Final answer:
Leucine is the amino acid most likely to be found in the transmembrane alpha helix of a single-pass transmembrane protein because of its nonpolar, hydrophobic nature.
Step-by-step explanation:
The question asks about the type of amino acid that would most likely reside in the transmembrane alpha helix of a single-pass transmembrane protein. Within the hydrophobic environment of the lipid bilayer, the amino acids that make up transmembrane regions tend to be non-polar and hydrophobic themselves because they are compatible with the lipid tail regions of the bilayer.
Based on the given options, Leucine is the amino acid most likely to reside in the transmembrane alpha helix of a single-pass transmembrane protein. Leucine is hydrophobic, which means it is more likely to be found in the membrane-spanning regions of proteins. In comparison, Glycine is small and flexible, which might disrupt the helix, Proline induces kinks in the helix due to its ring structure, and although Alanine is hydrophobic, Leucine is more nonpolar and commonly found in transmembrane helices.