Fibroin
Also searched as: silk fibroin, fibroin heavy chain, H-chain, silk II, brin
The structural core protein of silk; in Bombyx mori it is a complex of heavy and light chains whose glycine- and alanine-rich repeats crystallize into antiparallel beta sheets.
In detail
Each cocoon filament contains two fibroin strands, called brins, cemented together by a coating of sericin. Bombyx mori fibroin combines a very large heavy chain, a much smaller light chain joined to it by a disulfide bond, and a glycoprotein called P25 that associates with the pair without covalent links. The heavy chain is dominated by short repeating motifs of glycine, alanine and serine. As the silkworm spins, these repeats stack into tightly packed beta sheets that form crystalline domains, separated by less regular segments that give the filament some extensibility.
The science and numbers
The Bombyx mori heavy chain has about 5,263 amino acid residues and a mass of roughly 391 kDa, compared with about 26 kDa for the light chain. Its composition is roughly 46% glycine, 30% alanine, 12% serine and 5% tyrosine. The repetitive core consists mainly of the hexapeptides GAGAGS and GAGAGA, with some alanine or serine positions replaced by tyrosine or valine, so the motif can be written broadly as Gly-Ala-Gly-X. Because glycine carries only a hydrogen side chain and alanine a methyl group, adjacent sheets can pack very closely. The stored, unspun form, called silk I, has been assigned a type II beta-turn structure rather than an alpha-helix, while the spun form, silk II, is the antiparallel beta-sheet crystal. Irregular, non-repetitive segments of the heavy chain form the less ordered regions.
A practical example
A conservator compares a degummed silk thread with a thread of similar diameter under polarized light and sees that the silk glows brightly when rotated at certain angles. The class explains this birefringence as a sign of molecular alignment: beta-sheet crystallites in fibroin lie largely parallel to the filament axis, so light travels differently along the fiber than across it. The same alignment underlies silk's strength along its length.
What to distinguish
Fibroin sequences differ between species, so figures for Bombyx mori do not transfer directly to wild silks such as tussah or eri, or to spider silks. Fibroin is also not the whole raw filament: sericin forms a substantial share of cocoon mass until it is removed by degumming. Regenerated fibroin used in biomaterials has been dissolved and reprocessed, so its structure depends on that processing rather than on the silkworm's spinning.
Origins and history
The word combines the Latin fibra, fiber, with -in, a common ending for protein names, and it distinguishes the fiber-forming protein from the gummy sericin around it. Silk's diffraction pattern made it a classic example of the extended beta structure first identified in stretched keratin. Detailed sequence knowledge came much later, with the cloning and characterization of the fibroin genes; the structure of the light-chain gene was reported in the early 1990s.
Related terms
Sources & further reading
- PMC: Molecular organization of fibroin heavy chain and mechanism of fibre formation in Bombyx mori
- PMC: Structure of silk I (Bombyx mori silk fibroin before spinning), type II beta-turn, not alpha-helix
- PubMed: Structural study of irregular amino acid sequences in the heavy chain of Bombyx mori silk fibroin
- Gene (ScienceDirect): Structure of the Bombyx mori fibroin light-chain-encoding gene
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.