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Despite recent progress in elucidating the regulation of methionine (Met) synthesis, little is known about the catabolism of this amino acid in plants. In this article, we present several lines of evidence indicating that the cleavage of Met catalyzed by Met [gamma]-lyase is the first step in this process. First, we cloned an Arabidopsis cDNA coding a functional Met [gamma]-lyase (AtMGL), a cytosolic enzyme catalyzing the conversion of Met into methanethiol, [alpha]-ketobutyrate, and ammonia. AtMGL is present in all of the Arabidopsis organs and tissues analyzed, except in quiescent dry mature seeds, thus suggesting that AtMGL is involved in the regulation of Met homeostasis in various situations. Also, we demonstrated that the expression of AtMGL was induced in Arabidopsis cells in response to high Met levels, probably to bypass the elevated [K.sub.m] of the enzyme for Met. Second, [[sup.13]C]-NMR profiling of Arabidopsis cells fed with [[sup.13]C]Met allowed us to identify labeled S-adenosylmethionine, S-methylmethionine, S-methylcysteine (SMC), and isoleucine (lie). The unexpected production of SMC and lie was directly associated to the function of Met [gamma]-lyase. Indeed, we showed that part of the methanethiol produced during Met cleavage could react with an activated form of serine to produce SMC. The second product of Met cleavage, [alpha]-ketobutyrate, entered the pathway of lie synthesis in plastids. Together, these data indicate that Met catabolism in Arabidopsis cells is initiated by a [gamma]-cleavage process and can result in the formation of the essential amino acid lie and a potential storage form for sulfide or methyl groups, SMC. methionine [gamma]-lyase | NMR profiling | plant

Threonine deaminase from Arabidopsis thaliana has two isoleucine binding sites on each monomer. The binding of the first isoleucine to the high-affinity binding site induces a conformational change that enhances the binding of the second isoleucine to a lower-affinity binding site.

Research has been conducted on the lipoate containing H-proteins. The role of the lipoate in the glycine decarboxylase complex's catalytic cycle has ben investigated.

The Arabidopsis thaliana threonine synthase was overexpressed in Escherichia coli, purified to homogeneity and then applied in kinetic and enzyme-bound pyridoxal 5'-phosphate fluorescence equilibrium-binding experiments. The purpose was to further elucidate the role of S-adenosylmethionine in activating plant threonine synthase through a noncovalent interaction. Findings showed that the activating effect of S-adenosylmethionine is due to an 8-fold rise in the rate of catalysis and from a 25-fold reduction of the K(sub m) value for the O-phosphohomoserine substrate.

A study was conducted to examine the functions of the quaternary structure of the homodimer plant acetolaydroxy acid isomeroreductase. The Escherichia coli strain BMH-71-18 mutS was utilized during the mutagenesis procedure with a site elimination mutagenesis kit. Results indicated that the quaternary structure supports an important function in stabilizing a high-affinity magnesium binding site.

Plant acetohydroxy acid isomeroreductase catalyzes the second step in the biosynthesis of branched chain amino acids. This enzyme is strongly inhibited by several compounds belonging to a class under the thiadiazole family. The noncompetitive inhibition can be reversible or irreversible depending on the side chain attached to the thiadiazole ring. Mass spectrometric studies reveal that the cysteine residue at position 498 is the target of the covalent bonding of irreversible inhibitors of this enzyme.