Skin keratin, amino acid composition

Although a helices are abundant in proteins, the average length is fairly short, that is, 17 A long containing about 11 amino acids or three turns. Therefore, a helices are typically found in short domains within proteins and not as continuous stretches. Keratins that make up hair and the most superficial layer of skin contain a central domain with an a helical component of 310 amino acids or about 46.5 nm in length. Keratin is an example of a protein with a fairly long a helix. The amino acid composition that favors a helix formation is fairly broad with the exception of proline and serine. 

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