Studies on the biosynthesis of cholesterol XIX. Steric course of hydrogen eliminations and of C—C bond formations in squalene biosynthesis

Abstract

1. Three molecules of isopentenyl pyrophosphate are used in the biosynthesis of one molecule of farnesyl pyrophosphate. In this process, one hydrogen atom is added and three are eliminated : these three were all originally attached to C-4 of an earlier precursor, mevalonate. It is demonstrated that these eliminations are stereospecific and that all three occur in the same stereochemical sense. The absolute configuration of the eliminated hydrogen is defined. 2. In the association of three isopentenyl pyrophosphate molecules to form farnesyl pyrophosphate, two new C—C bonds are formed with expulsion of two pyrophosphate ions. It is demonstrated that both these processes occur with inversion of configuration at the pyrophosphate-bearing carbon atom. 3. In the biosynthesis of squalene from two molecules of farnesyl pyrophosphate, one farnesyl residue suffers exchange of one hydrogen atom and one new C—C bond is formed. It is demonstrated that this bond is formed with inversion of configuration at that farnesyl residue which does not exchange hydrogen.

Cyclization of farnesyl diphosphate to pentalenene. Orthogonal stereochemistry in an enzyme-catalyzed SE′ reaction

“…(Scheme I). By contrast, the closely analogous prototropic rearrangement of isopentenyl diphosphate (IPP, 2) to dimethylallyl diphosphate (DMAPP,3) has been shown to occur with net anti stereochemistry (4). In more recent studies of the cyclization of farnesyl diphosphate (I) to the sesquiterpene hydrocarbon pentalenene (4), we have shown that theinitial SEl reaction takes place with net anti stereochemistry (5) (Scheme 2).…”

“…This allylic addition%limi-nation formally corresponds to an SE process. Cornforth and Popjak have shown that the key chain elongation step of polyprenol biosynthesis, typified by the farnesyl diphosphate (FPP, 1) synthase reaction, involves a pair of net syn allylic additionelimination reactions (3). (Scheme I).…”

“…Following a 1,2-hydride shift, the resultant cation 7 attacks the remaining double bond, leading, after loss of one of the original H-8 protons of FPP, to formation of pentalenene (4). We (3). The required samples of IPP labeled with tritium at either the 4E or 4 2 positions were themselves prepared by a modification of the method of Ogura and coworkers (12), initially using deuterium in order to optimize both the incorporation of label and the degree of stereospecific-I ity (Scheme 4).…”

Biosynthetic pathways from acetate to natural products

“…The detailed chemical mechanism of squalene synthesis from two molecules of farnesyl pyrophosphate is still obscure3. But the beautiful work of Cornforth et al 15 has not only elucidated the stereochemistry of this process but also presented unequivocal proof that the condensation is an asymmetric process in the sense that one of the farnesyl pyrophosphate molecules condensing is subject to reactions different from those of the other.…”

Biosynthesis of 5α-Cholestan-3β-ol in Cerebrotendinous Xanthomatosis

“…Since significant counts were not found in these compounds, this possibility is considered excluded. (6,13). These workers proved by degradative studies the precise locations of the 14C and 'H atoms in cholesterol, lanosterol, and squalene that were synthesized by liver homogenates from the mevalonate mixture (13).…”