Biosynthesis of Sterols

The dwarf mutants of pea (Ikb) [34] and Arabidopsis dim) [35,36] accumulate 24-methylenecholesterol and isofucosterol and are deficient in campesterol and sitosterol. In the Ikb mutant, the levels of endogenous BRs are significantly reduced. Thus, blocked synthesis of campesterol causes dwarfism by reducing the level of endogenous BRs (Eig. 2). 

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Since carotenoids are derived for the central isoprenoid pathway (Fig. 13.3), the regulation of their formation must involve a co-ordinated flux of isoprenoid imits into this branch of the pathway as well as into others such as the biosynthesis of sterols, gibberellins, phytol and terpenoid quinones. An imderstanding of the complexities of regulation of the pathway is necessary in order to target the regulatory steps for genetic manipulation. 

Biosynthesis of Sterol-Related Triterpenes - De Novo Sterol Biosynthesis... 

Rearrangement has never been observed without the accompanying reduction.310-313 More complex rearrangements that occur during biosynthesis of sterols are described in Chapter 22. 

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

As discussed y)xevio xs, y,Saccharomyces cerevisiae does not need oxygen to obtain energy when fermenting grape juice. However, there are some essential biosynthetic pathways that use oxygen as substrate. This is the case for the biosynthesis of sterols and unsaturated fatty acids (Ratledge and Evans 1989). 

H]Squalene, 155, and [3- H]-2,3-oxidosqualene, 156, the key compounds in studies of the biosynthesis of sterols have been obtained according to the route shown in equation 53, which involves the modified Wittig reaction of [l- H]trisnorsqualene aldehyde 157 with phosphorus ylide to give 155 or with sulphur ylide to give 156 in high radiochemical yield and high purity. 

The biosynthesis of cholesterol, related steroids, and phytosterols is dealt with in this section whereas the further metabolism of these classes and the remaining nonsteroidal triterpenoids are covered in the following two sections. Reviews have appeared on the biosynthesis of sterols and higher terpenoids, the in vivo metabolism of steroids in primates" and in plant tissue culture," and dietary feedback control of cholesterol synthesis." The latter contains a reasoned defence of the hypothesis that HMG-CoA reductase is controlled by alterations to its supporting microsomal membrane. Abstracts of a symposium on all aspects of steroid biosynthesis have appeared." ... 

For obvious reasons, inhibitors of cholesterol biosynthesis have aroused intense interest. Studies on a variety of species (mostly rats) have been made using 1-alkylimidazoles," colchicine," and chenodeoxycholic acid, the last work being particularly interesting as the metabolite affected HMG-CoA reductase but not cholesterol 7a-hydroxylase, the steps believed to be rate-limiting for the biosynthesis of sterols and of bile acids respectively. Arsenite, /8-mercaptoethanol, dithiothreitol, and ethanethiol all inhibited the biosynthesis of cholesterol from MVA in rat liver homogenates. The accumulation of 4,4-dimethyl-5a-cholest-8-en-3/3-ol together with the corresponding A -diene supported the view that... 

A major breakthrough was achieved in the elucidation of the biosynthesis of sterols. Strong evidence has been obtained showing that squalene 2,3-oxide is an intermediate in the conversion of squalene to cholesterol. ... 

Sterols have been reported in cyanobacteria, with A5 C27 and C29 usually dominating (Volkman et al. 1998). However, the biosynthesis of sterols by cyanobacteria is debatable (Ourisson et al. 1987 see Section 2.4.3f), and recent studies have shown that the absolute abundance of sterols is usually low and probably attributable to contamination (Summons et al. 2002). 

The biosynthesis of sterols is extremely complicated and a good textbook in biochemistry should be consulted (e.g., Nelson and Cox, 2000). Let us recapitulate the process ... 

The biosynthesis of sterols takes place via the protracted sterol/isoprenoid biosynthetic pathway (Chapter 1). Although the major portion of the carbon flux through this pathway is normally directed into sterols, several branches exist leading to the production of other isoprenoid compounds needed by the cell, such as ubiquinone, dolichol and isopentenyl adenine. Total carbon flux is regulated through the enzymes of the early, or common, portion of the pathway of which the most important is HMG-CoA reductase. Distribution of carbon between the various end products is regulated at later stages of the pathway. 

It has also been proposed that hpids, Upid metabolites and cytosolic lipid-binding proteins interact to regulate sterol biosynthesis [199] (see also Chapter 3). Under this model, the accumulation in cells of certain hpids or free hpid intermediates inhibits sterol biosynthetic enzymes. The level of the binding protein for a particular Upid would determine the threshold concentration of Upid needed to produce inhibition [199]. Such a mechanism would allow coordination between the biosynthesis of sterols and other Upids. Fatty acyl-CoAs and Z-protein, the fatty acyl-CoA-binding protein, may modulate HMG-CoA reductase activity [199-201] since physiological... 

Biosynthesis of sterols and of dolichol are independently controlled in developing rat brain [214], in maturing mouse spermatocytes [215] and in mouse spleen cells during phenylhydrazine-induced erythropoiesis [216]. 

Compactin, a potent inhibitor of HMG-CoA reductase activity, inhibits biosynthesis of sterols, but not of ubiquinone, in heterozygous FH patients [218]. 

Recent progress on the analytical techniques of steroids by gas chromatography (GC) and high-performance liquid chromatography (HPLC) and their combination with mass spectrometry (MS) has opened a new field for studies on natural sterols. Thus, during the past decade, a great deal of information has become available on the structure and biosynthesis of sterols in invertebrates. 

The biosynthesis of the juvenile hormones is not yet fully known. Their similarity to famesol, an intermediate product in the biosynthesis of steroles in mammals, leads one to assume that their formation is analogous to that of farnesol. This obvious assumption seems to be supported by publications that have appeared in the early 1970s. Barnes and Goodfellow (1971) showed that isoprenoid biosynthesis in the larva of Sarcophaga bullata proceeds with the participation of mevalonate kinase. This enzyme regulates the formation of mevalonic acid pyrophosphate, an important intermediate product in steroid biosynthesis of mammals. Isopentenyl pyrophosphate, the C, unit of isoprenoid biosynthesis, is formed from mevalonic acid pyrophosphate by decarboxylation and, with the participation of ATP, by dehydration. 

The mevalonate pathway in the cytosol is responsible for biosynthesis of sterols, sesquiterpenes, and triterpenoids. After conversion of mevalonic acid to isopentenyl pyrophosphate, three C5 units can be joined head to tail to produce a C15 compound, famesyl pyrophosphate. Two famesyl pyrophosphates are then united head to head to form squalene, the progenitor of the C30 isoprenoids from which sterols are derived. The plant squalene synthetase, like its mammalian homologue, is found in the ER and the reaction proceeds via a presqualene pyrophosphate intermediate (Chapter 14). In the last step prior to cyclization, squalene is converted to squalene 2,3-epoxide. 

Alcaide, A.M., M. Devys, J. Bottin, M. Fetizon, and M. Barbier Biosynthesis of sterols in Nicotiana tabacum leaves particularly 24-methylene cholesterol and 24-methylene dihydrolanosterol Phytochemistry 7 (1968) 1773-1777. 

Various aspects of steroid biosynthesis were included in a Royal Society Symposium. The published proceedings and other reviews have dealt with cyclase enzymes,water-soluble steroids and triterpenoids, the involvement of a 14(15)- or 8(14)-double bond and its reductionin cholesterol biosynthesis, biosynthesis of sterols, steroid metabolism in insects, pregnane steroids, cardenolides, and bufadienolides. ... 

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