35. Structure of Silk

Structurally, five types (coiled coil, β-strand, cross β-sheet, collagen triple helix, and polyglycine) of silk have been identified that vary in the amount of crystalline and amorphous regions and the arrangement (sequence and orientation) of the amino acids along the axis of the fiber. Each type of structure has a specific sequence of amino acids. For instance, the coiled-coil structure has seven amino acid residues, and the protein chains form a right-handed α-helix with 3.2 amino acids per turn. In the coiled-coil silks produced by some insect species such as honeybees and weaver ants, each fibroin contains 210 amino acid residues in the coiled-coil region with alanine-rich cores [07Sut]. A structural model for the coiled-coil silk is shown in Fig. 35.1. Coiled-coil silks were also found to contain unusually high levels of alanine and large hydrophobic residues. The high levels of alanine were required to stabilize the helices and facilitate coiled-coil formation [07Sut]. In a β-strand structure, alternating amino acid side chains form opposite faces of the sheet and in a cross-β sheet, the protein chains form β-strands of uniform length and alternating turns at which the direction of the protein chain reverses. In a collagen triple helix, three 32 helices intertwine and form a superhelix, and finally in a polyglycine structure, the protein chains form a right-handed helix with three amino acids per turn. Figure 35.2 illustrates the five different types of structures discussed here.