β-Amyrin Biosynthesis: The Methyl-30 Group of (3S)-2,3-Oxidosqualene Is More Critical to Its Correct Folding To Generate the Pentacyclic Scaffold than the Methyl-24 Group.

Abstract

Oxidosqualene cyclases catalyze the transformation of oxidosqualene (1) into numerous cyclic triterpenes. Enzymatic reactions of 24-noroxidosqualene (8) and 30-noroxidosqualene (9) with Euphorbia tirucalli β-amyrin synthase were conducted to examine the role of the branched methyl groups of compound 1 in the β-amyrin biosynthesis. Substrate 8 almost exclusively afforded 30-nor-β-amyrin (>95.5 %), which was produced through a normal cyclization pathway, along with minor products (<4.5 %). However, a lack of the Me-30 group (analogue 9) resulted in significantly high production of premature cyclization products, including 6/6/6/5-fused tetracyclic and 6/6/6/6/5-fused pentacyclic skeletons (64.6 %). In addition, the fully cyclized product (35.4 %) having the 6/6/6/6/6-fused pentacycle was produced; however, the normally cyclized product, 29-nor-β-amyrin was present in only 18.6 % of these products. The conversion yield of substrate 8 possessing a Z-Me group at the terminus was approximately twofold greater than that of compound 9 with an E-Me group. Thus, the Me-30 group is essential for the correct folding of a chair-chair-chair-boat-boat conformation of compound 1 for the production of the β-amyrin scaffold, whereas the Me-24 group exerts little influence on the normal polycyclization cascade. Here, we show that the Me-30 group plays critical roles in constructing the ordered architecture of a chair-chair-chair-boat-boat structure, in facilitating the ring-expansion reactions, and in performing the final deprotonation reaction at the correct position.