Protein packing in globular proteins — Typical coordination number: buried hydrophobic side chains usually fit into a hole formed by how many other side chains?

Introduction:
Protein cores are tightly packed like a three dimensional jigsaw. This question tests understanding of typical packing density in the hydrophobic interior of globular proteins, where nonpolar side chains interlock with neighbors to exclude water and maximize stabilizing contacts.

Given Data / Assumptions:

• Context: a folded, soluble globular protein under physiological conditions.
• Focus: how many neighboring side chains typically surround one buried hydrophobic side chain.
• No requirement to identify a specific residue; the topic is general core packing statistics.

Concept / Approach:
In protein interiors, packing approaches that of organic crystals but with some voids. Each buried side chain makes multiple van der Waals contacts with nearby residues. Empirical analyses of high resolution structures show a typical local coordination number around six, often described as a side chain fitting into a hole made by about five to seven neighbors.

Step-by-Step Solution:

Verification / Alternative check:
Consider common core residues such as Leu, Ile, Val, and Phe. Rotamer libraries and contact maps show each of these typically engages many short range contacts that sum to about half a dozen distinct neighbors, consistent with the 5–7 range rather than very low or very high counts.

Why Other Options Are Wrong:

• 1–3: underestimates real packing; would leave excessive cavities.
• 9–12 and 13–15: unrealistically high in a sterically crowded core.
• 2–4: still too low compared to observed coordination numbers.

Common Pitfalls:
Confusing the number of atomic contacts with the number of distinct neighboring residues. One neighbor can contribute many atom–atom contacts, but the count of distinct side chains remains near six on average.

Final Answer:
5–7 other amino acids.