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a simple protein, the principal component of the natural silk and spider-silk fibers secreted by the silk-secreting glands of certain insects and the silk glands of spiders and other arthropods. Fibroin is a viscous, syrupy fluid, that hardens in the air to a tough, insoluble filament. Fibroin threads, coated with the protein sericin, form a silk fiber, as in silkworm cocoons.
Fibroin belongs to a group of structural proteins called scleroproteins. It is resistant to organic solvents, dilute acids and bases, and proteolytic enzymes. The fibroin molecule consists of an elongated polypeptide chaino with repeating structural units, measuring approximately 7 Å. The group of amino acids in fibroin includes a high content of glycine (43 percent), alanine, serine, and tyrosine. The content of the amino acid tyrosine in fibroin is so much higher than in other proteins that fibroin is usually used as the prime source for obtaining tyrosine.