When boiling eggs, the smell of sulfur is due to the presence of sulfur-containing amino acids, such as cysteine and methionine. These amino acids are found in the egg white and, when heated, release hydrogen sulfide gas, which has a strong, unpleasant odor.

Cysteine and methionine are essential amino acids, meaning that they cannot be synthesized by the body and must be obtained from food. They play important roles in many bodily functions, including protein synthesis, tissue repair, and the production of hormones and enzymes.

In addition to cysteine and methionine, other sulfur-containing amino acids include homocysteine, taurine, and glutathione. These amino acids are also found in eggs, but they are present in smaller amounts and do not contribute as much to the smell of sulfur when eggs are boiled.