Presqualene diphosphate

Isopentenyl diphosphate is isomerized by a shift of the double bond to form dimethylallyl diphosphate, then condensed with another molecule of isopentenyl diphosphate to form the ten-carbon intermediate ger-anyl diphosphate (Figure 26-2). A further condensation with isopentenyl diphosphate forms farnesyl diphosphate. Two molecules of farnesyl diphosphate condense at the diphosphate end to form squalene. Initially, inorganic pyrophosphate is eliminated, forming presqualene diphosphate, which is then reduced by NADPH with elimination of a further inorganic pyrophosphate molecule. 

This enzyme [EC 2.5.1.21], also known as farnesyltransf-erase, presqualene di-diphosphate synthase, and squa-lene synthase, catalyzes the condensation of two molecules of farnesyl diphosphate to form presqualene diphosphate and diphosphate (or, pyrophosphate). The entire enzyme complex catalyzes the NADPH-depen-dent reduction of presqualene diphosphate to yield squalene. 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 16-Oximino-17a-benzyl-17//-hydroxy derivatives in the androstane and estrane series have been converted into 16-oxo-17//-benzyl-17a-hydroxy derivatives with inversed configuration at C(17), on treatment with titanium trichloride. It has been suggested468 that the rearrangement occurs through the key intermediate (401) (see Scheme 97). 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 

Catalysis with [Rh2(55-mepy)4] was extended from styrene to other olefins, for which excellent results were dso obtained. By using this chiral catalyst, it has recently been possible to fix the three stereocenters in the cyclopropane ring of (+)-( R,2R,3/ )presqualene diphosphate (Structure 9), an intermediate in the biosynthesis of squalene from famesyl diphosphate [25]. 

The taU-to-tail coupling of two farnesyl diphosphate molecules leads via a cyclopropane intermediate to squalene. The primary cyclopropane derivative, presqualene diphosphate, cleaves off the diphosphate residue, and the resulting cyclopropylmethyl carbocation opens the ring again. The aUyl cation is reduced by NADPH to squalene. 

Two molecules of FPP, one acting as prenyl donor and one as prenyl acceptor, are the direct precursors for squalene [165]. The squalene synthase catalyzes the reductive dimerization of FPP under consumption of one equivalent NADPH via the stable intermediate presqualene diphosphate (36a, Scheme 87.11) [166-168]. Its relative and absolute configurations were established by degradation studies and total synthesis [169-172]. The formation of presqualene diphosphate (36a) proceeds with inversion of configuration at Cl of the prenyl donor and with attack of... 

Scheme 87.11 Biosynthesis of presqualene diphosphate (36a) and prephytoene diphosphate (36b)...

The second step in the squalene biosynthesis is the reductive rearrangement of presqualene diphosphate (36a) to squalene. Initial diphosphate abstraction formally gives a primary cyclopropylcarbinyl catimi 37a that rearranges via a secondary cyclobutylium intermediate 38a to the tertiary cyclopropylcarbinyl cation 39a (Scheme 87.12) [176-178]. Hydride attack from NADPH at C3 terminates the squalene biosynthesis. Evidence for the existence of the tertiary... 

Squalene synthase (EC 2.5.1.21) known as a membrane-bound enzyme condenses two molecules of famesyl diphosphate (FPP) in a stable intermediate, the presqualene diphosphate (PS) which is then reduced by NAD(P)H to form squalene (S). In spite of various hypotheses concerning the binding sites and reaction centers, the catalytic machinery for the two reactions and the mechanism(s) responsible for the sterol mediated regulation of squalene synthase (SQS), have not been elucidated. Sterol auxotrophic mutant strains (1) can be used as tools for such studies. Therefore, before going to purify SQS from plant sources (2), we have undertaken the purification and comparative characterization of squalene synthase (SQS) from three yeast strains the wild-type FLIOO, the mutant strain erglOB and the recombinant strain FK5188 pMF13. 

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