Cysteine chemistry: The sulfur-containing side chain of cysteine exhibits which property profile and reactivity toward another cysteine residue?

Introduction / Context:
Cysteine (Cys) plays an outsized role in protein structure and catalysis due to its thiol (–SH) side chain. Understanding its polarity and reactivity explains disulfide bond formation, metal binding, redox regulation, and active-site chemistry.

Given Data / Assumptions:

• Cysteine side chain = sulfhydryl (thiol).
• Disulfide formation occurs via oxidation of two cysteine residues to cystine (–S–S–).
• Physiological aqueous environment makes thiols relatively polar/ionizable.

Concept / Approach:
The –SH group is polarizable and can be deprotonated to a thiolate (–S^−), increasing nucleophilicity. Two cysteines can oxidize to a disulfide, stabilizing extracellular proteins and secreted peptides; intracellularly, reducing environments often keep cysteines in thiol state.

Step-by-Step Solution:
Classify polarity: the thiol participates in hydrogen bonding and ionization → broadly hydrophilic relative to aliphatic residues.Identify reactivity: thiol/thiolate is nucleophilic and redox-active.Recognize structural consequence: two cysteines form disulfide bridges that stabilize tertiary/ quaternary structure.Select the description that matches these properties.

Verification / Alternative check:
Many enzymes (e.g., cysteine proteases) rely on the thiolate’s nucleophilicity; secreted proteins (e.g., antibodies, insulin) contain stabilizing disulfide bonds.

Why Other Options Are Wrong:

• Amphiphilic/hydrophobic: mischaracterizes the thiol’s polar/ionizable nature.
• None of the above: invalid because cysteine does form disulfides and is reactive.

Common Pitfalls:
Assuming all sulfur-containing residues are hydrophobic; confusing methionine (thioether, less reactive) with cysteine (thiol, reactive).

Final Answer:
Hydrophilic, highly reactive, and forms a disulfide with another cysteine