Identification of a flavin‐containing S‐oxygenating monooxygenase involved in alliin biosynthesis in garlic

Abstract

SUMMARYS-Alk(en)yl-L-cysteine sulfoxides are cysteine-derived secondary metabolites highly accumulated in the genus Allium. Despite pharmaceutical importance, the enzymes that contribute to the biosynthesis of S-alk-(en)yl-Lcysteine sulfoxides in Allium plants remain largely unknown. Here, we report the identification of a flavin-containing monooxygenase, AsFMO1, in garlic (Allium sativum), which is responsible for the S-oxygenation reaction in the biosynthesis of S-allyl-L-cysteine sulfoxide (alliin). Recombinant AsFMO1 protein catalyzed the stereoselective S-oxygenation of S-allyl-L-cysteine to nearly exclusively yield (R C S S )-S-allylcysteine sulfoxide, which has identical stereochemistry to the major natural form of alliin in garlic. The S-oxygenation reaction catalyzed by AsFMO1 was dependent on the presence of nicotinamide adenine dinucleotide phosphate (NADPH) and flavin adenine dinucleotide (FAD), consistent with other known flavin-containing monooxygenases. AsFMO1 preferred S-allyl-L-cysteine to c-glutamyl-S-allyl-L-cysteine as the S-oxygenation substrate, suggesting that in garlic, the S-oxygenation of alliin biosynthetic intermediates primarily occurs after deglutamylation. The transient expression of green fluorescent protein (GFP) fusion proteins indicated that AsFMO1 is localized in the cytosol. AsFMO1 mRNA was accumulated in storage leaves of pre-emergent nearly sprouting bulbs, and in various tissues of sprouted bulbs with green foliage leaves. Taken together, our results suggest that AsFMO1 functions as an S-allyl-L-cysteine S-oxygenase, and contributes to the production of alliin both through the conversion of stored c-glutamyl-S-allyl-L-cysteine to alliin in storage leaves during sprouting and through the de novo biosynthesis of alliin in green foliage leaves.

Untargeted analysis to monitor metabolic changes of garlic along heat treatment by LC–QTOF MS/MS

“… and Yoshimoto et al. pathways for synthesis of organosulfur compounds in garlic are proposed in this study as supplementary information (see Supporting Information Fig. ) to interpret metabolic changes occurring during fermentation.…”

l‐lysine metabolism to N‐hydroxypipecolic acid: an integral immune‐activating pathway in plants

“…Phylogenetic analyses based on protein sequences have divided plant FMOs into three clades (Olszak et al ., ; Schlaich, ; Yoshimoto et al ., ). Arabidopsis FMO1 falls into clade I; as described above it monooxygenates the N–H group in the piperidine ring of Pip to generate the N ‐hydroxylated amino acid NHP (Figure a).…”

Comparative evaluation of antioxidant properties of methanol extracts of allium cepa bulb, allium cepa bulb peels and allium fistulosum

“…The name A. fistulosum comes from the fact that the leaves are hollow, ("fistulosum" means "hollow"). Allium species are rich in two secondary metabolites that have perceived health benefits (YOSHIMOTO et al, 2015). These are the flavonoids (quercetin, anthocyanins and allixin) and the odorless organosulphur compounds, alk (en)yl cysteine sulphoxides (ACSOs).…”