Active isoprene

During the second step, mevalonic acid is implicated in a number of enzymic reactions involving ATP, and is converted to isopentyl pyrophosphate and to its isomer 3,3-dimethylallyl pyrophosphate. Actually, the two compounds constitute the active isoprene , which... 

Formally, isoprenoids are derived from a single common building block, isoprene (2-methyl-l,3-butadiene), a methyl-branched compound with five C atoms. Activated isoprene, isopentenyl diphosphate, is used by plants and animals to biosynthesize linear and cyclic oligomers and polymers. For the isoprenoids listed here—which only represent a small selection—the number of isoprene units (1) is shown. 

From activated isoprene, the metabolic pathway leads via dimerization to activated geraniol (1 = 2) and then to activated farnesol = 3). At this point, the pathway divides into two. Further extension of farnesol leads to chains with increasing numbers of isoprene units—e.g., phytol (1 = 4), dolichol (1 = 14-24), and rubber = 700-5000). The other pathway involves a head-to-head linkage between two farnesol residues, giving rise to squalene (1 = 6), which, in turn, is converted to cholesterol (1 = 6) and the other steroids. 

Cholesterol is one of the isoprenoids, synthesis of which starts from acetyl CoA (see p. 52). In a long and complex reaction chain, the C27 sterol is built up from C2 components. The biosynthesis of cholesterol can be divided into four sections. In the first (1), mevalonate, a Ce compound, arises from three molecules of acetyl CoA. In the second part (2), mevalonate is converted into isopen-tenyl diphosphate, the active isoprene. In the third part (3), six of these C5 molecules are linked to produce squalene, a C30 compound. Finally, squalene undergoes cycliza-tion, with three C atoms being removed, to yield cholesterol (4). The illustration only shows the most important intermediates in biosynthesis. 

Synthesis takes place in four stages, as shown in Figure 21-33 (D condensation of three acetate units to form a six-carbon intermediate, mevalonate (2) conversion of mevalonate to activated isoprene units (3) polymerization of six 5-carbon isoprene units to form the 30-carbon linear squalene and ( ) cyclization of squalene to form the four rings of the steroid nucleus, with a further series of changes (oxidations, removal or migration of methyl groups) to produce cholesterol. 

FIGURE 21-35 Conversion of mevalonate to activated isoprene units. Six of these activated units combine to form squalene (see Fig. 21-36). The leaving groups of 3-phospho-5-pyrophosphomevalonate are shaded pink. The bracketed intermediate is hypothetical. 

Stage (J) Condensation of Six Activated Isoprene Units to Form Squalene Isopentenyl pyrophosphate and dimethylallyl pyrophosphate now undergo a head-to-tail condensation, in which one pyrophosphate group is displaced and a 10-carbon chain, geranyl pyrophosphate, is formed (Fig. 21-36). (The head is the end to which pyrophosphate is joined.) Geranyl pyrophosphate undergoes another head-to-tail condensation with isopentenyl pyro-... 

Cholesterol is formed from acetyl-CoA in a complex series of reactions, through the intermediates /3-hydroxy-/3-methylglutaryl-CoA, mevalonate, and two activated isoprenes, dimethylallyl pyrophosphate and isopentenyl pyrophosphate. Condensation of isoprene units produces the noncyclic squalene, which is cyclized to yield the steroid ring system and side chain. 

The biosynthesis of monoterpenoids and camphor has been described by several authors (108-llU). Ruzicka (115,116) proposed a unified biogenetic scheme for terpenes. The biosynthetic building blocks for these terpenes are iso-prene units. The biosynthetically active isoprene units are isopentenyl pyrophosphate [l] and dimethyl allyl pyrophosphate [2] the compounds that are derived from acetate via mevalonic acid (Scheme V). Geranyl pyrophosphate [3] is the C-10 precursor for the terpenes (117). Banthorpe and Baxendale (ll8) confirmed the biosynthetic pathway of (iamphor via acetate mevalonate by conducting degradation study of camphor, biosynthesized from l c labelled mevalonic acid. The biosynthesis of camphor is summarised in Scheme VI. 

