The amino acid that contains sulfur in its side chain and is often categorized as nonpolar is __________.

### Why this is right Methionine contains sulfur in a thioether group in its side chain and is generally classified as nonpolar. Cysteine also contains sulfur, but its thiol group gives it different chemical reactivity. ### The concept Side-chain composition helps identify amino acids. Sulfur-containing side chains are especially relevant because cysteine can form disulfide bonds that stabilize protein structure, whereas methionine commonly contributes a hydrophobic side chain. ### Terms - **Thioether**: a sulfur atom bonded to two carbon-containing groups. - **Thiol**: a sulfur-containing group written –SH. ### Remember Methionine and cysteine contain sulfur, but their sulfur groups behave differently.