Polyisoprenoid synthesis

Mevalonic acid was discovered by Folker s group at Merck, Sharpe, and Dohme. The initial isolation was based upon the fact that it acted as a growth factor, or vitamin, for a strain of bacteria [35]. Once the structure had been determined, it was apparent that the molecule might well be the isoprenoid precursor that had been sought for many years. Subsequent experiments demonstrated that the sole (or nearly so) fate of the molecule was polyisoprenoid synthesis. In examining the role of cofactors necessary for the synthesis of cholesterol from mevalonate, only ATP and NADPH were found to be required. Experiments with a solubilized preparation from yeast demonstrated that there were 3 phosphorylated intermediates that could be isolated. These were shown to be mevalonic-5-phosphate, mevalonic-5-pyrophos-phate, and isopentenyl pyrophosphate [9]. These intermediates are derived from mevalonate in a sequence of phosphorylations, and the enzymes for all reactions have been obtained in homogeneous form. These enzymes, as well as the rest that lead to the synthesis of famesyl pyrophosphate, are cytosolic proteins. 

A central role in the biosynthesis of isoprenoids is filled by the isopentenyl diphosphate-dimethylallyl diphosphate isomerase (IDl) that catalyzes the interconversion of IPP and DMAPP. The necessity for such an enzyme was suggested in the 1950s when only IPP was known as a monomeric isoprenoid precursor, but an allylic diphosphate such as DMAPP was assumed to have the higher intrinsic reactivity for polyisoprenoid synthesis [22, 88, 89]. The first enzymatic isomerization of IPP to DMAPP was observed in 1959 from a cell-free extract of baker s yeast [90, 91]. Two types of IDI with essentially no amino acid sequence or structural similarities are able to catalyze this interconversion by completely different enzyme mechanisms. The well-known IDI-I have been identified in animals, plants, fungi, and bacteria, whereas the IDI-II can be found mainly in archaea but also in some bacteria [92, 93]. 

The biomimetic-type cyclization of polyisoprenoids is an important industrial process for terpene synthesis. In most cases, a large excess of coned. H SO and SnCl. has been employed For example, ionone, a precursor of vitamin A, is prepared by coned. H2SO4 catalyzed cyclization of pseudoionone. The disadvantage of this process is undoubtedly the requirement of bases to neutralize the large excess of acid. The EGA method offers a promising alternative for this purpose. Thus, Electrolysis of 15 and 17 in a ClCHjCH Cl—LiClO —Et NClO — (Pt) system provides 16 and 18, respectively in reasonable yields and the neutralization of the reaction solution can be performed simply by addition of a small amount of pyridine... 

Stereoselectivity in the synthesis of trisubstituted olefins is necessary for the study of biosynthetic routes to polyisoprenoids, the nonenzymatic cyclization of polyolefinic substrates, and the study of insect hormones. 

BertoUno A, Altman LJ, Vasak J, Rilling HC. Polyisoprenoid amphiphiUc compounds as inhibitors of squalene synthesis and other microsomal enzymes. Biochem. Biophys. Acta -Lipids and Lipid Metab. 1978 530 17-23. 

Johnson, Faulkner, et al.B have developed another approach to synthesis of polyisoprenoids which also utilizes the Claisen rearrangement to establish trans-trisubstituted double bonds. In a model experiment, the allylic alcohol (5) was heated with 7 equivalents of triethyl orthoacetate and 0.06 equivalent of propionic acid at 138° for 1 hour with provision for distillative removal of ethanol the diene ester (6) was obtained in 92% yield. Analysis by vpc indicated that (6) is the trans isomer to the extent of > 98% with less than 2% of the cis isomer. If the classical... 

Kauss has similarly observed the enzymic synthesis of D-man-nosyl-lipid by use of a particulate enzyme from mung-bean shoots. The results suggested that only the D-mannosyl group of GDP-d-mannose is transferred, because GDP, but not GMP, was incorporated into GDP-D-mannose by an exhange reaction. There is some evidence suggesting that the lipid is a polyisoprenoid compound, because prior incubation of the cells with mevaIonic-5-t acid resulted in the labeling of the acceptor lipid. 

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). 

Biosynthesis of Mammalian Glycoprotein Glycosylation Pathways in the Synthesis of the Non-reducing Terminal Sequences , Journal of Biological Chemistry, 254, 12531-41 Beyti, E.D. Porter, J.W. (1976) Biochemistry of Polyisoprenoid Biosynthesis , Annual Reviews of Biochemistry, 45,113-42... 

Dolichols (98), a family of polyisoprenoid alcohols, have isolated from all eukaryotic cells or archaebacteria." They play a role in the co-translational modification of proteins and are known in iV-glycosylation in the form of dolichol phosphate. The synthesis of the photochemical probes 118 and 119 bearing a photoreactive group [3-(trifiuoromethyl)-3-aryldiazirine], analogues of dolichol and dolichol phosphate, is described in Scheme 5.19. The synthetic strategy involves the sequential alkylation of a monoterpenoid hydroxysulfonyl dianion with allyl chlorides, and MBH adduct 102 as a starting material. 

So far, potential eubacterial and archaeal Man-T seem to fall into either the P-Man-T [34] or the a-Man-T ( E(X7)E ) [18] classes. Members of both families participate in EPS synthesis in eubacteria as well as in cytoplasmic, sugar nucleotide-dependent steps in the dolichol-linked pathway for A -glycosylation in the ER. Indeed, the similarity between the biochemical steps and enzymes in these two polyisoprenoid-linked pathways strongly suggests that they have a common evolutionary origin [3]. 

A major control of secondary metabolite production would be the availability of substrates. At the end of primary growth, acetyl-CoA is likely to be in excess, because its main utilizer (the Kreb s cycle) will be slowing down. As primary growth ends, this acetyl-CoA is utilized along with Trp to yield cAATrp. This metabolite in turn deflects excess DMAPP (whose synthesis requires 3 molecules of acetyl-CoA) away from isoprenoid biosynthesis. Therefore, it appears that secondary metabolism is an interlock mechanism, linking in this case protein biosynthesis and energy metabolism, and perhaps polyisoprenoid biosynthesis, which is called into play when a restraint is placed on a specific area of primary metabolism. 

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