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.

The interation between the acidic residues of FibNT may play an important role in this behavior. Some of these may have their pKa values up-shifted near the transition point and therefore ionize at neutral pH. This would hinder the protein folding and assembly processes due to charge repulsion. For instance, the hydrogen bonds formed by Glu56 and Asp100 with Asp44 and Glu98 respectively could be broken in higher pH environements, destabilizing the β-sheets conformations.

Relevance

Structural highlights

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