USA Biology Olympiad (USABO) Practice Exam

Which amino acid would you MOST expect to find in the transmembrane region of integral transmembrane proteins?

1. Alanine

2. Serine

3. Tryptophan

4. Glycine

The correct answer is: Tryptophan

Tryptophan would most likely be found in the transmembrane region of integral transmembrane proteins due to its unique properties. This amino acid is classified as an aromatic, hydrophobic residue, which aligns well with the hydrophobic environment typically found within the lipid bilayer of cell membranes. The transmembrane region of integral proteins is generally composed of stretches of hydrophobic amino acids that interact favorably with the lipid components of the membrane. Tryptophan, containing an indole side chain, provides not only hydrophobic characteristics but also has the ability to participate in π-π stacking interactions, which can stabilize the structure within the membrane. Furthermore, its bulkier structure allows for specific interactions that can contribute to protein folding and functionality in the membrane setting. In contrast, other amino acids listed have different properties that make them less suitable for this region. For example, alanine is also hydrophobic but less bulky and less structurally diverse than tryptophan. Serine contains a hydroxyl group that makes it polar and, therefore, less compatible with the lipid bilayer environment. Glycine is versatile but predominantly found in flexible regions of proteins and does not provide the hydrophobic interaction necessary for embedding in the membrane. Thus, trypt