Plant Sterol Biosynthesis. Identification of the Component Reactions of Oxidative Sterol C4-Demethylation

Abstract

Sterols play a crucial role in membrane function [1]. Studies using in vivo or model systems for structure-activity correlations have shown that they are specific structural requirements of the sterol molecule to function in the bulk role, including a 3$β$-hydroxyl group at C3 and a planar tetracyclic moiety. Particularly, lanosterol (3) and cycloartenol (1), the products of squalene-oxide cyclization in animals and fungi on one hand, and higher plants on the other, become functional only after enzymic removal of the three nuclear methyl groups (fig. 1) [1,2,3]. It has been postulated that the protuding methyl groups at C14 and C4 interfere severely with lipid-lipid interactions in the membrane resulting in the inefficiency of C4 or C14 methylated sterols for membrane function. In addition, recent works suggest that the 3$β$-hydroxy group is essential for membrane function of sterols and cannot be replaced by a ketone without destabilization of the lipid bilayer structure. Higher plants have elaborated sophisticated enzymic systems which are able to elegantly cleave the non-activated and stable carbon-carbon bonds at C4. Until the work of our group, nothing was known about the enzymology of this process because of the difficulty of obtaining the enzymic complexes catalyzing these demethylations.