Homocitrate synthase (HCS1) (acetyl-coenzyme A: 2-ketoglutarate C-transferase; E.C. 2.3.3.14) catalyzes the first and regulated step in the a-aminoadipate pathway for lysine synthesis. The pH dependence of the kinetic parameters, isotope effects, and dissociation constants for competitive inhibitors are used to probe the chemical mechanism of HCS. A general acid-base chemical mechanism is proposed. site-directed mutagenesis was used to change the three active site residues of HCS, and the resulting mutant enzymes were characterized using initial velocity studies, the pH dependence of the kinetic parameters and isotope effects. Data combined with a constant pH molecular dynamics simulation study suggest a catalytic dyad, comprised of Glu-155 and His-309, functions as a general base to deprotonate the methyl group of AcCoA.

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