For 10 points, name this amino acid that can form disulfide bonds, unlike methionine, and is symbolized C.
An N-acetyl derivative of this amino acid is used in over-the-counter expectorants, and TGF-beta has a characteristic group of nine of these residues, eight of which form a namesake knot. The catalytic site of caspases contains histidine and this amino acid, and along with histidine, it coordinates with zinc ions in zinc finger motifs. This amino acid is present in high concentrations in keratin and thus in human hair. For 10 points, name this amino acid that can form disulfide bonds, unlike methionine, and is symbolized C.
An N-acetyl derivative of this amino acid is used in over-the-counter expectorants, and TGF-beta has a characteristic group of nine of these residues, eight of which form a namesake knot.
{cysteine}