Search

The creosote bush (Larrea tridentata) harbors members of the monolignol acyltransferase, allylphenol synthase, and propenylphenol synthase gene families, whose products together are able to catalyze distinct regiospecific conversions of various monolignols into their corresponding allyl- and propenyl-phenols, respectively. In this study, co-expression of a monolignol acyltransferase with either substrate versatile allylphenol or propenylphenol synthases in Escherichia coli established that various monolignol substrates were efficiently converted into their corresponding allyl/propenyl phenols, as well as providing proof of concept for efficacious conversion in a bacterial platform. This capability thus potentially provides an alternate source to these important plant phytochemicals, whether for flavor/fragrance and fine chemicals, or ultimately as commodities, e.g., for renewable energy or other intermediate chemical purposes. Previous reports had indicated that specific and highly conserved amino acid residues 84 (Phe or Val) and 87 (Ile or Tyr) of two highly homologous allyl/propenyl phenol synthases (circa 96% identity) from a Clarkia species mainly dictate their distinct regiospecific catalyzed conversions to afford either allyl- or propenyl-phenols, respectively. However, several other allyl/propenyl phenol synthase homologs isolated by us have established that the two corresponding amino acid 84 and 87 residues are not, in fact, conserved., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The creosote bush (Larrea tridentata) accumulates a complex mixture of 8-8' regiospecifically linked lignans, of which the potent antioxidant nordihydroguaiaretic acid (NDGA) is the most abundant. Its tetra-O-methyl derivative (M4N) is showing considerable promise in the treatment of refractory (hard-to-treat) brain and central nervous system tumors. NDGA and related 9,9'-deoxygenated lignans are thought to be formed by dimerization of allyl/propenyl phenols, phenylpropanoid compounds that lack C-9 oxygenation, thus differentiating them from the more common monolignol-derived lignans. In our ongoing studies dedicated towards elucidating the biochemical pathway to NDGA and its congeners, a pinoresinol-lariciresinol reductase homologue was isolated from L. tridentata, with the protein obtained in functional recombinant form. This protein efficiently catalyzes the conversion of p-coumaryl and coniferyl alcohol esters into the corresponding allylphenols, chavicol and eugenol; neither of their propenylphenol regioisomers, p-anol and isoeugenol, are formed during this enzyme reaction.

Phenylpropenes such as chavicol, t-anol, eugenol, and isoeugenol are produced by plants as defense compounds against animals and microorganisms and as floral attractants of pollinators. Moreover, humans have used phenylpropenes since antiquity for food preservation and flavoring and as medicinal agents. Previous research suggested that the phenylpropenes are synthesized in plants from substituted phenylpropenols, although the identity of the enzymes and the nature of the reaction mechanism involved in this transformation have remained obscure. We show here that glandular trichomes of sweet basil (Ocimum basilicum), which synthesize and accumulate phenylpropenes, possess an enzyme that can use coniferyl acetate and NADPH to form eugenol. Petunia (Petunia hybrida cv. Mitchell) flowers, which emit large amounts of isoeugenol, possess an enzyme homologous to the basil eugenol-forming enzyme that also uses coniferyl acetate and NADPH as substrates but catalyzes the formation of isoeugenol. The basil and petunia phenylpropene-forming enzymes belong to a structural family of NADPH-dependent reductases that also includes pinoresinol-lariciresinol reductase, isoflavone reductase, and phenylcoumaran benzylic ether reductase.