Isopentenoid compounds

Sterols. Sterols and terpenes are both isopentenoid compounds. 

Like the fatty acids, their biosynthesis begins with acetate, d)ich undergoes a series of reactions forming acetoacetate, hydroxy-methylglutarate and finally mevalonate. Mevalonic acid is the precursor to all the isopentenoid compounds. Through a further series of reactions mevalonic acid is converted to Isopentenyl pyro-... 

Other lipids. Waxes are major lipids in a few organisms (e.g., jojoba, sperm whale). Cutins (condensation polymers of hydroxy fatty acids) are discussed in a later chapter (Kolattukudy et el.. Chap. 10). Hydrocarbons other than isopentenoid compounds occur in a variety of species. 

A basic question in which our laboratory has been interested over the last few years revolves around the problem of MVA synthesis and the flow of this compound to the major isopentenoid compounds in the plant cell. The pathway for the biosynthesis of polyisoprenoids and the position of the key-resulatins enzyme HMG-CoA reductase is summarized in Figure 1. Our studies and those of other groups have revealed that the regulatory role of HMG-CoA reductase does not seem to be confined only to mammals (1-8). but can also be extended to plants (9-22) and fungi (23-27). 

Isoprenoids (isopentenoids). The name for a group of natural products made up of isoprene units (e.g., ses-qui-, di-, and triterpenes, iridoids, carotinoids, steroids, natural rubber, etc.). Many non-isoprenoid compounds, however, do possess isoprenoid side chains, e.g., tocopherols, ubiquinones, chlorophyll, or contain isoprenoid structures incorporated into their skeletons, e.g., monoterpenoid indole alkaloids, penitrems, Cinchona alkaloids. 

Since the level of cholesterol was highest in the vegetative apices before their transition to flowering (Table 1), and cholesterol levels actually decreased during floral differentiation, it is also possible that cholesterol is in some way associated with the process of floral initiation. For instance, there is some evidence in the literature that the flower-inducing hormone may be an isopentenoid or steroid-like compound Various steroids occur in plants, and cholesterol has been shown to be a key intermediate in their biosynthesis Ojll. Thus, it is possible that cholesterol was being used to make steroids, which may be involved in floral induction. 

Compounds without biochemical precedent may derive from hitherto unknown or defunct biosynthetic pathways, or be geological transformation products. The geological record of isopentenoids reflects biochemical evolution. The diverse range, including steroids and hopenoids, in sediments billions of years old demonstrates the antiquity of their biosynthetic pathways and other occurrences coincide with the appearance of their putative source organisms. 

Efforts to evaluate ancient records prompts an exploration of the factors that influence the sources of organic matter and their alteration during and after deposition (9). Toward this objective, studies of contemporary depositional settings can provide evidence of the principal influences on the production and preservation of organic matter in the rock record. Most importantly, a detailed perspective of contem rary processes permits consideration of the varied controls that determine the survival of both total organic matter and specific organic compounds like isopentenoids. 

Sulfur Incorporation. It is also possible for unsaturated and polyunsaturated lipids to survive in sediments by intra- or inter-molecular incorporation of sulfur (75, 34). The wide variety of organo-sulfur compounds (OSC) recognized in sediments in recent years (34,35) demonstrates the viability and prevalence of such reactions. The structures of OSC and the inferred sites of sulfur incorporation demonstrate that this process preserves molecular characteristics that would otherwise be expected to be lost by degradation. For example, specific OSC derive from and retain the structural features of carotenoids and other polyunsaturated isopentenoids (34,55). 

Overall, this small selection of compounds whose occurrences can be well defined temporally provides an illustration of the extent to which molecular analyses of sediments and petroleums can provide evidence of evolutionary developments in isopentenoid biosynthesis. 

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