The sequences for three different tripeptides are written out below. Indicate whether you expect to find them in the inner core or on the surface of a cytosolic protein, and explain your answer

Question

A. Serine-Threonine-Tyrosine
B. Alanine-Glycine-Leucine
C. Proline-Serine-Alanine

Answer 1

A. This tripeptide is made up of entirely polar amino acids, which means it will most likely be found on the surface of the protein, interacting with the aqueous environment of the cytosol.
B. This peptide is made up of nonpolar amino acids. The side chains are most likely buried in the interior of the protein, which would promote interactions with other hydrophobic side chains and avoid unfavorable interactions with the aqueous environment of the cytosol.
C. This peptide is made up of both polar and nonpolar amino acids, and one of the nonpolar amino acids is proline. Proline residues have a restricted degree of conformational freedom because the side chain is covalently linked to the backbone nitrogen as well as the ?-carbon. So, a likely scenario is that the proline is at the surface of the protein, providing a structural turn between two secondary structure elements (? strands or ? helices), the serine is still close enough to the surface to interact with water, and the alanine is close enough to the interior of the protein to interact with other hydrophobic side chains.