Editing Thiol (section)

=== Cysteine and cystine ===
As the functional group of the [[proteinogenic amino acid]] [[cysteine]], the thiol group plays a very important role in biology. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of [[protein]] folding, an [[oxidation|oxidation reaction]] can generate a [[cystine]] unit with a [[disulfide bond]] (−S−S−). Disulfide bonds can contribute to a protein's [[tertiary structure]] if the cysteines are part of the same [[peptide]] chain, or contribute to the [[quaternary structure]] of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. A physical manifestation of cysteine-cystine equilibrium is provided by [[hair straightening]] technologies.<ref>{{cite book|last=Reece |first=Urry |display-authors=etal |title=Campbell Biology |url=https://archive.org/details/campbellbiology00reec |url-access=limited |edition=Ninth |publisher=Pearson Benjamin Cummings |location=New York |date=2011 |pages=[https://archive.org/details/campbellbiology00reec/page/n112 65], 83}}</ref>

Sulfhydryl groups in the [[active site]] of an [[enzyme]] can form [[noncovalent bond]]s with the enzyme's [[substrate (biochemistry)|substrate]] as well, contributing to covalent [[catalysis|catalytic activity]] in [[catalytic triad]]s. Active site cysteine residues are the functional unit in [[cysteine protease]] [[catalytic triad]]s. Cysteine residues may also react with heavy metal ions (Zn<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, Hg<sup>2+</sup>, Ag<sup>+</sup>) 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]].