Multiple Choice Question
Select the statement that is true of transmembrane domains.
Transmembrane domains are composed of hydrophobic amino acids.
Transmembrane domains are attached to the surface of the membrane.
Transmembrane domains are composed of hydrophilic amino acids.
The Correct Answer and Explanation is:
Correct Answer:
Transmembrane domains are composed of hydrophobic amino acids.
Explanation:
Transmembrane domains are regions of proteins that span across the lipid bilayer of cell membranes. These domains play a critical role in anchoring proteins within the membrane and facilitating interactions between the inside and outside of the cell.
The lipid bilayer that makes up the cell membrane consists largely of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. The interior of the membrane is a hydrophobic environment due to the fatty acid tails. Because of this, only certain types of molecules can comfortably reside or pass through the membrane.
Transmembrane proteins typically span the membrane with one or more alpha-helical segments, known as transmembrane domains. These domains must interact stably with the hydrophobic interior of the lipid bilayer. Therefore, they are predominantly composed of hydrophobic amino acids, such as leucine, isoleucine, valine, and phenylalanine. These non-polar amino acids allow the protein segment to be embedded in the membrane without being energetically unfavorable.
The hydrophobic nature of these amino acids ensures that the transmembrane domain can remain stably inserted in the membrane. If the domain were composed of hydrophilic amino acids, which are polar and interact well with water, it would be unfavorable for them to reside in the hydrophobic core of the membrane.
The other two options are incorrect:
- “Transmembrane domains are attached to the surface of the membrane” is incorrect because that describes peripheral membrane proteins, not transmembrane domains.
- “Transmembrane domains are composed of hydrophilic amino acids” is also false, as explained above, due to the incompatibility of hydrophilic amino acids with the membrane’s hydrophobic core.
Thus, the correct and most accurate statement is:
Transmembrane domains are composed of hydrophobic amino acids.