Isoprenoids/Terpenes

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Abstract

This largest class of natural products, with >75 000 known structures, arises from a pair of five-carbon isopentenyl diphosphate isomers, one acting as a π-electron double bond carbon nucleophile, the other as an allylic cation electrophile in C–C bond alkylations. Isoprene/terpene chain growth thus occurs five carbons at a time in head-to-tail couplings by prenyl transferase enzymes. At both the C15 or C20 chain length stages, enzymes can carry out related head-to-head chain couplings to generate the C30 hexaene squalene or the C40 nonaene phytoene. Squalene is the precursor to cyclase-mediated conversion to tetracyclic sterol frameworks and pentacyclic plant systems, such as amyrin and cycloartenol. The C10 (geranyl-PP = monoterpene), C15 (farnesyl-PP = sesquiterpene), and C20 (geranylgeranyl = diterpene) head-to-tail coupled metabolites can undergo many variations of internal carbocation-mediated cyclizations to generate a large array of mono- to tetracyclic olefins and alcohols. The predominant animal sterol is the C27 membrane lipid cholesterol, available from the initial C30 biosynthetic tetracyclic lanosterol by oxygenative removal of three C–CH3 groups. This phase of sterol metabolism marks a shift from carbocation-based reactions, to radical chemistry by oxygenases, as nine O2 molecules are consumed. In further conversion of cholesterol to the female sex hormone estradiol, another eight O2 molecules are consumed, for a total of 17 O2 being reductively split in the metabolic traverse from lanosterol to cholesterol. Meroterpenoid assembly involves the intersection of isoprene biosynthetic machinery with polyketide- or indole-processing enzymes.

Publisher

The Royal Society of Chemistry

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