Two important classes of proteins that have similar amino acid sequences and biological function are called - and -keratins. The -keratins are the major proteins of hair and fingernails and compose a major fraction of animal skin. The -keratins are members of a broad group of intermediate filament proteins, which play important structural roles in the nuclei, cytoplasm, and surfaces of many cell types. All of the intermediate filament proteins are predominantly -helical in structure; in fact, it was the characteristic x-ray diffraction pattern of -keratin that Pauling and his colleagues sought to explain by their -helix model.

The structure of a typical -keratin, that of hair, is depicted in Figure 6.11. The individual molecules contain long sequences-over 300 residues in length-that are wholly -helical. Pairs of these helices twine about one another in the left-hand coiled-coil structure shown in the lower portion of Figure 6.11. The mutual wrapping is such that the amino acid side chains (most of which are small in -keratin-see Table 6.2) interdigitate. In hair, two of the coiled coils then further twist together to form a 4-molecule protofibril, as shown in the upper portion of Figure 6.11. Finally, eight protofibrils combine in either a circular or a square arrangement to make the microfibril that is the basis of hair structure. Such twisted cables are stretchy and flexible, but in different tissues -keratin is hardened, to differing degrees, by the introduction of disulfide cross-links within the several levels of fiber structure. (Note that -keratin has an unusually high content of cysteine-see Table 6.2.) Fingernails have many cross-links in their -keratin, whereas hair has relatively few. The process of introducing a "permanent wave" into human hair involves reduction of these disulfide bonds, rearrangement of the fibers, and reoxidation to "set" the waves thus introduced.

The -keratins, as their name implies, contain much more -sheet structure. Indeed, they represented the second major structural class described by Pauling and co-workers. The -keratins are found mostly in birds and reptiles, in structures like feathers and scales.