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Fibroin is an insoluble protein present in silk created by spiders, the larvae of Bombyx mori, other moth genera such as Antheraea, Cricula, Samia and Gonometa, and numerous other insects. Silk in its raw state consists of two main proteins, sericin and fibroin, with a glue-like layer of sericin coating two singular filaments of fibroin called brins.[1][2]

Primary structure of fibroin, (Gly-Ser-Gly-Ala-Gly-Ala)n

The fibroin protein consists of layers of antiparallel beta sheets. Its primary structure mainly consists of the recurrent amino acid sequence (Gly-Ser-Gly-Ala-Gly-Ala)n. The high glycine (and, to a lesser extent, alanine) content allows for tight packing of the sheets, which contributes to silk's rigid structure and tensile strength. A combination of stiffness and toughness make it a material with applications in several areas, including biomedicine and textile manufacture.

Fibroin is known to arrange itself in three structures, called silk I, II, and III. Silk I is the natural form of fibroin, as emitted from the Bombyx mori silk glands. Silk II refers to the arrangement of fibroin molecules in spun silk, which has greater strength and is often used in various commercial applications. Silk III is a newly discovered structure of fibroin.[3] Silk III is formed principally in solutions of fibroin at an interface (i.e. air-water interface, water-oil interface, etc.).


Many species of Amycolatopsis and Saccharotrix bacteria are able to degrade both silk fibroin and polylactic acid.[4]


  1. ^ O. Hakimi et al., “Spider and mulberry silkworm silks as compatible biomaterials”,Composites Part B: Engineering, vol. 38 (3), pp. 324--337, article doi:10.1016/j.compositesb.2006.06.012
  2. ^ T. Dyakonov et al., “Design and Characterization of a Silk-Fibroin-Based Drug Delivery Platform Using Naproxen as a Model Drug”, Journal of Drug Delivery, vol. 2012 (2012), article ID 490514
  3. ^ Valluzzi, Regina; Gido, Samuel P.; Muller, Wayne; Kaplan, David L. (1999). "Orientation of silk III at the air-water interface". International Journal of Biological Macromolecules. 24 (2–3): 237–242. doi:10.1016/S0141-8130(99)00002-1.
  4. ^ Yutaka Tokiwa; Buenaventurada P. Calabia; Charles U. Ugwu; Seiichi Aiba (September 2009). "Biodegradability of Plastics". International Journal of Molecular Sciences. 10 (9): 3722–3742. doi:10.3390/ijms10093722. PMC 2769161. PMID 19865515.