Disulfide bonds most commonly stabilize the native structures of which class of proteins?

Introduction:
Disulfide bonds are covalent links formed by oxidation of cysteine thiols. Their prevalence varies across cellular compartments and protein classes. This question tests the association between disulfide bonding and the environment in which proteins fold and function.

Given Data / Assumptions:

• Intracellular cytosol is generally a reducing environment.
• Extracellular and certain organellar spaces are more oxidizing.
• Disulfide bond formation requires oxidation of two cysteine residues.

Concept / Approach:
Because disulfides are stabilized in oxidizing settings, they are far more common in secreted and extracellular proteins (e.g., antibodies, many hormones). In the reducing cytosol, disulfides are less stable unless formed in specific compartments (e.g., endoplasmic reticulum) before secretion.

Step-by-Step Solution:

Verification / Alternative check:
Proteomic surveys reveal a high density of disulfides in secreted proteins and low frequency in cytosolic proteins, consistent with redox chemistry and protein trafficking pathways.

Why Other Options Are Wrong:

• Dimeric or hydrophobic proteins: dimerization or hydrophobicity does not guarantee disulfides.
• Intracellular proteins: reducing cytosol disfavors disulfide stability.
• Membrane proteins: many lack disulfides unless extracellular loops require them.

Common Pitfalls:
Assuming that multimeric state implies disulfides or overlooking the role of cellular redox conditions.

Final Answer:
extracellular proteins