Notes on the Biogenesis of Essential Oils
Copyright ©Tony Burfield May 2005.

Many full-time aromatherapy college study courses now require a rudimentary knowledge of the biogenesis of essential oils. A recent non-scientific survey of aromatherapy teaching materials by this author revealed a poor & outdated comprehension of the subject (hence the generation of these notes and references).

Essential oils are the fragrant volatile products of the secondary metabolism of plants, and are generally are composed of:

1. Volatile materials synthesised via the biogenic precursor isopentenyl pyrophophosphate (IPP), consisting of complex mixtures of mono- and sesqui-terpene hydrocarbons, and (usually monofunctional) oxygenated materials biogenically derived from them [i.e. put simply: terpenoids].

2. Phenyl propanoids from the shikimic acid pathway, and their corresponding biotransformation products [put simply: many aromatics].

3. Other substances arising from the metabolism of fatty acids and amino acids, nitrogen & sulphur compounds, and artefacts arising from the degradation of natural components during the isolation of essential oils from plant material, and those arising during storage. Taking a few examples, aliphatic (unsaturated, non-terpenoid) undecatriene hydrocarbons contribute to the odour of galbanum oil Ferula galbaniflua, and the heterocyclic compound indole contributes importantly to the odour profiles of several floral oils and absolutes, including jasmine absolute from Jasminum grandiflorum & J. sambac. The odour profile of Coriander oil Coriandrum sativum includes contributions from substituted pyridines, thiazoles and pyrazines, such as 2,5 and 2,6-dimethyl pyrazines, whose odour notes can be characterised as roasted, nutty and chocolate-like, with green undertones. Cis-3-hexenol with its´ intense green grass odour, a major component of green tea essential oil Camellia sinensis, and a minor component of many herbaceous oils, together with other unsaturated C6 aldehydes, arises in green tissues whenever they are cut or attacked by insects. It is produced by the enzymatic degradation of the linolenic acid produced from a branch of the oxylipin pathway & catalyzed by the hydroperoxide lyase enzyme (Hatanaka et al. 1987).

In many plants essential oil components are either found free, or bound up as glycosides, which are thought to play a role in biogenesis of terpenoids as transport forms or as accumulation forms in undifferentiated cell cultures.

Older Theories of Terpenoid Biogenesis.

Isoprene Rule: Wallach (1887) proposed that monoterpenoids were hypothetically constructed by linkage of isoprene units (in head to tail form).

(Head)   (Tail)

Problems with this theory include the fact that satisfactory evidence of a biogenic route from isoprene, although it is a natural product, has never been put forward. Secondly, the isoprene rule does not easily explain the formation of irregular terpenoids, such as the tropolones found in oil of Western Red Cedar Thuja plicata, or some cyclopentane iridoids (monoterpene lactones) – which Banthorpe (1994) describes as derivable from GPP by any simple cation route [by now Jensen & Schripsema (2002) have described the biosynthesis of iridoid glycosides via iridodiol in the Gentiaceae]. Similarly the biogenesis of some common irregular compounds, such as lavandulol in lavender oil, could not be explained [now explained via the cleavage of a substituted cyclopropane  compound formed by the condensation of DMAPP with DMAPP]. 

Ruzicka´s Biogenic Isoprene Rule (1959): proposed that terpenes are built up from the biogenic 5-carbon compound isoprene (2-methyl-1,3 butadiene).

This rule fitted in with the elucidation of the structures of camphor and alpha-pinene by head to tail addition of biogenic isoprene units. The word turpentine, in which alpha-pinene occurs, gave rise to the compound class: ´terpene´. Ruzicka’s biogenic isoprene rule can now be taken to mean that sub-groups of terpenoids were built from a single parent compound (permit the possibility of rearrangements during biosynthesis), and that parent compounds were related in a homologous fashion (see table 1).

e.g. alpha-pinene, a bicycyclic monoterpene hydrocarbon based on a pinane  structure, which has 10 carbon units and can be built up from 2 the attachment of two isoprene units (represented by thick lines in diagrams below); and the sesquiterpene b-caryophyllene, based on the caryophyllane 15 carbon skeleton, can be built from the attachment of  3 isoprene units (see thick lines below):

Diagram 1: Building up Terpenoid Structures [after Pybus & Sell (1999)].

