Cryptoregiochemistry of the Delta(11)-myristoyl-CoA desaturase involved inthe biosynthesis of Spodoptera littoralis sex pheromone

Many moth species biosynthesize their sex pheromones by the action of uniqu e desaturases. These membrane-bound family of enzymes are especially intere sting, since some of them produce (E)-unsaturated fatty acids either exclus ively or along with the (Z)-isomer. In this article we present the first me chanistic study on one of these enzymes, namely, the Delta(11)-myristoyl-Co A desaturase of the moth Spodoptera littoralis. Intermolecular primary isot ope effect determinations were performed in competition experiments. The un usual use of odd-number fatty acids, tridecanoic acid and deuterium-labeled tridecanoic acid, in these experiments showed the existence of a large iso tope effect for the carbon-hydrogen bond cleavage at C11, but no isotope di scrimination occurred in the removal of C12-H. The results of the competiti ve experiments are consistent with the hypothesis that this Delta(11)-desat urase involves a first slow, isotope-sensitive C11-H bond cleavage, with pr obable formation of an unstable intermediate, followed by a second fast C12 -H bond removal. We suggest that a single enzyme may be responsible for the formation of both (Z)- and (E)-11-tetradecenoic acids by accommodating bot h gauche and anti conformers of the substrate, respectively. It is also pos sible that two mechanistically identical discrete enzymes are involved in e ach desaturation. In this case, the geometry of the resulting double bond w ould result from the different conformation adopted by the acyl substrate a t each enzyme active site.