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  • Title: Threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Kinetic and spectroscopic effects upon binding of serine and threonine.
    Author: Costrejean JM, Truffa-Bachi P.
    Journal: J Biol Chem; 1977 Aug 10; 252(15):5332-6. PubMed ID: 328500.
    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.
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