Editing Thujone (section)

== Biosynthesis ==

The biosynthesis of thujone is similar to the synthesis of other monoterpenes and begins with the formation of [[geranyl diphosphate]] (GPP) from [[dimethylallyl pyrophosphate]] (DMAPP) and [[isopentenyl diphosphate]] (IPP), catalyzed by the enzyme geranyl diphosphate synthase.<ref name=Dewick2008>{{cite book |last=Dewick |first=Paul M |date=2009 |title=Medicinal natural products: a biosynthetic approach |url=https://archive.org/details/medicinalnatural00dewi_880 |url-access=limited |edition=3rd |publisher=John Wiley & Sons Ltd |pages=[https://archive.org/details/medicinalnatural00dewi_880/page/n201 195]–197 |isbn=978-0-470-74167-2 }}</ref> Quantitative [[Carbon-13 nuclear magnetic resonance|<sup>13</sup>C NMR]] spectroscopic analysis has demonstrated that the [[Isoprene rule|isoprene units]] used to form thujone in plants are derived from the [[MEP pathway|methylerythritol phosphate pathway]] (MEP).<ref>{{cite journal |last1=Umlauf |first1=Dirk |last2=Zapp |first2=Josef |date=September 2004 |title=Biosynthesis of the irregular monoterpene artemisia ketone, the sesquiterpene germacrene D and other isoprenoids in ''Tanacetum vulgare'' L. (Asteraceae) |journal=Phytochemistry |volume=65 |issue=17 |pages=2463–2470 |doi=10.1016/j.phytochem.2004.08.019 |pmid=15381410|bibcode=2004PChem..65.2463U }}</ref>

The reactions that generate the thujone skeleton in [[sabinene]] from GPP are mediated by the enzyme [[(+)-sabinene synthase|sabinene synthase]] which has GPP as its substrate.<ref name=Dewick2008 /> GPP ('''1''') first isomerizes to linalyl diphosphate (LPP) ('''2''') and neryl diphosphate (NPP) ('''3'''). LPP preferentially forms a delocalized allylic cation-diphosphate ('''4'''). The ion-pair intermediate then cyclizes in an electrophilic addition to yield the α-terpinyl tertiary cation ('''5''').<ref name=Dewick2008 />

[[File:The-conversion-of-gpp-to-alpha-terpinyl-cation.png|center|400px|The conversion of GPP to alpha-terpinyl cation.]]

The α-terpinyl cation ('''5''') then undergoes a 1,2 hydride shift via a [[Wagner-Meerwein rearrangement|Wagner–Meerwein rearrangement]], leading to the formation of the terpinen-4-yl cation ('''6'''). This cation undergoes a second cyclization to form the thujyl cation intermediate ('''7''') before loss of a proton to form the thujone precursor, [[Sabinene|(+)-sabinene]] ('''8''').

[[File:The-conversion-of-alpha-terpinyl-cation-to-sabinene.png|center|700px|The conversion of alpha-terpinyl cation to (+)-sabinene]]

From [[Sabinene|(+)-sabinene]] ('''8'''), the proposed biosynthetic route to generate thujone follows a three-step pathway: (+)-sabinene is first oxidized to an isomer of (+)-sabinol ('''9-1'', ''9-2''') by a [[Cytochrome P450|cytochrome P450 enzyme]], followed by conversion to (+)-sabinone ('''10''') via a [[dehydrogenase]]. Finally, a [[reductase]] mediates the conversion to α-thujone ('''11-1''') and β-thujone ('''11-2''').<ref>{{cite journal |last1=Foster |first1=Adam J. |last2=Hall |first2=Dawn E. |date=Apr 2013 |title=Identification of Genes in ''Thuja plicata'' Foliar Terpenoid Defenses |journal=Plant Physiology |volume=161 |issue=4 |pages=1993–2004 |doi=10.1104/pp.112.206383 |pmc=3613470 |pmid=23388118}}</ref> The isomerism of the (+)-sabinol intermediate varies among thujone-producing plants; for instance, in the [[western redcedar]] (''[[Thuja plicata]]''), thujone is derived exclusively from the (+)-''trans''-sabinol intermediate ('''9-1''') whereas in the [[Garden sage|common garden sage]] (''[[Salvia officinalis]]''), thujone is formed from the (+)-''cis''-sabinol intermediate ('''9-2''').<ref>{{cite journal |last1=Gesell |first1=Andreas |last2=Blaukopf |first2=Markus|date=May 2015 |title=The Gymnosperm Cytochrome P450 CYP750B1 Catalyzes Stereospecific Monoterpene Hydroxylation of (+)-Sabinene in Thujone Biosynthesis in Western Redcedar |journal=Plant Physiology |volume=168 |issue=1 |pages=94–106 |doi=10.1104/pp.15.00315 |pmc=4424034 |pmid=25829465}}</ref>

[[File:Proposed-synthesis-of-thujone-from-sabinene.png|center|700px|Proposed synthesis of thujone from sabinene]]