Definition

1. Noun:
   • A sulfur-containing amino acid: Cysteine is an organic compound, specifically an α-amino acid, that contains a thiol (-SH) functional group. It is a semi-essential amino acid found in most proteins.
   • A biochemical building block: It is a crucial component in the synthesis of proteins and other important biomolecules like glutathione. It is notable for its ability to form disulfide bonds, which contribute to the three-dimensional structure of proteins.

Usage Examples

• Noun:
  • Cysteine is important for protein structure because it can form disulfide bonds.
  • The antioxidant glutathione is synthesized from cysteine, glutamate, and glycine.
  • Some hair straightening treatments work by breaking the disulfide bonds between cysteine residues in keratin.

Advanced Usage

• "Cysteine residue": Refers to a single unit of cysteine incorporated into a polypeptide or protein chain. The chemical properties of this residue are critical for protein folding and stability.

  • The enzyme's activity depends on a critical cysteine residue in its active site.

• "Free cysteine": Refers to cysteine molecules that are not bound in a protein or peptide, often discussed in metabolic or nutritional contexts.

  • The concentration of free cysteine in the cell is tightly regulated.

Variants and Related Words

• Cystine (noun): The oxidized, dimeric form of cysteine, where two cysteine molecules are joined by a disulfide bond (-S-S-).

  • Cystine is less soluble than cysteine and can form crystals.

• Cysteinyl (adjective): Pertaining to or derived from cysteine, often used in biochemical terminology.

  • The cysteinyl group is highly reactive.

Synonyms

• 2-Amino-3-mercaptopropanoic acid: The systematic IUPAC name for cysteine.
• L-cysteine: The specific, biologically active form (enantiomer) of the amino acid commonly found in proteins and supplements.

Related Phrases and Concepts

• Disulfide bond/bridge: A covalent bond formed between the thiol groups of two cysteine residues, crucial for stabilizing protein structure.

  • Insulin's structure is stabilized by disulfide bonds between cysteine chains.

• Thiol group: The -SH functional group present in cysteine, responsible for its reactivity and antioxidant properties.

  • The thiol group of cysteine can act as a nucleophile in enzymatic reactions.

Noun

1. an amino acid containing sulfur that is found in most proteins; oxidizes on exposure to air to form cystine