The 3-dimensional structure of Fibroin protein, a beta pleated sheet

"Silkworms" (the larvae of the silk moth Bombyx mori) produce silk in an aqueous environment, then exude it to the outside of their body via openings called spinnerets, forming fibres which are then spun - oriented into strands which dry and are used to cover the larva when it pupates. Other moth larvae produce similar products, as do various spiders, although these include other sticky proteins for the purposes of trapping insects.

Primary level of protein structure: The protein fibroin in silk has a repeating sequence of amino acids : mostly Gly-Ser-Gly-Ala-Gly-Ala, extending in length to over 5000 residues.

Secondary level of protein structure: These amino acids have small R groups, so the side chains fit together easily and allow many long straight parallel fibres to be formed, each facing in the opposite direction to the ones above and below, forming a beta pleated sheet .

The protein fibroin from silk arranged as a beta pleated sheet

This (constructed) graphic shows 3 layers of 5 (fairly short) strands, forming quite a compact sheet ("silk 1" arrangement).
Each strand is simply formed of alternating glycine and alanine residues (no serine).
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Primary structure

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Secondary structure

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Tertiary structure

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