Cysteine

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14 atoms, 13 bonds.

IUPAC name: (2R)-2-amino-3-sulfanylpropanoic acid
Molar mass: 121.154(30) g/mol
Atoms: 14
SMILES: C(C(C(=O)O)N)S
InChIKey: XUJNEKJLAYXESH-REOHCLBHSA-N
PubChem CID: 5862

Made of

Structural formula of cysteine. Each unlabelled corner is a carbon atom, and the three hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

At the end of cysteine’s side chain, a CH₂SH arm, sits a thiol group, –SH: a sulfur atom bonded to a single hydrogen. The Medical Subject Headings (MeSH) of the US National Library of Medicine describe it as an amino acid carrying a thiol, one that oxidation turns into cystine. In proteins, pairs of cysteines join through their sulfur atoms, yet each is always built into the chain as cysteine itself, according to HSDB, the Hazardous Substances Data Bank.

Its International Union of Pure and Applied Chemistry (IUPAC) name, (2R)-2-amino-3-sulfanylpropanoic acid, calls the thiol a sulfanyl group and fixes it on the third carbon. Carbon, nitrogen and oxygen make up the rest of the formula, C₃H₇NO₂S, which comes to 121.15 g/mol. Its colourless crystals dissolve freely in water and decompose at 260 °C, according to HSDB. As a food additive it is described as white crystals with a sulfurous smell. Commercially it is available as L(+)-cysteine hydrochloride.

Structure and bonding

Three arms leave the second carbon, ending in the lone S, the N and the pair of O atoms. The Ball-and-stick view shows the second carbon holding four different groups: a hydrogen, an amine group (NH₂), the acid group (COOH) and the CH₂SH arm. Its angles in the model, 107.5° to 112.3°, sit near the tetrahedral geometry Chem1 calls usual for carbon in organic compounds.

In the Stick view the sulfur end looks stretched. The computed model’s C–S bond is 1.82 Å, against 1.36 Å for the C–O bond carrying the acid hydrogen. The model’s angle at sulfur is about 97°, where its C–O–H angle is about 112°. In the Space-filling view the yellow sulfur is the largest sphere.

The drawing shows the neutral molecule; the glycine page explains the charged forms an amino acid can also take. Nor is the free S–H permanent. In neutral or slightly alkaline solution air oxidises cysteine to cystine, two molecules bonded S to S, while acid solutions keep it more stable, according to HSDB. The model is one of two mirror-image forms, the (2R) arrangement of the IUPAC name, which ChEBI, the Chemical Entities of Biological Interest database, identifies as L-cysteine.

Where it turns up

Keratin, the main protein of nails, skin and hair, contains cysteine, according to the National Cancer Institute’s thesaurus (NCIt). In hair, cysteine’s sulfur atoms pair up as cystine disulfide links. Permanent-wave and hair-straightening products work on those links. The thioglycolates in them chemically reduce the cystine disulfide bonds inside the hair, which weakens the keratin, the National Toxicology Program reports.

Other proteins carry the same link. Each insulin unit has two chains, A of 21 amino-acid units and B of 30, joined by two disulfide bonds, according to MeSH. The LOTUS natural-products database also records L-cysteine in Indigofera hirsuta.

Bakeries meet cysteine as an additive. The US Food and Drug Administration (FDA) lists L-cysteine as a dough strengthener and flour treating agent, and also as a flavouring agent, flavour enhancer and nutrient supplement. HSDB calls it a reducing agent in bread doughs.

Why it matters

Disulfide bonds between cysteine thiol groups matter for a protein’s folded structure and enzyme activity, according to HSDB. Linking cysteines in different parts of a chain helps a protein fold and keeps its shape, MedlinePlus Genetics explains. In the receptor protein TNFR1, cysteines swapped for other amino acids leave those bonds unformed and the protein misfolded. Hydrogen bonds shape proteins too, as the alanine page describes.

Glutathione, found in most mammalian tissue, joins one unit each of cysteine, glutamic acid and glycine. The National Cancer Institute (NCI) describes glutathione as an antioxidant that also helps form and maintain disulfide bonds in proteins. Acetylcysteine, a synthetic derivative the body converts into L-cysteine, thins mucus by reducing disulfide bonds in mucus proteins. It is also used against acetaminophen overdose because it can restore the depleted glutathione of liver cells.

HSDB calls L-cysteine the central compound of human sulfur metabolism. Its breakdown can yield taurine and carbon dioxide. Its thiol sulfur must also be oxidised so that it does not accumulate to toxic levels.

Properties

Properties of cysteine Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 5862.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₇NO₂S
Molar mass	121.154(30) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	7
Nitrogen atoms	1
Oxygen atoms	2
Sulfur atoms	1
Bonds	13
Single bonds	12
Double bonds	1
PubChem molecular weight	121.16 g/mol	g/mol	PubChem

Safety

Pure cysteine carries the Globally Harmonized System (GHS) signal word Warning. It is classed as harmful if swallowed in most company reports, and as a skin, eye and respiratory irritant in a smaller share. Decomposed by heat, it gives off very toxic sulfur oxide and nitrogen oxide fumes. The PubChem Laboratory Chemical Safety Summary for cysteine is the reference sheet for handling it.

Questions

What is cystine?

Cystine is two cysteine molecules joined through their sulfur atoms by a disulfide bridge, according to MeSH. Cystine also gives its name to a kind of kidney stone. In cystinuria, an inherited disorder caused by changes in the SLC3A1 or SLC7A9 gene, the kidneys cannot properly return cystine to the blood. It builds up in the urine and, as the urine concentrates, crystallises into stones. MedlinePlus puts its frequency at about 1 in 10,000 people.

Where is the sulfur atom in a cysteine molecule?

It caps the side chain, bonded to the third carbon and to a single hydrogen as the thiol group, –SH. With Labels on, it is the one S among the 3D model's 14 atoms, with three carbon, seven hydrogen, one nitrogen and two oxygen. A CH₂ group links it to the second carbon, which holds four different groups; the model draws their (2R) arrangement, the form known as L-cysteine.

Is L-cysteine made from hair?

It used to be. L-cysteine was once produced almost entirely by breaking down hair or other keratins, according to HSDB. The amino acid recovered was cystine, which was then reduced to cysteine by electrolysis. Biological processes are now the main route, and a direct fermentation has been developed that makes L-cystine with a modified Escherichia coli bacterium. US Chemical Data Reporting for 2012 put national production at 28,660 pounds a year.

Is cysteine safe?

As a food ingredient, L-cysteine is affirmed by federal regulations as generally recognised as safe (GRAS) for use as a dough strengthener in yeast-raised baked goods. As a food flavouring, Haz-Map counts it as safe. The pure chemical is classified differently. Under the Globally Harmonized System it is classed as harmful if swallowed in 55.6% of company reports, while 33.8% of reports find it meets no hazard criteria.

Made of the same elements

C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.