Disulfide chemistry: Which amino acid name is used when referring to a ‘‘half-cystine’’ residue participating in a disulfide bond within proteins?

Introduction / Context:
Disulfide bonds stabilize protein tertiary and quaternary structures. The terminology around cysteine/cystine and “half-cystine” can be confusing but is routinely used in biochemistry, structural biology, and proteomics.

Given Data / Assumptions:

• Cysteine side chain contains a thiol (–SH).
• Two cysteine residues can oxidize to form a disulfide bond (–S–S–), commonly called cystine.
• “Half-cystine” refers to each cysteine residue within a disulfide linkage.

Concept / Approach:
When a disulfide forms, the individual residues are still cysteine residues by sequence, but in structural/analytical contexts each partner within the –S–S– is often called a half-cystine. The term cystine names the covalent dimer, not the individual amino acid.

Step-by-Step Solution:
Identify residue forming disulfides → cysteine. Name of the oxidized dimer → cystine (two cysteines linked). Each participant within the disulfide → “half-cystine.” Therefore, the amino acid referred to is cysteine.

Verification / Alternative check:
Protein sequence annotations and mass-spec modifications list “cysteine (disulfide-linked)” or “half-cystine” for residues engaged in disulfides.

Why Other Options Are Wrong:
Isoleucine, valine, histidine cannot form disulfides; “cystine” denotes the linked pair, not a single residue.

Common Pitfalls:
Confusing the term cystine with half-cystine; assuming half-cystine is a different amino acid.

Final Answer:
Cysteine.