What is the signature amino acid sequence of amphipathic helices?

The signature amino acid sequence of amphipathic helices is characterized by the arrangement of polar and non-polar residues that creates a structure with hydrophobic and hydrophilic faces. In this case, the correct answer indicates that the sequence starts with two polar residues followed by two non-polar residues. This configuration ensures that one side of the helix, dominated by polar residues, interacts favorably with the aqueous environment, while the opposite side, populated by non-polar residues, remains hydrophobic.

This arrangement is crucial because it allows amphipathic helices to integrate into membrane structures or interact with other biomolecules in a selectively permeable manner. By placing polar residues at one end of the helix, these structures can effectively anchor in lipid bilayers, while the adjacent non-polar residues help maintain the integrity of the membrane.

Comparatively, other configurations, such as having alternating polar and non-polar residues or exclusively polar residues, do not effectively produce the necessary amphipathic character required for this type of helix. These alternative arrangements would either lead to a fully hydrophilic or fully hydrophobic surface, which would not fulfill the functional roles of amphipathic helices in biological systems.