The phrase "an oil drop with a polar coat" metaphorically describes the three-dimensional organization of which class of proteins?

Introduction:
The metaphor “an oil drop with a polar coat” captures the essence of how many soluble proteins arrange their side chains. This question evaluates understanding of the hydrophobic effect and how it drives folding in aqueous environments, particularly for globular proteins.

Given Data / Assumptions:

• Aqueous cellular environments favor burying hydrophobic groups.
• Polar and charged groups remain solvent-exposed to interact with water.
• Protein function often requires a compact, dynamic fold.

Concept / Approach:
Globular proteins fold so that nonpolar side chains cluster in the interior (like oil), while polar and charged residues form a shell that interfaces with water (the polar coat). This arrangement maximizes favorable interactions with solvent and minimizes the entropic cost associated with hydrophobic side chains disrupting water structure.

Step-by-Step Solution:

Verification / Alternative check:
Structural analyses of enzymes and carriers (typical globular proteins) consistently show hydrophobic cores and hydrophilic surfaces, validating the metaphor.

Why Other Options Are Wrong:

• Fibrous proteins, collagen, silk, elastin: predominantly extended or specialized structural roles; they lack a compact hydrophobic core with a polar shell typical of soluble globular proteins.

Common Pitfalls:
Assuming every protein follows this pattern; membrane proteins often invert this logic, presenting hydrophobic residues outward to interact with lipid bilayers.

Final Answer:
globular proteins