Cysteine (Cys, C)
Cysteine is a sulfur-containing amino acid with a reactive thiol side chain. It forms disulfide bonds that cyclise and stabilise peptides, and it is the main oxidation-sensitive residue alongside methionine and tryptophan.
Structure and properties
Cysteine carries a thiol group on a short side chain. The thiol is a strong nucleophile, binds metal ions such as zinc and copper, and is readily oxidised. Two cysteines can oxidise to form a covalent disulfide bond (cystine), which links distant parts of a chain or separate chains. Free cysteine is fairly hydrophobic and is often buried in folded proteins. Its reactivity makes it the most common site for selective chemical modification, such as attaching labels, polyethylene glycol or linkers through maleimide or haloacetamide chemistry. Unlike other residues, L-cysteine has the R configuration under CIP rules because of the sulfur atom's priority.
Role in peptides
Disulfide bonds are the classic way nature constrains small peptides. Oxytocin is cyclised by a disulfide between its first and sixth residues, and many toxins and hormones use several disulfides to hold a defined fold. Correct pairing is essential for activity, so multi-disulfide peptides are often made with orthogonal protecting groups so that each pair forms in a controlled step. Glutathione (gamma-Glu-Cys-Gly) relies on its free cysteine thiol for its redox chemistry. Engineered cysteines are also widely added to peptides as conjugation handles.
Stability and analysis
Free thiols oxidise in air, especially at neutral or basic pH and in the presence of trace metals, forming dimers, mixed disulfides or higher oxidation states. Peptides with several cysteines can form scrambled or intermolecular disulfides, so HPLC and mass spectrometry should confirm both the expected mass and the disulfide state. Cysteine also racemises readily during activation in synthesis. Lab practice is to store under inert gas, use degassed solvents, and add reducing agents only where the free thiol is intended.
In PeptideWiki compounds
Compounds whose sequence contains cysteine, with the number of occurrences.
- IGF-1 LR3 (6)
- AOD-9604 (2)
- Calcitonin (salmon) (2)
- Oxytocin (2)
- Pramlintide (2)
- Setmelanotide (2)
- Glutathione (1)
- Humanin (1)
Frequently asked questions
What is the difference between cysteine and cystine?
Cysteine is the single amino acid with a free thiol. Cystine is the disulfide-linked pair formed when two cysteine thiols are oxidised. In a peptide, a cystine bridge can be intramolecular, closing a ring as in oxytocin, or intermolecular, joining two chains. The two states differ by the loss of two hydrogens, which high-resolution mass spectrometry can detect, and often by HPLC retention.
How is disulfide pairing confirmed?
For a single disulfide, intact mass and HPLC comparison with a reduced sample are often enough. For peptides with several disulfides, the usual approach is partial reduction and alkylation or non-reducing enzymatic digestion followed by mass spectrometry, which identifies which cysteines are linked. Comparison with a reference standard of known connectivity is also common. Mispaired isomers have the same mass as the correct form, so mass alone cannot confirm pairing.
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