Which amino acids contain sulfur?

Cysteine and methionine are the only amino acids that contain sulfur in their structure. Cysteine contains a thiol (-SH) group, which makes it a crucial amino acid for protein structure as it can form disulfide bonds. These disulfide bridges play a significant role in stabilizing the three-dimensional conformation of proteins, particularly those secreted outside of cells.

Methionine, on the other hand, includes a thioether group in its side chain. It is often the initiating amino acid in protein synthesis and also serves as a methyl donor in various metabolic processes.

The other options listed do not contain sulfur in their side chains. Serine and threonine are hydroxyl-containing amino acids, while leucine and isoleucine are branched-chain amino acids. Aspartate and glutamate, being acidic amino acids, contain carboxyl groups instead. Therefore, only cysteine and methionine are categorized as sulfur-containing amino acids.