Crying…over you…

Why does chopping onions bring tears to your eyes? The breaking open of onion cells leads to the release of a volatile compound known as "lachrymatory factor", (Z)-propanethial S-oxide. How this compound is produced by lachrymatory factor synthase (LFS) in onions has not been clear. Now, Marcin Golczak of Case Western Reserve University, Cleveland, Ohio, and colleagues have solved crystal structures of LFS in an apo-form and in complex with a substrate analogue, crotyl alcohol. Their structures allowed them to devise a plausible mechanism of catalysis that involves sequential proton transfer and formation of a carbanion intermediate. This mechanism reconciles experimental and theoretical studies and ties up a loose end in science that had biochemists crying in frustration.