Enzyme-catalyzed acylation of homoserine: Mechanistic characterization of the Escherichia coli metA-encoded homoserine transsuccinylase

Timothy L. Born, John S. Blanchard

Research output: Contribution to journalArticle

35 Scopus citations

Abstract

The first unique step in bacterial and plant methionine biosynthesis involves the activation of the γ-hydroxyl of homoserine. In Escherichia coli, this activation is accomplished via a succinylation reaction catalyzed by homoserine transsuccinylase. The activity of this enzyme is closely regulated in vivo and therefore represents a critical control point for cell growth and viability. We have cloned homoserine transsuccinylase from E. coli and present the first detailed enzymatic study of this enzyme. Steady-state kinetic experiments demonstrate that the enzyme utilizes a ping-pong kinetic mechanism in which the succinyl group of succinyl-CoA is initially transferred to an enzyme nucleophile before subsequent transfer to homoserine to form the final product, O-succinylhomoserine. The maximal velocity, V/K(succinyl-CoA), and V/K(homoserine) all exhibited a bell-shaped pH dependence with apparent pK's of 6.6 and ~7.9. The enzyme was inhibited by iodoacetamide in a pH-dependent manner, with an apparent pK of the group being inactivated of 6.4. This suggests the presence of an active site cysteine which forms a succinyl-cysteine intermediate during enzymatic turnover. Solvent kinetic isotope effect studies yielded inverse effects of 0.7 on V and 0.61 on V/K in the reverse reaction only. On the basis of these observations, we propose a detailed chemical mechanism for this important member of the acyltransferase family.

Original languageEnglish (US)
Pages (from-to)14416-14423
Number of pages8
JournalBiochemistry
Volume38
Issue number43
DOIs
StatePublished - Oct 26 1999

ASJC Scopus subject areas

  • Biochemistry

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