Characterization and partial purification of squalene-hopene cyclase from Bacillus acidocaldarius

Abstract

A membrane-bound enzyme activity from Bacillus acidocaldarius converted squalene to two pentacyclic triterpenes, hop-22(29)-ene and hopan-22-ol. The products were formed in a constant molar ratio of hopene:hopanol, 5:1, probably through parallel, and not successive, reactions. The conversion was independent of oxygen, in contrast to the biosynthesis of sterols from epoxysqualene in eukaryotes. The squalene-hopene cyclase was pufified 270-fold by extraction from B. acidocaldarius membranes at low concentrations of Triton X-100 followed by DEAE-cellulose chromatography. The enzyme showed optimal rates of squalene conversion at pH 6 and 60°C, corresponding to the intracellular pH and the optimal growth temperature of the bacterium. The apparent K m for squalene is 3 μM. Effective inhibitors of the enzyme were some sulfhydryl reagents and the histidyl reagent diethyl pyrocarbonate. The squalene-hopene cyclase, like several eukaryotic epoxysqualene cyclases, was strongly inhibited by AMO 1618 and by high ionic strength. On the basis of these and other similarities a phylogenitic relationship between the dey enzyme of steroid and hopanoid biosynthesis was envisaged.