title: Threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Kinetic and spectroscopic effects upon binding of serine and threonine.
abstract: The two threonine-sensitive activities aspartokinase and homoserine dehydrogenase are inhibited by L-serine. The inhibition of the aspartokinase by L-serine displays homotropic cooperative effects and is competitive versus aspartate. The inhibition by L-serine of the homoserine dehydrogenase displays Michaelis-Menten kinetics which are of a competitive nature versus homoserine. Characteristic effects of L-serine on the protein include a perturbation of its absorption and fluorescence spectra, with an increase in the fluorescence of the protein-NADPH complex. L-serine shifts the allosteric equilibrium of the protein to a "T-like" conformation to which L-threonine binds noncooperatively. L-Serine, a threonine analog, is not capable, as the physiological effector, of inducing a complete R to T transition of the enzyme; the aspartokinase globules show a cooperative conformation change upon serine binding, but this conformation change is not found in the homoserine dehydrogenase globules.
mesh_terms: D001225:Aspartokinase Homoserine Dehydrogenase / Q000378:metabolism*; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D009097:Multienzyme Complexes / Q000378:metabolism*; D011485:Protein Binding; D012694:Serine / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D013912:Threonine / Q000494:pharmacology*
chemical_list: D009097:Multienzyme Complexes; D013912:Threonine; D012694:Serine; D001225:Aspartokinase Homoserine Dehydrogenase