Yeast squalene synthase. A mechanism for addition of substrates and activation by NADPH.

Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. Previous studies of the mechanism of addition of FPP to the enzyme have led to conflicting interpretations of initial velocity measurements (Beytia, E., Qureshi, A. A., and Porter, J.W. (1973) J. Biol. Chem. 248, 1856-1867; Agnew, W.S., and Popjak, G. (1978) J. Biol. Chem. 253, 4566-4573). Initial velocities for synthesis of PSPP and squalene were measured over a wider range of FPP and NADPH concentrations than previously reported, using a soluble form of recombinant enzyme. In the absence of NADPH, PSPP formation was activated by FPP at concentrations above approximately 0.5 microM. At fixed levels of NADPH, the dependence of initial rates of PSPP and squalene synthesis on FPP concentrations indicated that the C15 substrate added by a sequential mechanism. In addition, NADPH stimulated synthesis of PSPP by 40-fold at saturating levels of the cofactor. This stimulation is, at least in part, by reduction of PSPP to squalene.