When we cut an onion, the synthase enzyme converts the amino acid sulfoxides of the onion into which acid?

Sulfenic acid

The correct answer is option 2. Sulfenic acid.

When an onion is cut, the enzyme responsible for the reaction is not "synthase" but rather "alliinase." When the onion cells are damaged by cutting, the alliinase enzyme catalyzes the conversion of sulfur-containing compounds called sulfoxides (specifically, amino acid sulfoxides) into sulfenic acids. The primary compound involved in this reaction is called "alliin," which is converted into "sulfenic acid."

Here's a step-by-step explanation:

1. Alliin in intact onion cells: In the intact onion cells, the amino acid alliin is present. Alliin does not have any noticeable odor or irritant properties.

2. Cutting the onion releases alliinase: When the onion is cut, the cells are damaged, and the enzyme alliinase is released from the cell vacuoles.

3. Conversion of alliin to sulfenic acid: Alliinase catalyzes the conversion of alliin into sulfenic acid, which is an unstable intermediate compound.

4. Formation of volatile compounds:  The sulfenic acid formed in the reaction undergoes further chemical reactions, leading to the formation of various volatile sulfur compounds. One of the major compounds formed is syn-propanethial-S-oxide, which is responsible for the characteristic pungent odor of onions. This compound diffuses into the air and irritates the eyes, causing tears.

Overall, when an onion is cut, the enzyme alliinase converts the amino acid sulfoxides (alliin) of the onion into sulfenic acid, which then undergoes further reactions to produce the characteristic volatile compounds responsible for the strong odor and eye irritation associated with cutting onions.