Silkworm fibroin (Figure 6.12) contains long regions of antiparallel sheet, with the polypeptide chains running parallel to the fiber axis. The sheet regions comprise almost exclusively multiple repetitions of the sequence

[Gly - Ala - Gly - Ala - Gly - Ser - Gly - Ala - Ala - Gly - (Ser - Gly - Ala - Gly - Ala - Gly)8]

Almost every other residue in the sheet region of fibroin is glycine and between them lie either alanine or serine residues. This alternation allows the sheets to fit together and pack on top of one another in the manner shown in Figure 6.12. The arrangement results in a fiber that is strong and relatively inextensible, because the covalently bonded chains are stretched to nearly their maximum possible length. Yet the fibers are very flexible, because bonding between the sheets involves only the weak van der Waals interactions between the side chains, which provide little resistance to bending.

Not all of the fibroin protein is in -sheets. As the amino acid composition in Figure 6.12 shows, fibroin contains small amounts of other, bulky amino acids like valine and tyrosine, which would not fit into the structure shown. These are carried in compact folded regions that periodically interrupt the -sheet segments, and they probably account for the amount of stretchiness that silk fibers have. In fact, different species of silkworms produce fibroins with different extents of such non - -sheet structure and corresponding differences in elasticity. The overall fibroin structure is a beautiful example of a protein molecule that has evolved to perform a particular function-to provide a tough, yet flexible fiber for the silkworm's cocoon or the spider's web.