What does cysteine mean?
Cysteine is one of the twenty standard amino acids that serve as the building blocks of proteins. What sets it apart chemically is its sulfur-bearing thiol group, which gives the molecule unusual reactivity: neighboring cysteine residues within a protein can link together through disulfide bonds, locking chains into stable three-dimensional shapes. This property makes cysteine essential to the architecture of antibodies, enzymes, and structural proteins such as keratin in hair and skin. In nutrition, cysteine is classified as non-essential because the body manufactures it from other amino acids, yet it remains biologically important as a precursor to glutathione, a key cellular antioxidant. It is also familiar to many people under its modified supplement form, N-acetylcysteine, used clinically and popularly for various purposes. Though chiefly a scientific term, cysteine surfaces regularly in conversations about diet, hair care, and medicine.
nounA sulfur-containing amino acid that is a common constituent of proteins and can form disulfide bonds that help stabilize protein structure.
- A naturally occurring amino acid containing sulfur, found in most proteins and notable for its thiol (-SH) side chain.
- (Biochemistry) The specific residue within a protein chain whose thiols pair up to form disulfide bonds.
"The disulfide bridges formed between cysteine residues give the antibody its rigid three-dimensional shape."
"Eggs, poultry, and dairy are all good dietary sources of cysteine."
"Mutations that replace a cysteine with another amino acid often disrupt the protein's folded structure."
The plural appears mainly in technical writing when referring to individual cysteine molecules or residues within a protein.
"The active site contains two closely spaced cysteines."
Two strands of hair hold together because cysteine molecules in keratin link up hand-in-hand — it's why perms work and why hair smells burnt when singed.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cysteine
The word cysteine entered English in the mid-nineteenth century, derived from cystine, the related compound from which it was first obtained, with the suffix -eine marking it as an amine-containing substance. Cystine itself traces back to Greek kystis, meaning bladder or sac, because the compound was originally isolated from bladder stones. The same Greek root underlies medical terms such as cyst and cystic.
Related word forms
How cysteine is actually used
Cysteine is a technical term used mainly in biochemistry, nutrition, and medicine; in everyday speech it appears mostly in discussions of supplements such as N-acetylcysteine. It is a non-essential amino acid in humans, since the body can synthesize it from methionine and serine.
Easily confused with cysteine
Cysteine is the free amino acid itself, while cystine is an oxidized dimer made of two cysteine molecules joined by a disulfide bond.