Stereochemistry of Δ⁴ dehydrogenation catalyzed by an ivy (Hedera helix) Δ⁹ desaturase homolog

The stereochemistry of palmitoyl-ACP Δ⁴ desaturase-mediated dehydrogenation has been examined by tracking the fate of deuterium atoms located on stereospecifically monodeuterated substrates-(4S)- and (4R)-[4-²H₁]-palmitoyl-ACP and (5S)- and (5R)-[5- ²H₁]-palmitoyl-ACP. It was found that the introduction of the (Z)-double bond between C-4 and C-5 of a palmitoyl substrate occurs with pro-R enantioselectivity - a result which matches that obtained for a closely related homolog-castor stearoyl-ACP Δ⁹ desaturase. These data show that despite the difference in regioselectivity between the two enzymes, the stereochemistry of hydrogen removal is conserved.