Final answer:
Amphipathic alpha helices have hydrophobic and hydrophilic faces but do not have a fixed signature amino acid sequence. Instead, their amphipathic nature depends on the spatial arrangement of polar and non-polar amino acids.
Step-by-step explanation:
Amphipathic alpha helices in proteins have two distinct faces due to their structure; one side is hydrophilic and interacts with aqueous environments, while the other side is hydrophobic and tends to avoid water. These alpha helices are a type of secondary protein structure. The hydrophilic face interacts with the aqueous environment, and the hydrophobic face is oriented towards the interior of the protein or the lipid bilayer, if the protein is membrane-bound. A common signature amino acid sequence for amphipathic alpha helices does not exist as such because the amphipathic nature depends on the specific distribution of polar and non-polar amino acids along the helix.
Every helical turn in an alpha helix has 3.6 amino acid residues, with the R groups protruding outward. In contrast, the β-pleated sheet is another form of secondary structure in proteins, where backbone hydrogen bonds form the 'pleats' with R groups extending above and below the folds. Alpha helices and β-pleated sheets are crucial in the structure of both globular and fibrous proteins.