Care guide
The Fibroin Complex: H-Chain, L-Chain and P25
Three Proteins, One Fixed Ratio
Fibroin is the structural half of silk. Look closer and fibroin itself is not a single protein either. It is an assembly of three:
- H-fibroin, the heavy chain — the large structural component
- L-fibroin, the light chain
- P25, also called fibrohexamerin, a small associated protein
They are secreted together as a single high-molecular-mass unit, and the proportions are not approximate. The molar ratio is 6:6:1 — six heavy chains and six light chains to every one P25.
The Detail That Makes It Convincing
That 6:6:1 ratio is found both in the lumen of the posterior silk gland and in the finished cocoon.
The proportion set at the point of manufacture survives spinning, extrusion, and solidification into fibre. Whatever the assembly process is, it is precise, and it is not disturbed by the fairly violent physical business of being drawn into a thread.
Where Each Is Made
All three fibroin components come from the posterior silk gland. The sericin coating is made further forward, in the middle gland. Silk is assembled along an organ in the order it will be used.
When the Genes Switch On
The silk protein genes — heavy chain, light chain, P25 and others — are not running throughout the animal's life. Expression rises sharply around day three of the fifth instar and stays high to day five, then eases as the larva enters the wandering stage and prepares to spin.
The animal spends its final larval days doing very little except manufacturing silk protein, which is why a fifth-instar silkworm eats as much as it does and why the whole life cycle is arranged around getting it there.
Why Anyone Needs This Detail
Because engineering silk means reproducing it. Anything aiming to make fibroin outside the animal — scaffolds, sutures, engineered fibres — has to reckon with the fact that natural silk is a three-protein complex in a fixed ratio, not a single molecule you can simply express and spin.
Related: inside the silk gland · what silk is made of