There are a large number of possible terpenoid skeletons which are outside the scope of this brief summary, but which can be seen in publications such as that of Teisseire (1994).

More Modern theories : Mevalonic Acid Pathway to Terpenoids.

An enzymically modulated pathway route is envisaged to proceed via the formation of the 6-carbon compound 3R(+)-mevalonic acid, thought as the precursor of all terpenoids.

Isopentenyl pyrophosphate (syn. 3-methylbutenyl pyrophosphate) can be regarded as the biogenic equivalent to isoprene. e.g. for monoterpenoids:

Glucose -> Acetyl CoA (from citric acid cycle) -> Acetylacetyl CoA + Acetyl CoA -> 3-hydroxy-3-methylglutaryl CoA -> mevalonic acid -> 5-phoshomevalonate -> 5 pyrophospho mevalonate -> IPP (isopentenyl pyrophosphate) + DMAPP (dimethylallyl pyrophosphate) -> geranyl pyrophosphate (GPP) -> monoterpenes.

The formation of neryl pyrophosphate from GPP provides the carbocation A (see below) which is easily deprotonated or rearranged to a wide range of cyclic, acylic or bicyclic skeletons. The skeletal terpene framework can be re-arranged, oxidised, reduced, hydrated etc. to produce range of terpene products, according to the activity of individual terpene cyclases. In higher plants, monoterpene synthesis in generally acknowledged to occur in plastids, whereas sesquiterpene biosynthesis takes place in the cytosol (Kesselmeier & Staudt 1999).

  ⁺⁺   ->

  ^(or) 

Diagram 2. Showing GPP built up from IPP & DMAPP.

  -> -> -> ->

Diagram 3. Production of alpha-pinene from GPP

GPP + IPP ->

Diagram 4. Showing FPP built up from GPP.

 Sesquiterpenes are thus generated from farnesyl pyrophosphate; diterpenes from geranylgeranyl pyrophosphate (see below).

Table 1:  Terpenoids, Precursors and their Natural Occurrence, modified from  Bancroft (1994).

[Key: IPP = isopentenyl pyrophophate; DMAPP = 3,3-dimethylallyl pyrophoshate; GPP= geranyl pyrophosphate; FPP = 2E,6E-farnesyl pyrophosphate; GGPP =  2E, 6E, 10E-geranylgeranylpyrophosphate; GFPP = 2E,6E,10E-14E-geranylfarnesyl pyrophoshate].

The Mevalonate Independent Pathway.

Criticisms of the mevalonate pathway have included the poor incorporation of radio-labelled ¹⁴C into carotenoids from plant chloroplasts (Rohmner 1999). Recently a new pathway (a plastid-localised ‘mevalonate independent’ pathway or ‘MEP pathway’) -> IPP, via the condensation of pyruvate and glyceraldehydes-3-phoshate to generate 1-deoxy-D-xylulose 5-phosphate, followed by intramolecular rearrangement and reduction to 2-C-methylerythritol 4-phosphate, has been discovered in higher plants and certain eubacteria including Escherichia coli. It is thought that the overall formation of IPP in higher plants runs independently in these two pathways, the mevalonate pathway being confined to the cystostolic compartment only.  An article discussing the state of the art knowlege of the MEP pathway was published by Rohmer (2003) available at http://www.iupac.org/publications/pac/2003/pdf/7502x0375.pdf. According to Kesselmeir & Staudt, the following pathway can be postulated:

Pyruvate -> Hydroxy ethyl thiamine pyrophosphate (+ 3-glycerylaldehyde-3-phoshate) -> 1-deoxy-D-xylulose 5-phosphate -> IPP.

Thus the MEP pathway to IPP formation mechanism can potentially facilitate the formation of isoprene, carotenoids, and other isoprenoids in higher plants (Schwender et al., 1996; Lichtenthaler et al., 1997; Zeidler et al., 1997).

Shikimic Acid Pathway.

