Serione Silk Proteins

Diving Deep into the Composition: Sericin & Fibroin

In our previous post, we introduced you to the magical world of silk proteins, setting the stage for our deep dive into two of the most critical components of silk: Sericin and Fibroin. Together, these two silk proteins create the extraordinary properties that make silk a material of choice in various industries. Let’s explore what makes these two silk proteins truly unique.

A Closer Look at Sericin and Fibroin

Silk, as we know, is primarily composed of two proteins – Sericin, the adhesive that holds the silk fibers together, and Fibroin, the fibrous protein that forms the core structure. Here’s a closer look at each of these.


Sericin is a water-soluble glycoprotein making up 20-30% of silk. It serves as the “glue” that holds the silk fibers together. This “silk gum” is removed during the silk degumming process to reveal the underlying fibroin fibers. But don’t be fooled by the process; Sericin has impressive properties of its own.


Fibroin, the structural core of silk, makes up about 70-80% of the material. Composed of tightly bound protein chains, it provides silk with its unique strength and sheen. Fibroin fibers, when unwoven, can reach incredible lengths, adding to the desirability of this silk protein.


Silk fibroin and sericin are both proteins, which are naturally formed as chains of amino acids. These chains then fold in various ways to form the final three-dimensional structure of the protein. The structure of a protein is determined by the sequence of amino acids it contains and the environmental conditions under which it folds.

Fibroin, the core component of silk, has a unique structure. It’s a block copolymer, consisting of heavy-chain fibroin (~350kDa), light-chain fibroin (~25kDa), and a glycoprotein, P25 (~30kDa), which associate in a 6:6:1 molar ratio.

  1. Heavy-chain fibroin (H-chain, ~350kDa): This high molecular weight protein is the primary structural component of fibroin. The H-chain is composed of a repeating GAGAGS hexapeptide sequence, contributing to the crystalline regions that give silk its impressive tensile strength.
  2. Light-chain fibroin (L-chain, ~25kDa): The L-chain is a smaller, globular protein that interacts with the H-chain. It’s hypothesized to stabilize the structure of the H-chain, though its exact role isn’t fully understood.
  3. Glycoprotein P25 (~30kDa): This glycoprotein is thought to act as a glue between the H-chain and L-chain. It aids in the assembly of the fibroin complex, ensuring the overall structure of fibroin is strong and stable.

The fibroin molecule is known to fold into a primarily beta-sheet structure, which is then assembled into higher-order structures that give silk its strength and flexibility. In this case, the chains of fibroin proteins align and tightly pack together, allowing for strong intermolecular interactions. This leads to a structure that’s more rod or fiber-like than round.

On the other hand, sericin, being a globular protein, folds into a more spherical or globular shape. Sericin consists of random coils and beta-sheets. However, it’s important to note that these globular structures can vary greatly in shape, from near-spherical to more elongated forms, depending on the exact sequence of amino acids and the conditions during folding. The structure of sericin also varies significantly depending on the species of silkworm, diet, and environmental conditions.

It’s also worth noting that these descriptions are simplifications. In reality, proteins often contain both elements of order (such as the beta-sheets in fibroin) and disorder (such as the random coils in sericin), and the exact three-dimensional structure of these proteins can be extremely complex.

The exact atomic structures of these proteins have not been fully elucidated due to the challenges posed by their size and complexity.

The Amino Acid Breakdown

The silk proteins are formed of numerous amino acids. Here’s a simplified tabular breakdown of their composition:

Amino AcidFibroin (%)Sericin (%)

Let’s dive into why each of these amino acids is important to the human body:

  1. Glycine: This is the simplest amino acid and acts as a building block for proteins. It aids in the production of collagen, which is crucial for skin, bone, and joint health.
  2. Alanine: This amino acid plays a key role in transferring nitrogen from peripheral tissue to the liver. It’s also involved in glucose metabolism, helping the body maintain energy levels.
  3. Serine: It aids in the production of phospholipids, which are crucial components of cell membranes. Additionally, serine plays a role in DNA production, fat metabolism, and muscle formation.
  4. Tyrosine: This amino acid is essential for the production of neurotransmitters, which influence mood and sleep. It’s also a precursor to melanin, the pigment responsible for skin and hair color.
  5. Valine: This branched-chain amino acid helps stimulate muscle growth and regeneration and is involved in energy production.

The properties and composition of silk proteins – specifically sericin and fibroin – demonstrate their versatility and wide range of potential applications. From textiles to biomedicine, these silk proteins continue to unfurl their fascinating secrets, promising to drive significant advancements in various fields. Stay tuned to our blog at Serione for more deep dives into the world of silk proteins.

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