Introduction
Silk fibers from Bombyx mori (silkworm) have been utilized by mankind since ancient times due to their remarkable mechanical properties and comfort when woven into fabrics, with the earliest records dating to around 2700 BC [1]. They are thermally comfortable, elastic, strong, and soft—properties that make silk highly sought after as a luxury fabric for garments. Silk is also biocompatible, making it applicable as a medical biomaterial for sutures, drug delivery systems, and scaffolds, where it plays a vital role in tissue regeneration[2].
Fibroins are fibrous structural proteins composed of multiple subunits that assemble into high-strength materials like silk and byssal threads. The exact composition varies by organism but typically features core fibroin filaments bundled within a sericin coating. These core filaments consist of three key components: a heavy chain (FibH), a light chain (FibL), and glycoproteins, all stabilized by disulfide bonds. The N-terminal domain of FibH (FibNT) plays a crucial role in the pH-dependent assembly of fibroin molecules during fiber formation [3] (see pH-Dependent Structural Transition section).
Biological production
Bombyx mori fifth-instar larvae produce silk proteins in specialized silk glands, where synthesis is spatially organized - fibroins (FibH, FibL, and glycoproteins) are produced in the posterior silk gland while sericins are synthesized in the middle section. Both components are stored in the lumen of the middle silk gland prior to fiber formation. The spinning process is mediated by precise pH and ion concentration gradients along the anterior silk gland. As the silk proteins encounter this gradient, the FibNT domain undergoes a critical conformational transition from random coil to β-sheet structure. This pH-dependent structural change, driven by protonation of key acidic residues, confers stability and strength to the fiber while enabling the final assembly of mature silk[3].
Basic structure
The N-terminal domain of the fibroin heavy chain (FibNT 3UA0) is a homo-tetramer composed of 536 residues (134 for each monomer), most of which are hydrophilic ( in maroon). FibNT's is a homodimer with eight alternating β- and a disordered (Gly109-Ser126). Its (A and B) are nearly identical except for the (Phe26-Val35) conformation:
- Chain A: Adopts a loop conformation.
- Chain B: Forms a short α-helix.
The FibNT homodimer exhibits the following : β1A–β2A–β4B–β3B–β3A–β4A–β2B–β1B, where there are two β-hairpins (Thr36–Asn65 and Glu78–Ser107) connected with two type I β-turns (Asp49–Gly52 and Asp89–Gly92) for each monomer (animation highlights the chain A). The entire assembly is stabilized by an extensive network of 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 pKa 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, —such as those between Glu56–Asp44 and Asp100–Glu98—are disrupted, destabilizing β-sheet conformations until protonation occurs at lower pH.
