After 150 years, the structure of the enzyme responsible for onion’s eye irritation has been found.
It’s common knowledge that cutting onions can make you cry. Upon damage to the plant tissue, onions release a compound called lachrymatory factor (LF) as a chemical defence mechanism, irritating the eyes and causing them to water. What is less well known is the mechanism the onions use in order to generate the chemical. In fact, scientists have been stumped for over 150 years.
Despite the knowledge that LF is produced by a reaction catalysed by the enzyme lachrymatory factor synthase (LFS), analysing the conversion of the initial substrate – usually a sulfenic acid – to LF has been difficult to achieve. Unfortunately, the rapid reactivity of the substrate and the instability of the LF makes them very challenging to observe.
In order to surmount this problem, a team of US researchers determined the crystal structure of LFS. By analysing the crystal structure, they were able to observe the structure of the enzyme both as a whole and as its active site bound to another compound. Using this data in conjunction with known information about similar proteins, they were able to deduce the chemical mechanism used in the enzyme catalysed reaction – a sequential proton transfer accompanied by the formation of a carbocation intermediate (as illustrated in Figure 1).
The paper, entitled ‘Enzyme that makes you cry – crystal structure of lachrymatory factor synthase from Allium cepa‘ can be found in the journal ACS Chemical Biology [DOI: 10.1021/acschembio.7b00336]