Enzymic cyclization of 2,3-dihydrosqualene and squalene 2,3-epoxide by squalene cyclases: from pentacyclic to tetracyclic triterpenes

Abstract

Cell-free systems from the protozoon Tetrahymena pyriformis and the bacterium Alicyclobacillus acidocaldarius normally convert squalene into pentacyclic triterpenes of the gammacerane and hopane series. 2,3-Dihydrosqualene, a substrate analogue lacking one of the terminal double bonds of squalene and therefore making it impossible to form pentacyclic products, was converted unexpectedly into tetracyclic triterpenes, i.e. euph-7-ene by the T. pyriformis enzyme and a 1:1 mixture of (20R)-dammar-13(17)-ene and (20R)-dammar-12-ene by the system from A. acidocaldarius. Formation of a pentacyclic framework with six-membered D-ring might thus only depend on the assistance of the terminal double bond to the cyclization process, its lack of participation leading, probably spontaneously, to the formation of the thermodynamically favoured tetracyclic skeleton with a five-membered D-ring.

The cell-free system from T. pyriformis was further, for the first time, directly shown to induce cyclization of (3S)-[3-³H]squalene epoxide into gammacerane-3β,21α-diol, and the (3R)-enantiomer into gammacerane-3α,21α-diol. The (3R)-enantiomer also afforded a novel monocyclic product with a 2,3,4-trimethylcyclohexanone structure. The enzymic cyclization of squalene epoxide is apparently exclusively initiated by an oxirane ring-opening, and not by a proton attack on the remaining terminal double bond of the molecule.