Mevalonic acid, the precursor of the active isoprene, was used in the incorporation studies. Both mevalonic acid-2-C14 and mevalonic acid-2T or -4T were utilized by the fungus for the synthesis (131, 132). By means of a stagewise degradation of radioactive alkaloids formed with the aid of mevalonic acid-2-C14 and localization of the C14, it could be shown that mevalonic acid is incorporated into the molecule in the manner depicted by the accompanying reaction scheme (133, 134, 135). 

The addition of mevalonic acid-l-C14 to a pyroclavine- and festuclavine-producing fungus strain yielded inactive alkaloids which, in agreement with the scheme, showed that the carboxyl group of the mevalonic acid is not incorporated. Lowering of the assimilation of mevalonic acid-2-C14 by the addition of dimethylallylpyrophosphate or isopentenylpyrophos-phate supported the assumption that mevalonic acid enters the alkaloid molecule via one of these activated isoprene radicals. This was confirmed by the incorporation of deuterated isopentenylpyrophosphate in alkaloids of the clavine type in saprophytic cultures of a Claviceps strain (128). 

Stage one is the synthesis of isopentenyl pyrophosphate, an activated isoprene unit that is the key building block of cholesterol. 

Mevalonate is converted into 3-isopentenylpyrophosphate in three consecutive reactions requiring ATP (Figure 26.8). Decarboxylation yields isopentenyl pyrophosphate, an activated isoprene unit that is a key building block for many important biomolecules throughout the kingdoms of life. We will return to a discussion of this molecule later in the chapter. 

It is an important compound, its pyrophosphate sometimes called active isoprene being a major key compound in the biosynthesis of isoprenoids. 

Cholesterol is formed biosynthetically from isopentenyl pyrophosphate (active isoprene). The majority of cholesterol in the body derives from de novo biosynthesis in the liver [1,2]. Cholesterol synthetic pathway has been assumed to occur primarily in the cytoplasm and endoplasmic reticulum (ER). However, more recent evidences have suggested that the enzymes, except squalene synthase, squalene epoxidase and oxidosqualene cyclase, are partly localized in the peroxisomes, which are essential for normal cholesterol synthesis [11]. 

Fig. 34.5. The formation of activated isoprene units (A -isopentenyl pyrophosphate and dimethylallyl pyrophosphate) from mevalonic acid. Note the large ATP requirement for these steps.

E) Condensation of six activated isoprene units to form squalene... 

Biosynthesis The biosynthesis of the C. proceeds in chloroplasts or chromoplasts in analogy to other iso-prenoids by way of 3-isopentenyl diphosphate ( active isoprene ). The colorless 15-cis-phytoene (C40) is formed from 2 molecules of geranylgeranyl diphosphate, further transformations furnish, wiaall-trans- -carotene (colored), all-trans- ycopenc. Subsequent cy-clization reactions allow the formation of other carotinoids. Xanthophvlls are formed from C. by incorporation of oxygen... 

Biosynthesis See umbelliferone the furan ring originates from activated isoprene (dimethylallyl pyrophosphate). Key step in psoralene biosynthesis is the oxidative dealkylation of (+)marmesine, which in one step is converted to psoralene and acetone by a cytochrome P450 enzyme. 

In 1921, Leopold Ruzicka (1887-1976) established the isoprene rule for the biosynthesis of terpenes through a formal head-to-tail coupling of isoprene units. Deviations from the rule (as in the case of chrysanthemic acid), as well as Wagner-Meerwein rearrangements, occasionally complicate the analysis. Only in the mid-1950s it could be shown that terpenes are indeed formed by active isoprene emits . [24,25]... 

Lynen and co-workers (1959) were the first to realize that dimethylallyl pyrophosphate (6) corresponds to Ruzicka s active isoprene and that this compound (and homologous allylic pyrophosphates) is involved in repetitive condensations with isopentenyl pyrophosphate (4) (Poulter and Rilling, 1981). 

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