Cysteinyl

Cysteinyl is an adjective used in biochemistry and chemistry to denote a relation to the amino acid cysteine. Cysteine is a sulfur-containing, polar amino acid characterized by its thiol (-SH) group, which participates in redox chemistry and disulfide bond formation in proteins.

In peptide and protein chemistry, cysteinyl can describe cysteine-containing residues within a peptide or protein, as well as compounds that include a cysteine-derived moiety. For example, cysteinylglycine is a dipeptide consisting of cysteine linked to glycine. The term is also used in reference to cysteinyl residues in proteins, or to modifications such as cysteinylation, where a cysteine-derived group forms a covalent mixed disulfide with a protein thiol, affecting redox state and function.

The term also appears in pharmacology and physiology in reference to cysteinyl leukotrienes (LTC4, LTD4, LTE4). These inflammatory mediators are produced from arachidonic acid and contain a cysteine-containing moiety derived from glutathione. Cysteinyl leukotrienes act on cysteinyl leukotriene receptors (CysLT1 and CysLT2) and play roles in conditions such as asthma and allergic reactions. Therapeutic agents that block these receptors, such as montelukast, are used to mitigate symptoms driven by cysteinyl leukotrienes.

Cysteinyl is distinct from cystine, which is the disulfide-linked dimer of two cysteine molecules. While cysteinyl denotes relations to cysteine or its derivatives, cystine refers specifically to the cysteine disulfide pair.