Occurrence of Cationic Intermediates and Deficient Control during the Enzymatic Cyclization of Squalene to Hopanoids

Tetracyclic triterpenes (for example, 3) are formed as minor products of the cyclization of squalene (1) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2) in the cyclization process.