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Function

Basic structure

The N-terminal domain of the fibroin heavy chain (FibNT 3UA0) is a homo-tetramer composed of 268 residues, most of which are (hydrophilic amino acids in marron and hydrophobic in medium blue). FibNT's asymmetric unit is a homodimer with eight alternating β-sheets and a disordered C-terminus (Gly109-Ser126). Its two chains (A and B) are nearly identical except for the N-terminal segments (Phe26-Val35):

• Chain A: Forms a short α-helix.
• Chain B: Adopts a loop conformation.

The FibNT homodimer exhibits the following topology: β1_A–β2_A–β4_B–β3_B–β3_A–β4_A–β2_B–β1_B, where the β-sheets are connected via two β-hairpins (Thr36–Asn65 and Glu78–Ser107) and two type I β-turns (Asp49–Gly52 and Asp89–Gly92). The entire assembly is stabilized by an extensive network of hydrogen bonds between adjacent β-strands.

pH-Dependent Structural Transition

The structure of fibroin is highly pH-dependent. During the natural silk-spinning process, the fibroin solution experiences a steep pH gradient along the silk gland (from anterior to posterior), which triggers the gelation of condensed fibroin. Specifically, the N-terminal domain (FibNT) remains in a disordered random-coil state at neutral pH, preventing premature β-sheet formation. Only when the pH drops to approximately 6.0 does FibNT undergo a cooperative structural transition, adopting the stable β-sheet conformation essential for fiber assembly.

Interactions between acidic residues in FibNT are critical for pH-sensitive behavior. Near the transition point (pH ~6.0), some residues exhibit up-shifted pK_a values, allowing them to remain ionized at neutral pH. This sustained negative charge creates electrostatic repulsion, actively preventing premature folding and β-sheet assembly. For example, at higher pH, key hydrogen bonds—such as those between Glu56–Asp44 and Asp100–Glu98—are disrupted, destabilizing β-sheet conformations until protonation occurs at lower pH.

Relevance

Silk fibers from Bombyx mori have been utilized by mankind since ancient times due to its remarkable mechanical properties and comfort when woven into fabrics, its earliest record being around 2.700 BC. It is thermally comfortable, elastic, strong and soft and that is why it is highly sought after as a luxury fabric for garments. It is also biocompatible, making it even applicable as a medical biomaterial for sutures or scaffolds.

Bombyx mori silkworms naturally use their silk to construct a cocoon at the end of the final stage of larval development before cocoon formation, before they undergo metamorphosis into a moth. During the natural spinning process, the silkworm extrudes the silk dope (a water-soluble liquid crystalline state containing up to 30%wt/vol fibroin in water) from its spinnerets into the external environment. This process involves mechanical shearing, stretching, and water evaporation. The delicate gland conditions (silk dope acidification, concentration changes of metal ions, and water content reduction) are crucial for the proper folding of fibroin into micelles and then liquid crystals.