Membrane-spanning segments: which amino acids dominate? Which amino acids are most likely to reside within a hydrophobic membrane-anchoring domain of an integral membrane protein?

Introduction / Context:
Transmembrane helices of proteins traverse lipid bilayers. Their side chains must be compatible with the hydrophobic core of membranes rich in fatty acyl chains.

Given Data / Assumptions:

• Bilayer interior is nonpolar.
• Membrane-anchoring segments are typically alpha-helical and hydrophobic.
• We compare amino acid side chain properties.

Concept / Approach:
Hydrophobic residues (Leu, Ile, Val, Phe, Met, Ala) favor membrane cores. Polar/charged residues (Asp, Glu, Lys, Arg, His) are energetically unfavorable in the hydrocarbon interior unless specifically stabilized (for example, by pores or ion pairs).

Step-by-Step Solution:
Identify hydrophobic side chains: isoleucine, valine, phenylalanine are strongly hydrophobic.Contrast with options that include polar/charged residues (aspartate, lysine, histidine).Choose the set comprised entirely of hydrophobics.

Verification / Alternative check:
Kyte–Doolittle hydropathy plots reveal transmembrane segments enriched in these residues.

Why Other Options Are Wrong:
Inclusion of acidic/basic residues indicates water-exposed or functional sites, not core-spanning regions; serine/threonine/asparagine are polar.

Common Pitfalls:
Assuming aromatic residues are always surface-exposed; phenylalanine often stabilizes helices in membranes.

Final Answer:
Isoleucine, valine and phenylalanine.