What type of amino acid residue is likely to be prevalent in the middle region of the seven α-helical domains of Frizzled?

The presence of nonpolar amino acid residues in the middle region of the seven α-helical domains of Frizzled is linked to the structural and functional properties of transmembrane proteins. The α-helices that span cell membranes are typically composed of hydrophobic (nonpolar) amino acids.

This hydrophobic nature is crucial because it allows these segments to interact favorably with the lipid bilayer of the membrane, helping maintain the structural integrity of the protein and facilitating proper insertion into the membrane. In contrast, polar and charged residues tend to be found in regions of proteins that interact with the aqueous environment or form functional sites that require solubility and interaction with water or other polar molecules.

Thus, a predominance of nonpolar amino acids in the mid-regions of Frizzled’s α-helical domains suggests an adaptation to function effectively as a membrane receptor while minimizing unfavorable interactions with the lipid membrane itself.