domain has a fou r helix bundle structure providing the greater part of the interface f or dimer formation. There is an active site cavity between the domains . An experimentally identified nucleophilic residue, Asp-10, is locate d on a loop following the amino-terminal beta-strand in the core domai n, and other functional residues, Thr-14, Arg-41, Ser-118, Lys-151, Ty r-157, Ser-175, Asn-177, and Asp-180, detected by a site-directed muta genesis experiment, are arranged around the nucleophile in the active site. Although the enzyme is an alpha/beta-type hydrolase, it does not belong to the alpha/beta hydrolase fold family, from the viewpoint of the topological feature and the position of the nucleophile.