Named from a urinary calculus discovered in 1810, holds proteins together with molecular handcuffs — and is the reason your hair can be permanently curled.
Symbol
Cys · C
Discovered
1884
Mol. Weight
121.16 g/mol
Essential
Conditional
C
Discovery: From a Bladder Stone
L-Cysteine
The story of cysteine begins in 1810, when English chemist William Hyde Wollaston isolated a crystalline substance from urinary calculi and called it cystic oxide — now known as cystine, the oxidised dimeric form of cysteine. For several decades, cystine evaded recognition as a protein building block, partly because early analytical methods were incompatible with sulfur-containing compounds.
In 1884, German chemist Eugen Baumann treated cystine with a reducing agent and demonstrated that it was a dimer of a simpler monomer. He named the monomer cysteine — from kystis, the Greek word for bladder, honouring Wollaston's original urinary calculus source — by adding an 'e' to cystine to signal the relationship between the two forms. The distinction between cysteine (the individual amino acid, with a free –SH group) and cystine (two cysteines linked by a disulfide bond) would prove to be fundamental. Cystine forms spontaneously when two cysteine residues come close enough to share electrons — and this reaction, simple as it sounds, turns out to be one of the most powerful structural tools in all of biochemistry.
Two cysteine residues, brought close together in a folding protein, can lock their sulfur atoms into a bond — creating a molecular handcuff that can hold a protein's shape for decades.
Identifiers and Properties of Cysteine
Identity
IUPAC Name(2R)-2-amino-3-sulfanylpropanoic acid
FormulaC₃H₇NO₂S
Mol. Weight121.16 g/mol
CAS Number52-90-4
MDL NumberMFCD00064306
Physical
Melting point220 °C
Solubility280 g/L (25 °C)
pKa₁ (COOH)1.96
pKa₂ (NH₃⁺)10.28
pKa (–SH)8.18
pI5.07
Rf (BuOH/AcOH/H₂O = 12:3:5)0.40
Identifiers
Canonical SMILESC(C(C(=O)O)N)S
Isomeric SMILESC([C@@H](C(=O)O)N)S
InChIKeyXUJNEKJLAYXESH-REOHCLBHSA-N
CategoryPolar
ContainsSulfur (thiol –SH)
The Disulfide Bond: Molecular Handcuffs
Cysteine's defining feature is its side chain: a thiol group (–CH₂–SH). This group is unusual because the sulfur atom can donate electrons to form a covalent bond with another sulfur atom — specifically, with the sulfur of another cysteine somewhere else in the protein chain. The result is a disulfide bond (S–S), also called a disulfide bridge.
How a Disulfide Bond Forms
Cys − CH₂ − S − H
+ oxidation
Cys − CH₂ − S − H
↓
Cys − CH₂ − S − S − CH₂ − Cys
Two –SH groups lose their hydrogens and bond together, forming a strong covalent S–S link that locks protein structure in place.
Disulfide bonds are remarkably strong, and they are used by nature wherever a protein needs to be particularly stable. Antibodies rely heavily on disulfide bonds. Insulin — the hormone that regulates blood sugar — is held together by disulfide bonds between its two chains. The tough structural proteins in feathers and hooves all use cysteine's disulfide chemistry.
Why Your Hair Can Be Permanently Curled
Hair is made primarily of a protein called keratin. Keratin chains are held in their helical shape largely by cysteine disulfide bonds — lots of them. In naturally straight hair, these bonds hold the protein strands in a particular arrangement. In naturally curly hair, the bonds lock the strands into a different, curved arrangement.
The chemistry of a permanent wave (perm) is entirely about manipulating these bonds. The first chemical applied breaks the disulfide bonds using a reducing agent. The hair is then physically shaped around rollers. The second chemical re-forms the disulfide bonds in the new shape. The hair's keratin is now locked into the curled configuration by a fresh set of cysteine-cysteine bonds. Take away the rollers — the curl stays.
This is chemistry you can feel in your hands. Every permanent wave, every relaxer, every salon treatment that changes hair shape is a deliberate manipulation of cysteine's disulfide bonds.
