Structure of Allosteric Threonine Deaminase from E. Coli. Travis Gallagher, Edward Eisenstein and Gary L. Gilliland; Center for Advanced Research in Biotechnology of the University of Maryland Biotechnology Institute and the National Institute of Standards and Technology, 9600 Gudelsky Drive, Rockville, MD 20850, USA.

The biosynthetic threonine deaminase is an allosteric tetramer of MW 220 Kdal regulating the synthesis of the branched chain amino acids in E. coli. We have analyzed by diffraction several crystal forms of this enzyme, including two distinct forms that grow under identical conditions. The best diffracting crystals (2.3 A native data) belong to space group I222, where the asymmetric unit contains one 514-residue chain and one PLP group. The structure has been determined in this space group, and is currently in refinement with R=0.24 and Rfree=0.34. Datasets from other space groups and ligand complexes are on hand. Implications of the tertiary and quaternary structure for the allosteric mechanism will be discussed. Structure determination was by SIRAS and also utilized a novel technique wherein a confidently-fit domain was used to improve phases and thus maps of a more difficult domain. We call the technique "iterative structure factor combination" because it involves adding structure factors for the composite domains to improve the estimate of total phase.