Question

37. Proteins that form a ß-barrel pore in the membrane have several ß-strands that span the membrane. The amino acid side chains facing the inside of the pore would be hydrophilic whereas the amino acid side chains facing the lipid bilayer would be hydrophobic. Which of the three 10-amino acid sequences listed below is the most likely candidate for a transmembrane ß-strand in a ß-barrel protein? Explain your choice.

(a) Ala-Asp-Phe-Lys-Leu-Ser-Val-Glu-Leu-Thr
(b) Ala-Phe-Leu-Val-Leu-Asp-Lye-Ser-Glu-Thr
(c) Ala-Phe-Asp-Lys-Leu-Val-Ser-Glu-Leu-Thr

Answer 1

Sequence (b) is the most likely candidate for a transmembrane ß-strand because it contains a higher proportion of hydrophobic amino acids that can interact with the lipid bilayer's hydrophobic core.

Step-by-step explanation:

The student is asking which of the three given 10-amino acid sequences is the most likely candidate for a transmembrane ß-strand in a ß-barrel protein. The criteria for a transmembrane ß-strand is that it must have hydrophobic amino acids on the sides facing the lipid bilayer and hydrophilic amino acids on the inside of the pore. Therefore, it is essential to assess the polarity of the amino acids within each sequence.

Sequence (a) Ala-Asp-Phe-Lys-Leu-Ser-Val-Glu-Leu-Thr contains multiple charged and polar amino acids such as Asp, Lys, Ser, Glu, and Thr. Sequence (c) also contains several polar/charged residues. These would not be ideal for interacting with the hydrophobic core of the lipid bilayer. On the other hand, sequence (b) Ala-Phe-Leu-Val-Leu-Asp-Lye-Ser-Glu-Thr has a higher proportion of hydrophobic amino acids like Ala, Phe, Leu, and Val, which makes it the most suitable candidate for a transmembrane ß-strand that would be in contact with the lipid bilayer's hydrophobic environment.