What does cystein mean?
Cystein is a variant spelling — common in older texts and in German-influenced scientific nomenclature — of the amino acid more familiarly rendered as cysteine in English. Cysteine is a proteinogenic amino acid distinguished by a thiol (-SH) group on its side chain, which makes it unusually reactive among the standard amino acids. This reactivity allows two cysteine molecules to oxidise and join through a disulfide bridge, forming cystine and giving proteins such as keratin structural rigidity. In biochemistry, cysteine residues also serve catalytic roles at enzyme active sites and contribute to cellular antioxidant defences through compounds like glutathione. Although 'cystein' remains legible to any chemist, modern English-language publications almost universally adopt the 'cysteine' spelling, so readers encountering 'cystein' today are most likely looking at a translated, historical, or continental European source.
nounA variant spelling of cysteine: a sulphur-containing amino acid that is a constituent of many proteins and is notable for its thiol (-SH) side chain, which allows it to form disulfide bonds.
- As a variant spelling of cysteine: a sulphur-bearing amino acid found in proteins, capable of forming disulfide bonds.
"The disulfide bridges formed by cystein residues give the protein its stable three-dimensional shape."
"The keratin in hair owes its strength to disulfide links between adjacent cystein residues."
"Because of its reactive thiol group, cystein plays a key role in the folding of many enzymes."
Rarely pluralised except when referring to multiple residue instances within a protein sequence.
"The enzyme contains four conserved cysteins near its active site."
'Cystein' with no final 'e' is how much of continental Europe spells the same amino acid English speakers call cysteine — one silent letter separating two spelling traditions.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cystein
The name derives from the Greek 'kystis' (κύστη), meaning bladder or sac, because the related compound cystine was first isolated from bladder stones in the early nineteenth century; the amino acid was named accordingly. The '-ein' ending reflects German chemical nomenclature of the period, which is why 'cystein' persists in German usage while English settled on 'cysteine'. The word thus shares its root with 'cyst' and related medical terms such as 'cystic'.
How cystein is actually used
'Cystein' is a non-standard spelling in modern English; scientific literature overwhelmingly uses 'cysteine'. The '-ein' ending reflects German nomenclature conventions. Writers should prefer 'cysteine' unless quoting historical or European sources.
Easily confused with cystein
Cysteine is the standard English spelling of the amino acid, while 'cystein' is an older or German-influenced variant spelling of the same word.
Cystine is a distinct compound formed when two cysteine molecules are joined by a disulfide bridge.