The Biosynthesis of Volatile Sulfur Flavour Compounds

Abstract

Many natural biological flavours are products of secondary metabolism and pose a particular challenge in understanding both their biosynthetic pathways and roles within their native organism. The concepts of primary and secondary metabolism were adopted at the end of the nineteenth century to distinguish the pathways for the biosynthesis of the essential metabolites required in all cells (e.g. amino acids, lipids, carbohydrates, nucleotides), whether plant, animal or microbial, from the metabolites that were frequently species-specific and initially considered dispensable (e.g. isoprenoids, phenolics, steroids). These definitions have inevitably become looser as knowledge of metabolism and genetics has advanced and demonstrate both complexity and conservation in biosynthetic pathways for secondary metabolites. The biosynthesis of secondary metabolites is now seen as the result of a highly organized and controlled process, frequently requiring large numbers of enzymes and complex genetic regulation. Furthermore, many secondary metabolites are now known to be essential for survival. 1 The early appreciation that secondary metabolites are usually restricted to a particular species has remained true. In contrast to the substantial uniformity of primary metabolism, secondary metabolism is frequently species-and indeed cell type-specific and understanding their biosynthesis requires research within the specific producer organisms. The model organism approach, which proved so powerful in advancing biological knowledge during the late twentieth century, therefore has limited value because the

Proton Transfer Reaction Mass Spectrometry: Applications in the Life Sciences

“…This points to an induction process which involves the transcription of genes or activation of enzymes necessary for the synthesis of these sulfides. Methanethiol, DMS and DMDS have been studied in Brassica crops because they cause the typical 'cabbage odour' [122]. Indeed, several enzymes, mainly transferases and lyases, are present in the aboveground parts of Broccoli and other cultivated cabbages that produce sulfides from the amino acids methionine and cysteine ( [123], [124], [125]).…”

Isotope‐Guided Metabolomics Reveals Divergent Incorporation of Valine into Different Flavor Precursor Classes in Chives

“…[2] The molecular mechanisms by which these compounds are activated by the enzyme alliinase to produce volatile sulfur flavor compounds that exert various biological effects are already well understood, thanks to decades of vigorous investigation. [2,[4][5][6] In contrast, how these flavor precursors are produced in Allium plants is only partially (8), but the link to the saturated analog propiin (3) is not yet clear. The conversion of valine (8) to the C 3 side chains of alliin (1) and isoalliin (2) probably involves 2-carboxypropyl key intermediates such as 2carboxypropylglutathione (11).…”

“…However, the enzymatic hydrogenation of C=C double bonds with limited activation is biosynthetically unusual. Also, the existence of the other saturated S-alkyl cysteine sulfoxides methiin (4), ethiin ( 5) and butiin (6), whose origin from γ-GluÀ Cys(CP)À Gly ( 11) is improbable, raises the question if there is a different biosynthetic route to propiin (3) and other S-alkyl cysteine sulfoxides.…”

“…In addition, S ‐ethyl cysteine sulfoxide (ethiin, 5 ) and S ‐butyl cysteine sulfoxide (butiin, 6 ) occur in few Allium species as minor components [2] . The molecular mechanisms by which these compounds are activated by the enzyme alliinase to produce volatile sulfur flavor compounds that exert various biological effects are already well understood, thanks to decades of vigorous investigation [2,4–6] . In contrast, how these flavor precursors are produced in Allium plants is only partially understood.…”