STRUCTURE OF THE ALLOSTERIC TETRAMER THREONINE DEAMINASE. T.Gallagher, E.Eisenstein, D.Chinchilla, J.Zondlo and G.Gilliland, Center for Advanced Research in Biotechnology of the Maryland Biotechnology Institute, University of Maryland, Shady Grove and the National Institute of Standards and Technology, 9600 Gudelsky Dr., Rockville MD 20850

The biosynthetic threonine deaminase from E. coli is a 220-Kdal tetramer whose activity is regulated by the allosteric ligands isoleucine and valine. SIRAS phases to 3.1 Å resolution based on a uranyl derivative, and native data to 2.3 Å, have led to a model of the enzyme and suggested a mechanism for the allosteric transition. Each 514-residue chain folds into 3 [[alpha]]/[[beta]] domains, with the pyridoxal phosphate cofactor and the active site nestled between catalytic domains while the regulatory domain makes extensive quaternary interactions. Sigmoidal kinetics likely result from a structural transition in which the domains rearrange to modulate access to the active sites; biochemical evidence suggests that the transition involves a reduction in the symmetry of the tetramer from 222 to 2.

The unliganded structure, crystallized in space group I222 with lattice constants 84, 91, and 163 Å and one chain per asymmetric unit, is currently in refinement. Additional datasets from two other crystal forms, and from crystals grown in the presence of the allosteric ligands, have been collected. Thus a complete description of the tertiary and quaternary structure, and discussion of the catalytic and allosteric mechanisms of this key regulatory enzyme, will be possible. Further interest to the science of crystal growth rises from the fact that two distinct crystal forms grow under identical conditions; this phenomenon will be explored in light of the molecular contacts.