This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: citrol degradation
|Superclasses:||Degradation/Utilization/Assimilation → Secondary Metabolites Degradation → Terpenoids Degradation|
Expected Taxonomic Range: Fungi
Geraniol is a linear monoterpene that occurs in many terpene-containing essential oils derived from plants. It has a flowery, rose-like scent and is found in large quantities in palmrosa, geranium and rose oils. Citrol is a mixture of nerol and geraniol. Citral is a mixture of neral (cis-citral) and geranial (trans-citral) and is widely used in the flavor, fragrance and other industries. sulcatone (methylheptenone) has been described as having a green citrus-like odor and a bittersweet taste (in [Wolken01] and in [Wolken02]).
The biotransformation of geraniol to sulcatone (methylheptenone) by fungi and bacteria has been previously documented and pathways have been proposed, but they were not substantiated biochemically. However more recent biochemical studies have elucidated the pathway for this biotransformation in spore extracts of Penicillium digitatum ATCC 201167. This work was based on enzymatic studies as well as substrate degradation and product formation studies. This novel pathway involves citrol dehydrogenase and a novel citral lyase [Wolken01, Wolken02].
About This Pathway
Geraniol is converted to geranial by NAD+-dependent citrol (geraniol and nerol) dehydrogenase, which also converts nerol to neral. geranial (trans-citral) can undergo non-enzyme catalyzed cis-trans isomerization to produce neral (cis-citral) in the presence of amino acids [Wolken00]. Likewise, neral can also undergo this isomerization to geranial. The mixture of geranial and neral (known as citral) formed is deacetylated by citral lyase to form sulcatone (methylheptenone) and acetaldehyde. Further metabolism of sulcatone was not reported in this paper, although a pseudomonad has been shown to degrade it [Devi77].
Geranial can also be transformed to geranate (geranic acid) by NAD+-dependent citral dehydrogenase activity. Whole spores could catalyze the reverse reaction converting geranate to citral (geranial and neral). Over time, all reactants were converted to sulcatone (methylheptenone) and acetaldehyde by citral lyase activity. Citral lyase catalyzes a novel enzymatic activity involving the deacetylation of an α,β-unsaturated aldehyde [Wolken01].
Devi77: Devi JR, Bhattacharyya PK (1977). "Microbiological transformations of terpenes: Part XXIII--Fermentation of geraniol, nerol & limonene by a soil pseudomonad, pseudomonas incognita (linalool strain)." Indian J Biochem Biophys 14(3);288-91. PMID: 612543
Wolken00: Wolken WA, ten Have R, van Der Werf MJ (2000). "Amino acid-catalyzed conversion of citral: cis-trans isomerization and its conversion into 6-methyl-5-hepten-2-one and acetaldehyde." J Agric Food Chem 48(11);5401-5. PMID: 11087492
Luddeke12: Luddeke F, Wulfing A, Timke M, Germer F, Weber J, Dikfidan A, Rahnfeld T, Linder D, Meyerdierks A, Harder J (2012). "Geraniol and geranial dehydrogenases induced in anaerobic monoterpene degradation by Castellaniella defragrans." Appl Environ Microbiol 78(7);2128-36. PMID: 22286981
NOGE05: NOGE K, KATO M, IGUCHI T, MORI N, NISHIDA R, KUWAHARA Y (2005). "Biosynthesis of Neral in Carpoglyphus lactis (Acari: Carpoglyphidae) and Detection of Its Key Enzyme, Geraniol Dehydrogenase, by Electrophoresis." Journal of the Acarological Society of Japan 14(2);75-81.
Noge08: Noge K, Kato M, Mori N, Kataoka M, Tanaka C, Yamasue Y, Nishida R, Kuwahara Y (2008). "Geraniol dehydrogenase, the key enzyme in biosynthesis of the alarm pheromone, from the astigmatid mite Carpoglyphus lactis (Acari: Carpoglyphidae)." FEBS J 275(11);2807-17. PMID: 18422649
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