Why onions make us cry decoded

Ever wondered why chopping onions leave us teary-eyed? Scientists, including one of Indian origin, have decoded the natural defence mechanism that springs into action when we cut the vegetable, making our eyes sting and water.
Adding onions to a recipe can make a meal taste rich and savoury, but cutting up the onion can be brutal.
Onions release a compound called lachrymatory factor (LF), which makes the eyes water.
Scientists know that a certain enzyme causes this irritating compound to form but precisely how it helps LF form in the onion remained an open question.
When an onion is cut, it has a natural defence mechanism that springs into action, producing LF. This kind of compound is rare - only four known natural types exist.

An enzyme in the onion known as lachrymatory factor synthase (LFS) spurs production of LF in the onion.

"Tearless" onions, sold exclusively in Japan for a hefty price, do not make LFS so they also don't produce the irritant LF.
Scientists have been at a loss to explain exactly how LFS helps LF form. That is because it is extremely reactive, and LF evaporates or breaks down easily.
Researchers including Surajit Banerjee from Cornell University in the US wanted to take a different approach to solve this mystery once and for all.
The team determined the crystal structure of LFS and analysed it. With the crystal structure, they could finally see the architecture of the enzyme as a whole and its active site as it bound to another compound.

By combining this information with known information about similar proteins, the group developed a detailed chemical mechanism that could explain the precise steps involved in LF synthesis - and hence, why people wind up crying when they chop an onion.