Dissecting the Stereocontrolled Conversion of Short-Lived Sulfenic Acid by Lachrymatory Factor Synthase

Lachrymatory factor synthase (LFS) is responsible for the natural production of syn-propanethial S-oxide. Because its substrate and product are both transient, the mechanistic insight is currently poor. Here, we decipher the catalytic cycle of onion LFS (AcLFS) by employing crystallography, biochemical assays, and molecular simulations. Crystal structures complexed with solute compounds demonstrate not only the properties of the catalytic site but also a potent binding mode of the labile substrate, a mode which can fulfill the "syn-effect" of the reaction. Mutagenesis analyses identified the key residues in the active site, and the kinetic and energetic backgrounds were specified with computational approaches. We present a rational catalytic mechanism based on the intramolecular proton shuttling that is distinct from the canonical [1,4]-sigmatropic rearrangement reaction. Our investigations both in vitro and in silico provide the mechanistic basis explaining how AcLFS generates the lachrymatory agent in nature and provide insights into the molecular machinery concerning organic labile sulfur species.