Biosynthesis of Prelycopersene Pyrophosphate and Lycopersene by Squalene Synthetase

The conversion of [4,8,12,16-14C]geranylgeranyl pyrophosphate to prelycopersene pyrophosphate and lycopersene by purified bakers' yeast squalene synthetase is reported. The identification of the products of this reaction has been achieved through co-chromatography with authentic samples on thin layer plates and gas-liquid chromatography with authentic samples of either the original compound or a derivative. Further proof of the identities of these compounds has been obtained from their mass fragmentation patterns. Lycopersene is formed from [4,8,12,16-14C]geranylgeranyl pyrophosphate in the presence of the yeast enzyme, NADPH, and Mg2+. NADH is less than 10% as effective as NADPH in this reaction. The formation of prelycopersene pyrophosphate from [4,8, 12,16-14C]geranylgeranyl pyrophosphate by squalene synthetase is markedly increased in the absence of NADPH and the presence of Mg2+ or Mn2+. The product of this reaction is optically active, and it gives a normal optical dextrorotatory dispersion curve. The mass spectrum of this compound shows an intense peak at m/e 544+ which is due to the loss of the pyrophosphate moiety (178 mass units) in the mass spectrometer. The chemical reduction of the product, prelycopersene pyrophosphate, formed by squalene synthetase by lithium aluminum hydride yields an alcohol. A fragmentation pattern characteristic of a primary alcohol with a molecular ion at m/e 562+, which corresponds to a molecular formula of C40H65OH was obtained on mass spectral analysis. Further proof of this structure was obtained from the mass spectrum of the trimethylsilyl ether derivative. A molecular ion peak was obtained at m/e 634+. The presence and the position of a cyclopropane ring in prelycopersene pyrophosphate was established by the mass spectrum of its catalytically reduced alcohol. Two major losses, due to cleavage of the side chains of the cyclopropane ring, were observed from the molecular ion M+, 576+. The conversion of prelycopersene pyrophosphate to lycopersene occurs in the presence of NADPH and appears to be analogous to the conversion of presqualene pyrophosphate to squalene. A mechanism for the conversion of geranylgeranyl pyrophosphate to lycopersene is proposed. This mechanism is based upon the results obtained by Beytia et al. (Beytia, E. D., Qureshi, A. A., and Porter, J. W. (1973) J. Biol Chem. 248, in press) on the mechanism of synthesis of squalene from farnesyl pyrophosphate by yeast squalene synthetase.