Sterol Biosynthesis by a Prokaryote: Firstin VitroIdentification of the Genes Encoding Squalene Epoxidase and Lanosterol Synthase fromMethylococcus capsulatus

Sterol biosynthesis by prokaryotic organisms is very rare. Squalene epoxidase and lanosterol synthase are prerequisite to cyclic sterol biosynthesis. These two enzymes, from the methanotrophic bacterium Methylococcus capsulatus, were functionally expressed in Escherichia coli. Structural analyses of the enzymatic products indicated that the reactions proceeded in a complete regio- and stereospecific fashion to afford (3S)-2,3-oxidosqualene from squalene and lanosterol from (3S)-2,3-oxidosqualene, in full accordance with those of eukaryotes. However, our result obtained with the putative lanosterol synthase was inconsistent with a previous report that the prokaryote accepts both (3R)- and (3S)-2,3-oxidosqualenes to afford 3-epi-lanosterol and lanosterol, respectively. This is the first report demonstrating the existence of the genes encoding squalene epoxidase and lanosterol synthase in prokaryotes by establishing the enzyme activities. The evolutionary aspect of prokaryotic squalene epoxidase and lanosterol synthase is discussed.