Collagen is a strong, extensible, insoluble and chemically inert animal protein. Collagen is the primary componenent of connective tissue, and is the most abundant protein in vertebrates. Collagen gives strength to bones, teeth, cartilage, tendon and the fibrous matrices of skin, cornea, intervertebral discs and blood vessels. Three collagen strands form a parallel triple helix that is nearly one third glycine. The consensus amino acid sequence of collagen is (-Gly-Pro-Hyp-)n, where Hyp is 4-hydroxyproline. Each triple-helix associates into a right-handed superhelix called a microfibril. Each microfibril is interdigitated with its neighboring microfibrils. Collagen is heavily modified and cross-linked (at Lys and His), depending on the tissue type.
Alpha Keratin is a coiled-coil. It is strong, inextensible, insoluble and chemically inert. Alpha Keratin is found in hair, wool, horn, and tails. The consensus amino acid sequence is a repeating heptamer of (-a-b-c-d-e-f-g-)n where residues a and d are non-polar. The two alpha helicies coil, rather than form a ladder, because each alpha helix turns 700 degrees each (-a-b-c-d-e-f-g-)n repeat. Alpha keratin contains cysteine crosslinks (disulfides), which are reduced and reoxidized during a "perm". [The structure contained here is a coiled-coil from GCN4, not from alpha Keratin. The PDB does not appear to contain a model or structure of alpha Keratin.]
Silk Fibroin is strong, inextensible, insoluble and chemically inert. It is composed of antiparallel beta sheets. This fiber is made by insects and spiders. The strands of the beta sheets run parallel to the fiber axis. Long stretches of silk Fibroin are composed of microcrystalline arrays of (-Gly-Ser-Gly-Ala-Gly-Ala-)n interrupted by regions containing bulkier residues. In sum the fiber is composed of microcrystalline arrays alternating with amorphous regions.