Shikimic acid is formed from glucose in plants, and is the biogenic precursor of the amino acids L-phenylalanine, L-tyrosine and L-typtophan. Pathways from shikimic acid generate anthranilates (e.g. in mandarin oil Citrus reticulata), cinnamates (e.g. in peru balsam oil Myroxylon pereirae) and other phenylpropanoids, and from this point on to other metabolites such as lignans & flavononoids. In particular, phenyl propanoids (basically  compounds with a 3-carbon chain attached to a benzene ring) are formed from trans or (E)-cinnamic acid via the elimination of ammonia from L-phenylalanine. Common phenylpropanoids in essential oils include methyl chavicol, methyl eugenol, eugenol, methyl cinnamate, vanillin & anethole.

Curiously not too much is known about certain aspects of phenyl propnaol accumulation in plants. An investigation of accumulation of phenylpropenes in the two types of glandular trichomes (peltate & capitate) in two chemotypes of Sweet Basil (eugenol & methyl chavicol) oil leaves (Gang 2001) revealed that the eugenol and methyl chavicol accumulate almost exclusively in the peltate glands and the putative enzymes in the pathway are almost exclusively confined to the peltate class. A simplified pathway for the production of methyl chavicol from phenylalanine was given as follows:

 ->  ->  ->  ->

 ->   ->   ->   

References for further reading:

Banthorpe D.V. (1994) “Terpenes” In Mann J., Davidson RS., Hobbs J.B., Banthorpe D.V. & Harborne J.B. Natural Products: Longman Scientific & Technical 1994.

Bu’Lock J.D. (1965) The Biosynthesis of Natural Products McGraw-Hill.

Croteau R. (1981) in Biosynthesis of Isoprenoid Compounds, (eds J.W. Porter et al.), Wiley, New York, Vol. 1, p. 225.

Croteau, R. (1987) “Biosynthesis and Catabolism of Monoterpenoids” Chem. Rev., 87, 929.

Croteau, R. et al. (1994), Recent Adv. Phytochem., 28, 193.

Gang D.R. et al. (2001) “An Investigation of the Storage and Biosynthesis of Phenylpropenes in Sweet Basil” Plant Physiol 125, 539-555.

Hatanaka A., Kajiwara T., Sekiya J. (1987) “Biosynthesis pathway for C6-aldehydes formation from linolenic acid in green leaves.” Chem Phys Lipids 44, 341–361.

Hill, R.A. (1993) in The Chemistry of Natural Products, 2nd edn (ed. R.H. Thomson), Blackie, Glasgow, pp. 107.

Jensen S.R. & Schripsema J. (2002) “Chemotaxonomy and pharmacology of Gentianaceae” In Gentianaceae: Systematics & Natural History eds Struwe L. & Albert V. Cambridge Univ. Press 2002.

Kesselmeier J. & Staudt M. (1999) “Biogenic Volatile Organic Compounds (VOC): An Overview on Emission, Physiology and Ecology” Journal of Atmospheric Chemistry 33: 23–88, 1999.

Lichtenthaler H. K., Schwender L., Disch A. & Rohmer M. (1997) “Biosynthesis of isoprenoids in higher plant chloroplasts proceeds via a mevalonate independent pathway” FEBS Lett. 400, 271–274.

Pybus D.H. & Sell C.S. The Chemistry of Fragrances RSC 1999.

Rohmer M. (1999). In Comprehensive Natural Product Chemistry. Isoprenoids Including Carotenoids and Steroids, D. E. Cane (Ed.), Vol. 2, pp. 45–67, Pergamon, Oxford, UK (1999) and references therein (through Romer 2003).

Schwender J., Seemann M., Lichtenthaler H. & Rohmer M. (1996) “Biosynthesis of isoprenoids, carotenoids, sterols, prenyl sidechains of chlorophylls and plastoquinone) via a novel pyruvate/glyceraldehydes 3-phosphate non-mevalonate pathway in the green alga Scenedesmus, Biochem. J. 316, 73–80.

Teisseire P.J. (1994) Chemistry of Fragrant Substances VCH Publishers Inc.

Zeidler J. G., Lichtenthaler H. K., May H. U. & Lichtenthaler, F.W. (1997) “Is isoprene emitted by plants synthesized via the novel isopentenyl diphosphate pathway?” Z. Naturforsch. 52c, 15–23.