User:Vinícius M. Neto/Sandbox 1

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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: β1_A–β2_A–β4_B–β3_B–β3_A–β4_A–β2_B–β1_B. 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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