Methionine catabolism in Arabidopsis cells is initiated by a γ-cleavage process and leads to S -methylcysteine and isoleucine syntheses

Abstract

methionine ␥-lyase ͉ NMR profiling ͉ plant T he sulfur-containing amino acid methionine (Met) is an essential metabolite in all living organisms (1). Besides its role as protein constituent, Met is the precursor of S-adenosylmethionine (AdoMet), which is the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of biotin, polyamines, and the phytohormone ethylene. Met is synthesized de novo by plants, fungi, and microbes. It is the only sulfur-containing amino acid that is essential for human and monogastric livestock. In these organisms, Met is metabolized through the reverse transsulfuration pathway where the ␥-cleavage of the cystathionine intermediate, a reaction that does not exist in plants, allows synthesis of cysteine (Cys) (2). Therefore, Met must be supplied from the diet and, because it is the most limiting essential amino acid in legume seeds, metabolic engineering strategies have been developed to increase the carbon flux into free Met as well as the incorporation of this amino acid into proteins (3).In higher plants, the neo-synthesized Met molecule originates from three convergent pathways with the sulfur atom deriving from Cys, the nitrogen͞carbon backbone from aspartate, and the methyl moiety from the ␤-carbon of serine via the pool of folates (1). De novo Met synthesis consists of three consecutive reactions localized in plastids (4) and catalyzed by cystathionine ␥-synthase, cystathionine ␤-lyase, and methionine synthase. Several recent studies have indicated that Met synthesis and accumulation are subject to complex regulatory controls in which cystathionine ␥-synthase plays a crucial role (5-7).Studies of metabolic fates of Met using the aquatic plant Lemna paucicostata indicated that the synthesis and turnover of AdoMet accounts for Ϸ80% of Met metabolism, whereas the synthesis of proteins drives Ϸ20% of Met (for a review, see ref. 8). More than 90% of AdoMet then is used for transmethylation, leading to nucleic acid, protein, lipid, and other metabolites modifications. The resulting homocysteinyl moiety is recycled back to Met and AdoMet by a set of cytosolic reactions designated as the activated methyl cycle. The use of AdoMet for the synthesis of polyamines and, in some plant tissues, ethylene also is accompanied by recycling of the methylthio moiety and regeneration of Met. The last known metabolic fate of Met is unique to plants and leads to the production of S-methylmethionine (SMM), a compound resulting from the AdoMet-dependent methylation of Met. SMM then can donate a methyl group to homocysteine (Hcy) through the reaction catalyzed by Hcy S-methyltransferase, yielding two molecules of Met. The SMM cycle might serve to achieve short-term control of AdoMet levels in plant cells, thus controlling the commitment of one-carbon units into methyl-group synthesis (9-11).Despite recent progress in elucidating the synthesis of Met and its complex regulation, little is known about the catabolism of this amino acid in plant cells. Several authors have ...

The draft genome of tropical fruit durian (Durio zibethinus)

“…VSCs have been identified as major contributors to durian smell, and we observed upregulation of sulfur-related pathways in durian fruit arils in comparison to other durian plant organs and fruits from other species. At the genomic level, we also identified expansions in gene families in sulfur-related pathways, most notably in the MGL gene, whose product has been shown to catalyze the breakdown of methionine into VSCs in microbes and plants 38,39,49 . More specifically, our analysis suggests a distinct role for MGLb genes in the durian fruit, where these genes were found to be significantly upregulated.…”

“…4a). Studies in microbes and plants have shown that MGL is a major functional contributor to VSC biosynthesis, degrading the sulfurcontaining amino acids cysteine and methionine into methanethiol, which is further broken down into di-and trisulfides 1,38,39 (Fig. 4b).…”

Methionine gamma‐lyase: The unique reaction mechanism, physiological roles, and therapeutic applications against infectious diseases and cancers

“…The regulation of its expression in seeds is peculiar. It was shown that while MGL mRNA was highly accumulated but not translated in dry seeds, the protein was highly expressed in imbibed seeds (21). This unique regulation of MGL expression suggests that immediate production of MGL protein is essential in the early germination process.…”

“…(13), Citrobacter intermedius (14), Brevibacterium linens (15), Citrobacter freundii (16), Porphyromonas gingivalis (17), and Treponema denticola (18), parasitic protozoa such as Trichomonas vaginalis (19), Entamoeba histolytica (20), and a model plant Arabidopsis thaliana (21). MGL activity was also detected from archaeon Ferroplasma acidarmanus (22), cheese surface bacteria such as Micrococcus luteus, Arthrobacter sp., Corynebacterium glutamicum, and Staphylococcus equorum (23).…”

Biosynthesis of plant‐derived flavor compounds

“…In Arabidopsis, the cleavage of methionine is catalyzed by methionine c-lyase, resulting in the production of methanethiol, 2-ketobutyrate and ammonia (Ré beillé et al, 2006). In onion (Allium cepa) and garlic (A. sativum), a series of volatile sulfur compounds is generated by cleavage of odorless S-alk(en)yl cysteine sulfoxide flavor precursors catalyzed by the enzymes allinase and lachrymatory-factor synthase (Jones et al, 2004;Lanzotti, 2006).…”