Functions of L-Cysteine in the Body
Cysteine is a conditionally essential amino acid — the body synthesizes it from methionine, but under conditions of illness, metabolic stress, or inadequate methionine intake, dietary cysteine becomes important.
Glutathione synthesis
Cysteine is the rate-limiting precursor of glutathione, the cell's primary antioxidant tripeptide (glutamate + cysteine + glycine). Glutathione is present in all tissues of the human body and protects cells from oxidative damage, supports immune function, and plays a central role in the detoxification of harmful substances — including alcohol metabolites, certain drugs, and environmental toxins. The antioxidant activity of glutathione is specifically attributed to the cysteine thiol group within the molecule.
Taurine synthesis
Cysteine is also a biosynthetic precursor of taurine — a sulfur-containing compound found at high concentrations in the brain, heart, and skeletal muscle. Taurine is involved in bile salt conjugation, osmoregulation, and membrane stabilization. Its synthesis from cysteine provides an important metabolic link between amino acid metabolism and cardiovascular and neurological function.
Structural role in proteins
Beyond glutathione, cysteine's disulfide bonds are essential for the correct folding and stability of many structural and functional proteins: keratin (skin, hair, nails), immunoglobulins, and various hormones including insulin. Cysteine also contributes to the regeneration and maintenance of hair and nail tissue through its role as a sulfur donor in keratin cross-linking.
Detoxification and liver protection
Through its role in glutathione production, cysteine helps protect the liver and brain from damage caused by toxic substances, including alcohol and its metabolites. N-acetylcysteine (NAC) — a stable, bioavailable derivative of cysteine — is used clinically as a mucolytic agent in respiratory conditions (bronchitis, chronic obstructive pulmonary disease) and as an antidote in paracetamol (acetaminophen) overdose, where it replenishes hepatic glutathione stores.
Immune support
Cysteine contributes to immune function both through glutathione-mediated protection of immune cells and through its role in supporting white blood cell activity. Adequate cysteine availability is particularly important for proper immune response during illness or recovery from surgery.
Side Effects and Supplemental Considerations
Diabetes: People with diabetes should exercise caution with supplemental cysteine. At high doses, cysteine can reduce certain disulfide bonds in the insulin molecule, potentially impairing its activity. Supplementation in diabetic patients should only be undertaken under medical supervision.
Cystine kidney stones: Excess cysteine can be oxidized to cystine, which is poorly soluble and may contribute to the formation of kidney or bladder stones — a condition known as cystinuria. Individuals with a history of urinary stones should consult a physician before using cysteine supplements.
Did You Know?
A permanent wave works in two steps: first a reducing agent breaks cysteine's disulfide bonds in hair; then, while the hair is shaped on rollers, an oxidizing agent reforms the bonds in the new position. The curl is held by cysteine chemistry.
Interesting Facts
🔒
The protein lock. Disulfide bonds formed by cysteine are some of the strongest non-backbone interactions in protein structure. Proteins exposed to high temperatures or strong detergents unfold — but proteins with many disulfide bonds are far more resistant to these insults.
🧅
The smell of onions and garlic. Onions and garlic contain sulfur compounds derived from cysteine (like alliin in garlic). When cells are broken by cutting or crushing, enzymes convert these cysteine derivatives into volatile compounds that cause the distinctive smell and the eye-watering lachrymatory factor in onions.
🫁
Key precursor to glutathione. Cysteine is the rate-limiting building block of glutathione — one of the cell's primary antioxidant molecules. Glutathione is a three-amino-acid peptide (glutamate + cysteine + glycine) that protects cells from oxidative damage and is found in virtually every tissue of the body.
💊
Used in baking. L-cysteine (listed as E920 in food labeling) is used in bread production as a dough conditioner. It weakens gluten structure by breaking some disulfide bonds, making dough more pliable and easier to process mechanically.
🦂
In venom proteins. Many animal venoms — including scorpion toxins and cone snail peptides — contain high concentrations of cysteine, whose disulfide bonds create the rigid, stable structures needed for toxin function and durability in the environment.
Where to Find Cysteine in Food
Cysteine is a non-essential amino acid — the body can synthesize it from methionine. But dietary cysteine still contributes to overall availability, particularly when methionine intake is limited:
