title: Homoserine kinase of Escherichia coli: kinetic mechanism and inhibition by L-aspartate semialdehyde.
abstract: The kinetic mechanism of homoserine kinase, purified to homogeneity from Escherichia coli, was examined by initial velocity techniques at pH 7.6. Whereas ATP displayed normal Michaelis-Menten saturation kinetics (Km = 0.2 mM), L-homoserine showed hyperbolic saturation kinetics only up to a concentration of 0.75 mM (Km = 0.15 mM). Above this concentration, L-homoserine caused marked but partial inhibition (Ki approximately 2 mM). The kinetic data indicated that the addition of substrates to homoserine kinase occurs by a preferred order random mechanism, with ATP preferentially binding before L-homoserine. When the ATP concentration was varied at several fixed inhibitory concentrations of L-homoserine, the resulting inhibition pattern indicated hyperbolic mixed inhibition. This suggested a second binding site for L-homoserine. L-Aspartate semialdehyde, an amino acid analog of L-homoserine, proved to be an alternative substrate of homoserine kinase (Km = 0.68 mM), and was subsequently used as a probe of its kinetic mechanism. In aqueous solution, at pH 7.5, this analog was found to exist predominantly (ca 85%) as its hydrated species. When examined as an inhibitor of the physiological reaction, L-aspartate semialdehyde showed mixed inhibition versus both L-homoserine and ATP. Although the pH profiles for the binding of L-homoserine as a substrate (Km) and as an inhibitor (Ki) were identical, the kinetic data were best fit to a two-site model, with separate catalytic and inhibitory sites for L-homoserine.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D004926:Escherichia coli / Q000201:enzymology*; D006714:Homoserine / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D010770:Phosphotransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor); D013379:Substrate Specificity
chemical_list: D001224:Aspartic Acid; C006644:aspartic semialdehyde; D006714:Homoserine; D000255:Adenosine Triphosphate; D010770:Phosphotransferases; D017853:Phosphotransferases (Alcohol Group Acceptor); C021370:homoserine kinase