title: Aspartokinase I-homoserine dehydrogenase I of Escherichia coli K12. Concentration-dependent dissociation to dimers in the presence of L-threonine.
abstract: The concentration-dependent association-dissociation equilibrium of the bifunctional enzyme aspartokinase I-homoserine dehydrogenase I of Escherichia coli K12 has been investigated at pH 7.6 in the presence of 10 mM L-threonine and 0.1 M KCl by equilibrium gel permeation monitored by a single-photon counting spectrophotometer. The results obtained are consistent with the existence of a dimer-tetramer equilibrium with the association constant of 2.6 X 10(7) M-1 (deltaG0 = -9.9 kcal/mol of dimer). The limiting partition cross-sections estimated by a three-parameter least squares minimization procedure indicate that the molecular radii of the dimer and tetramer are 53.8 A and 70 A, respectively. Both the dimeric and tetrameric forms of the enzyme possess dehydrogenase activity. Treatment of the enzyme with the chaotropic salts, potassium thiocyanate or potassium trichloroacetate, generates a monomeric form that is devoid of dehydrogenase activity. The catalytically inactive monomeric form of the enzyme has a molecular radius between 43 and 45.5 A and a molecular weight of approximately 80,000 as determined by small zone gel chromatography and sedimentation equilibrium studies.
mesh_terms: D000429:Alcohol Oxidoreductases; D001222:Aspartate Kinase; D002151:Calorimetry; D004926:Escherichia coli / Q000201:enzymology*; D006715:Homoserine Dehydrogenase; D046911:Macromolecular Substances; D008970:Molecular Weight; D009097:Multienzyme Complexes; D010770:Phosphotransferases; D011487:Protein Conformation; D013912:Threonine
chemical_list: D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D013912:Threonine; D000429:Alcohol Oxidoreductases; D006715:Homoserine Dehydrogenase; D010770:Phosphotransferases; D001222:Aspartate Kinase