Editing Thiol (section)

==Biological importance==
[[File:RNR reaction.png|thumb|upright=1.8|The catalytic cycle for [[ribonucleotide reductase]], demonstrating the role of thiyl radicals in producing the genetic machinery of life.]]

=== 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]].

===Cofactors===
Many [[cofactor (biochemistry)|cofactors]] (non-protein-based helper molecules) feature thiols. The biosynthesis and degradation of fatty acids and related long-chain hydrocarbons is conducted on a scaffold that anchors the growing chain through a thioester derived from the thiol [[coenzyme A]]. [[Dihydrolipoic acid]], a [[dithiol]], is the reduced form of [[lipoic acid]], a cofactor in several metabolic processes in mammals.
The [[methane biosynthesis|biosynthesis of methane]], the principal [[hydrocarbon]] on Earth, arises from the reaction mediated by [[coenzyme M]] (2-mercaptoethyl sulfonic acid) and [[coenzyme B]] (7-mercaptoheptanoylthreoninephosphate). Thiolates, the conjugate bases derived from thiols, form strong complexes with many metal ions, especially those classified as soft. The stability of metal thiolates parallels that of the corresponding sulfide minerals.

===Drugs===
Drugs containing thiol group:
* [[6-Mercaptopurine]] (anticancer)
* [[Captopril]] (antihypertensive)
* [[D-penicillamine]] (antiarthritic)
*[[Sodium aurothiolate]] (antiarthritic)<ref>{{cite journal | doi = 10.1038/sj.bjp.0707358 | volume=152 | title=Myeloperoxidase: a target for new drug development? | year=2007 | journal=British Journal of Pharmacology | pages=838–854  | last1 = Malle | first1 = E| issue=6 | pmid=17592500 | pmc=2078229 }}</ref>

===In skunks===
The defensive spray of [[skunks]] consists mainly of low-molecular-weight thiols and derivatives with a foul odor, which protects the skunk from predators. Owls are able to prey on skunks, as they lack a sense of smell.<ref>{{cite web|url=https://theowlstrust.org/owl-hub/owl-education-programmes/understanding-owls/|title=Understanding Owls – The Owls Trust|website=theowlstrust.org|access-date=3 May 2018|url-status=live|archive-url=https://web.archive.org/web/20180205131745/https://theowlstrust.org/owl-hub/owl-education-programmes/understanding-owls/|archive-date=5 February 2018}}</ref>