Sulfhydryl

Cysteine and cystine

Sulfhydryl groups in the active site of an enzyme can form noncovalent bonds with the enzyme's substrate as well, contributing to covalent catalytic activity in catalytic triads. Active site cysteine residues are the functional unit in cysteine proteasecatalytic triads. Cysteine residues may also react with heavy metal ions (Zn, Cd, Pb, Hg, Ag) because of the high affinity between the soft sulfide and the soft metal (see hard and soft acids and bases). This can deform and inactivate the protein, and is one mechanism of heavy metal poisoning.