Absolute Configuration of Hydroxysqualene. An Intermediate in Bacterial Hopanoid Biosynthesis.

Squalene (SQ) is a key intermediate in hopanoid biosynthesis. Many bacteria synthesize SQ from farnesyl diphosphate (FPP) in three steps: FPP to (1R,2R,3R)-presqualene diphosphate (PSPP), (1R,2R,3R)-PSPP to hydroxysqualene (HSQ), and HSQ to SQ. Chemical, biochemical, and spectroscopic methods were used to establish that HSQ synthase synthesizes (S)-HSQ. In contrast, eukaryotic squalene synthase catalyzes solvolysis of (1R,2R,3R)-PSPP to give (R)-HSQ. The bacterial enzyme that reduces HSQ to SQ does not accept (R)-HSQ as a substrate.