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Function
Basic structure
The N-Terminal domain of the fibroin heavy chain (FibNT 3UA0) is a homo 4-mer composed of 268 residues, being most of them (hidrophilic amino acids in marron and hidrophobic in medium blue). FibNTs is a homodimer with 8 alternated β-sheets and a disordered C-terminus portion (Gly109-Ser126). Its two chains (chain A and chain B) are roughly the same but for the N-terminal segments (Phe26-Val35), that form a short helix packing in the chain A and a loop in the B.
The FibNT homodimer has the following topology: β1A–β2A–β4B–β3B–β3A–β4A–β2B–β1B. The β-sheets are conected with 2 β-hairpins (Thr36-Asn65 and Glu78-Ser107) and 2 type1 β-turns (Asp49-Gly52 and Asp89-Gly92). The whole assembly is packed toghether with several hidrogen bonds between the β-sheets.
Oligomerization
Fibroin oligomerization is deeply afected by the pH. Naturaly, during the silk spinning process, the fiber is subjected to a decreasing pH gradient from the anterior to the posterior part of the silk gland, which triggers the gelation of the condensed fibroin. In particular, the FibNT exists in a random coil state, which prevents premature formation of β-sheets. As the pH decreases to around 6.0, FibNT undergoes a structural transition to form β-sheets.
Relevance
Structural highlights
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