Isoprenoid derivatives

Bark beetles primarily utilize isoprenoid derived pheromones [100,101] and have been the most studied regarding their biosynthesis [8,98]. Earlier work indicated that the isoprenoid pheromones could be produced by the beetle altering host derived isoprenoids however more recent work indicates that for the most part bark beetles are producing pheromones de novo. The production of isoprenoids follows a pathway outlined in Fig. 4 which is similar to the isoprenoid pathway as it occurs in cholesterol synthesis in mammals. Insects cannot synthesize cholesterol but can synthesize farnesyl pyrophosphate. Insects apparently do not have the ability to cyclize the longer chain isoprenoid compounds into steroids. The key enzymes in the early steps of the isoprenoid... 

Ubiquinones and plastoquinones, also isoprenoid derivatives, function as electron carriers in mitochondria and chloroplasts, respectively. 

Goldstein, I. L., and M. S. Brown, Regulation of the mevalon-ate pathway. Nature 343 425-430, 1990. This article describes regulatory mechanisms for cholesterol biosynthesis within the context of the regulation of the biosynthesis of other isoprenoid derivatives. 

A similar cyclization sequence, incorporating a functionalized pyridine-containing unit rather than an isoprenoid derivative, is thought to give rise to the acromelic acids. It has been suggested that the various... 

B. IsopRENOiD Derivatives as Chemical Tools to Interrogate Prenyltransferases and Prenyl Cyclases... 

Since their discovery over 30 years ago, statins have proven to be remarkably useful agents in both the basic science and clinical arenas. Worldwide it is estimated that 25 million people are taking these agents. This number could further increase as we learn more about the potential use of statins in other disorders. Better understanding of the role of isoprenoids and isoprenoid-derivatives in human health and disease will undoubtedly lead to the identification of new therapeutic targets and pharmaceutical agents. 

Buriat, V., Oudin, A., Courtois, M., Rideau, M. and St. Pierre, B. (2004) Co-expression of three MEP pathway genes and geraniol-lO-hydroxylase in internal phloem parenchyma of Catharanthus roseus implicates multicellular location of intermediates during the biosynthesis of monoterpene indole alkaloids and isoprenoid-derived primary metabolites. Plant., 38,131 1. 

HMG-CoA reductase is an integral protein of the endoplasmic reticulum and the primary site of regulation of synthesis of cholesterol and nonsterol isoprenoid derivatives. Its activity has a well-defined diurnal rhythm in rats and mice, coinciding with that of the enzyme s synthesis and of the mRNA concentration. Activity is highest at about the middle of the dark period and lowest at about the middle of the light period. Its mechanism may be related to food consumption. Rats are nocturnal animals and consume food in the dark the increased bile production and excretion depletes liver cholesterol and may stimulate the increased synthesis of HMG-CoA reductase as a compensatory mechanism. 

It is unknown how the fatty chain carbonic acids of the lipids originally formed. It is noteworthy that fatty acids with up to six and seven C-atoms were detectable in the Murchison meteorite. Thus, it might be possible that membranes that were even more primitive existed in the past. However, dicarboxylic acids of chain lengths up to 17 were detectable after hydrothermal treatment. It is also remarkable that isoprenoid derivatives - more precisely, phytanyl and biphytanyl, that is CIO and C20 chains - linked to each other by monoether or diether bridges are part of membranes of the Archaea (Fig. 6.2, bottom structure). 

Cholesterol serves as the biosynthetic precursor for several vital compounds, including a variety of steroid hormones and bile acids. Many of these compounds, and many other polyisoprenoid compounds biosynthetically related to cholesterol, act biologically as important regulatory compounds [105]. In mammals such regulatory compounds include steroid hormones and vitamins A and D. Steroids and other isoprene derivatives also play important regulatory roles in other phyla. Several insect hormones, for example, are isoprenoid derivatives [106] (cf. Chapter 8). Memy of the floral scents of plants are isoprene derivatives. 

The classification of food colorants on a chemical basis is also not always clear. Although Proudlove (1994) and DeMann (1980) agree on three chemical classes for natural food colorants that is, isoprenoid derivatives, tetrapyrrole derivatives and benzopyran derivatives, DeMann (1980) also includes a fourth group called artefacts to include the melanoidins and caramels. Table 7.1 shows the classification based on chemical stmcture, the fourth group artefacts has been included as this is relevant to the discussion of chemical stmcture and light absorption. Table 7.1 also includes the names used by Dalzell (1997) in her classification. 

Fig. 7.1 Isoprenoid derivatives (a) formation of a terpene from 2 isoprene units (b) structure of lycoprene - a tetraterpene showing the isoprene units.

The diterpenoids are a large and ubiquitous family of isoprenoids derived from 2E,61% ] 0 -geranylgeranvl pyrophosphate. Of all the families of natural products, the diterpenoids have one of the widest ranges of biological activity. The clearest classification of the cyclic diterpenoids (abietane, labdane, clerodane,...) is based on biogenetic considerations. The variety of structural types found amongst the diterpenoids has led to their use as phytochemical markers. Indeed, many of the diterpenoids found in recent years have been isolated in the course of phytochemical surveys of the Compositae. 

All native cytokinins are derivatives of adenine with at least one substituent (at position). According to this substituent, these compounds may be classed into (1) isoprenoid (zeatin, A -A -isopentenyladenine and their derivatives), (2) isoprenoid-derived (dihydrozeatin and its derivatives) and (3) aromatic cytokinins. Native cytokinins and their derivatives are summarised in Fig. 1 together with abbreviations used here. 

De novo formation of isoprenoid and isoprenoid-derived cytokinins... 

To date there is no report about the biosynthesis of aromatic cytokinins. In view of the dissimilarity between the aromatic and the isoprenoid(-derived) A side chains it is likely that their biosynthetic pathways are quite different. Phenylalanine may be considered as a starting compound and benzaldehyde and/or hydroxylated benzaldehydes as immediate side chain precursors. However, the existence of some crossing-points between aromatic and isoprenoid side chain formation cannot be completely excluded. There is also the possibility that the enzymes of adenine and/or purine metabolism, which are not strictly specific, may catalyse some mutual conversions among BA-bases, nucleosides and nucleotides [81]. 

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