Biosynthesis of germacrene A carboxylic acid in chicory roots. Demonstration of a cytochrome P450 (+)-germacrene a hydroxylase and NADP+-dependent sesquiterpenoid dehydrogenase(s) involved in sesquiterpene lactone biosynthesis.
Sprouts of chicory (Cichorium intybus), a vegetable grown in the dark, have a slightly bitter taste associated with the presence of guaianolides, eudesmanolides, and germacranolides. The committed step in the biosynthesis of these compounds is catalyzed by a (+)-germacrene A synthase. Formation of the lactone ring is the postulated next step in biosynthesis of the germacrene-derived sesquiterpene lactones. The present study confirms this hypothesis by isolation of enzyme activities from chicory roots that introduce a carboxylic acid function in the germacrene A isopropenyl side chain, which is necessary for lactone ring formation. (+)-germacrene A is hydroxylated to germacra-1(10),4,11(13)-trien-12-ol by a cytochrome P450 enzyme, and is subsequently oxidized to germacra-1(10),4,11(13)-trien-12-oic acid by NADP+-dependent dehydrogenase(s). Both oxidized germacrenes were detected as their Cope-rearrangement products elema-1,3,11(13)-trien-12-ol and elema-1,3,11(13)-trien-12-oic acid, respectively. The cyclization products of germacra-1(10),4,11(13)-trien-12-ol, i.e. costol, were also observed. The (+)-germacrene A hydroxylase is inhibited by carbon monoxide (blue-light reversible), has an optimum pH at 8.0, and hydroxylates beta-elemene with a modest degree of enantioselectivity.
de Kraker
J W
JW
Plant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands.
Franssen
M C
MC
Dalm
M C
MC
de Groot
A
A
Bouwmeester
H J
HJ
D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D018651:Cichorium intybus / Q000201:enzymology*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D007783:Lactones / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D018517:Plant Roots / Q000201:enzymology*; D012717:Sesquiterpenes / Q000378:metabolism*; D045788:Sesquiterpenes, Germacrane
D007783:Lactones; D012717:Sesquiterpenes; D045788:Sesquiterpenes, Germacrane; C431525:germacra-1(10),4,11(13)-trien-12-oic acid; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D000429:Alcohol Oxidoreductases; C431679:sesquiterpenoid dehydrogenase; C431677:germacrene A hydroxylase