The Biosynthesis of C5–C20 Terpenoid Compounds

BY J. R. HANSON

1 Introduction

This chapter, covering 1973, follows the pattern of the previous Reports. A number of reviews on general aspects of terpenoid biosynthesis have appeared. Significant advances have been recorded in the establishment of cell-free systems that mediate stages of terpenoid biosynthesis and in the use of mutants in the investigation of biosynthetic sequences.

2 Mevalonic Acid

There are two pathways leading to the formation of acetoacetate. In the first acetoacetyl co-enzyme A is formed directly from two moles of acetyl co-enzyme A. It is this pathway which is implicated in HMG CoA biosynthesis. In the second, malonyl CoA is involved leading to an enzyme-bound acetoacetate as in fatty acid biosynthesis. Malonate has been shown to be incorporated into HMG CoA and into ergosterol, but the labelling patterns were consistent with decarboxylation prior to incorporation. The incorporation into mevalonate by rat liver preparations was related to the malonyl CoA decarboxylase activity present in this preparation. Mevalonate has not hitherto been identified in the invertebrates. However, its biosynthesis has now been demonstrated in a number of tissues from the fly, Sarcophaga bullata. The block in sterol biosynthesis appears to exist in the condensation of farnesyl pyrophosphate to squalene. Some attempts to distinguish different forms of mevalonate kinase from green leaves and etiolated cotyledons of the French bean, Phaseolus vulgaris, have been described.

3 Hemiterpenoids

The formation of the isoprenoid portion of the furanocoumarins has continued to receive attention. The positions of 7-demethylsuberosin and osthenol in furanocoumarin biosynthesis have been examined. 7-Demethylsuberosin (1), which is formed by prenylation of umbelliferone, was detected by trapping experiments and shown to be a precursor of the linear furanocoumarins such as psoralen (2) and bergapten (3) in Conium maculatum and Heracleum lanatum. Similar results had been obtained previously with Angelica archangelica. Osthenol (4) was a precursor of the angular furanocoumarins such as angelicin, isobergapten, and sphondin. The biosynthetic routes to psoralen (2), bergapten (3), and xanthotoxin (5) have been studied in cell-cultures obtained from Ruta graveolens. 7-Demethylsuberosin (1) and marmesin (6) were shown to be good precursors of these substances in this system confirming the previous results. A similar system derived from Ruta graveolens has been used to study the formation of edulinine and the furoquinoline alkaloids from quinoline derivatives.

Two different pathways for anthraquinone biosynthesis are known. One is entirely based on acetate whilst the other involves shikimic acid and glutamic acid, which afford o-succinoylbenzoic acid which is then converted into a naphthalene and prenylated. Labelling results suggest that ring C of alizarin (7), produced by Rubia tinctorum, and morindone (8), produced by Morinda citrifolia, are derived from mevalonate via dimethylallyl pyrophosphate. Isoprene has been reported as light-dependent natural plant emission from leaf discs of Hamamelis and its 'action spectrum' has been described.

4 Monoterpenoids

With a few exceptions, the incorporation of mevalonate into monoterpenes is poor and in a number of instances extensive randomization of the label has been observed. In several cases mevalonoid labels have been found predominantly in those parts of the molecule derived directly from isopentenyl pyrophosphate with virtually no label in the starter unit derived from dimethylallyl pyrophosphate. The non-uniform labelling of geraniol biosynthesized from 14C carbon dioxide in Perlargoniuum graveolens has been studied. The geraniol was isolated up to 24 h after an initial exposure of 2 h. Degradation showed a greater turn-over of label in the isopentenyl pyrophosphate portion than in the dimethylallyl pyrophosphate portion of geraniol. At first the label was approximately equally divided between the two portions but after 12 h the proportion of the label associated with the isopentenyl pyrophosphate derived half increased to 78%. Subsequently the preferential labelling decreased and approached an equal distribution. These results suggest that the geranyl pyrophosphate which was isolated at first arose as a result of leakage from sites of higher terpenoid biosynthesis, whereas the labelling of the later material was affected by the existence of a dimethylallyl pyrophosphate pool and compartmentalization of isoprenoid biosynthesis.

A cell-free system derived from Mentha piperita, has been established which mediates the cyclization of neryl pyrophosphate (9) to [apha]-terpineol (10). Similar systems were also obtained from Mentha spicata and Daucus carota. Competing phosphatase activity was inhibited by sodium fluoride. Changes in the monoterpene composition of the essential oil of Mentha aquatica have been brought about 18 by genetic substitution from a high limonene yielding strain of Mentha citrata extending studies reported previously with pulegone metabolism. It has been shown that one of the first steps in the formation of the iridoid monoterpenes and the monoterpene portion of the indole alkaloids, is the hydroxylation of geraniol and nerol at C-10. A microsomal mixed-function oxidase, capable of mediating this step, has now been isolated from Vinca rosea. Model systems have been studied 20 for the biological oxidation of monoterpene hydrocarbons based on the photo-oxygenation of α-thujene (11) and sabinene (12) in the presence of chloroplast preparations from Tanacetum vulgare or Juni perus sabina and various dyes.

trans-Verbenol (14) is the principal aggregating pheromone of the bark beetle, Dendroctonus ponderosa. Exposure to the oleoresin from the host-tree, Pinus monticola or to α-pinene (13) increases the production of this pheromone.

There has been considerable interest and speculation on the formation of the irregular monoterpenes. The various theories have been reviewed and a unified hypothesis based on an analogy with presqualene alcohol, has been proposed (Scheme 1). The initial step involves the head-to-head dimerization of two moles of dimethylallyl pyrophosphate to form a chrysanthemyl pyrophosphate. However, studies on the biosynthesis of chrysanthemum monocarboxylic acid from [4(R)-4-3H] mevalonic acid have shown that the label is unequally distributed between the two C5 moieties. The suggestion was made that the C5 isopentane unit leading to the cyclopropane part of the molecule is not derived directly from mevalonic acid....