What Is Silk Protein Made Of? The Dual Structure of Fibroin and Sericin

January 10, 2026 •

4 min read

Approximately 70-80% of raw silk is composed of a fibrous protein called fibroin, with the remaining 20-30% consisting of a glue-like protein, sericin. Understanding what is silk protein made of reveals the complex biochemical architecture behind this famously luxurious and durable natural fiber.

Quick Summary

Silk protein is primarily composed of two proteins, fibroin and sericin, secreted by silkworms like the Bombyx mori. Fibroin forms the strong fibrous core, while sericin acts as a protective, adhesive outer layer. Their distinct amino acid profiles and structures give silk its unique properties, from tensile strength to softness.

Key Points

Dual Protein System: Silk is composed of two primary proteins, the fibrous, core protein fibroin, and the sticky, outer coating protein sericin.
Fibroin's Strength: Fibroin is rich in glycine and alanine, forming highly organized beta-sheets that give silk its exceptional tensile strength and durability.
Sericin's Properties: Sericin has a high content of polar amino acids, including serine, which provides it with moisturizing, adhesive, and protective characteristics.
Degumming Process: Sericin is water-soluble and is typically removed from fibroin during textile processing by boiling the cocoons, revealing the soft, lustrous fiber beneath.
Biosynthesis: Silkworms produce fibroin and sericin in different parts of their glands, extruding them through a spinneret where the proteins transition into their final fibrous structure.
Application Variation: The specific properties of fibroin and sericin can be utilized separately, with pure fibroin used for textiles and biomaterials, and extracted sericin finding use in cosmetics and health products.

The Dual Nature of Silk Protein: Fibroin and Sericin

At its core, silk is not a single protein but a composite of two distinct proteins, each playing a crucial role in the fiber's structure and properties. These two main components are fibroin and sericin, which are secreted in a complex process by the silk glands of insects like the Bombyx mori silkworm. To truly answer the question, "what is silk protein made of?", one must understand these two constituent parts and their unique makeup.

Fibroin: The Structural Backbone

Fibroin is the principal structural protein, making up the bulk of the silk fiber, approximately 70-80% of its total weight. It is a fibrous protein composed of a heavy chain (about 390 kDa) and a light chain (about 25 kDa), linked together by a disulfide bond. Its composition is dominated by a repeating amino acid sequence that forms layers of anti-parallel beta-sheets.

Amino Acid Profile: Fibroin's heavy chain has a highly repetitive amino acid sequence, particularly rich in glycine (43%), alanine (30%), and serine (12%). The high proportion of small-residue amino acids like glycine and alanine allows for the tight packing of beta-sheets, which is fundamental to silk's mechanical strength.
Key Sequence: A common repeating sequence in silkworm fibroin is (Gly-Ser-Gly-Ala-Gly-Ala)n. These repetitive regions are interspersed with less crystalline, more amorphous segments that feature other amino acids.
Structural Contribution: The highly crystalline beta-sheet regions give silk its exceptional tensile strength and durability. When the silk is spun, these protein molecules align and become insoluble, creating a rigid and stable framework.

Sericin: The Adhesive Matrix

Sericin is a sticky, gummy protein that coats the two fibroin filaments, acting as a binding agent to hold them together in the cocoon. It is a family of hydrophilic glycoproteins with a variable molecular weight (10-400 kDa) and makes up about 20-30% of the raw silk fiber. Sericin is typically removed during the textile processing stage known as degumming, which involves boiling the cocoons in hot water or an alkaline solution.

Amino Acid Profile: Sericin has a more varied amino acid composition than fibroin, but is characterized by a high content of amino acids with polar side groups, including serine, glycine, and aspartic acid. The abundance of hydrophilic amino acids makes sericin soluble in hot water, allowing for its removal during processing.
Layers of Sericin: The sericin coat is not uniform but is composed of different layers, each with a slightly different composition and solubility. This includes Sericin A (outermost, water-soluble), Sericin B (middle), and Sericin C (innermost, less soluble).
Functional Role: Beyond its adhesive properties, sericin provides protection to the developing silkworm by guarding the fibroin fibers from environmental factors. It also possesses beneficial properties that have led to its use in cosmetics, such as antioxidant and moisture-retaining capabilities.

The Biosynthesis Process

The creation of silk protein is a fascinating biological process. It occurs within the specialized glands of the silkworm larva. Fibroin is synthesized in the posterior section of the glands, while sericin is produced in the middle section. These proteins are stored in a concentrated, liquid form within the gland's lumen. As the silkworm spins its cocoon, the liquid is pushed through a narrow opening called the spinneret. This physical force and exposure to air induce a change in the protein's secondary structure, transforming the soluble form (Silk I) into the robust, insoluble, beta-sheet structure (Silk II) of the finished fiber.

A Tale of Two Proteins: A Comparison Table


Feature	Fibroin	Sericin
Composition	Primarily Glycine, Alanine, Serine	High percentage of polar amino acids like Serine and Aspartic Acid
Structure	Predominantly beta-sheet crystallites	Primarily globular, random coils with some beta-sheets
Function	Provides core strength and elasticity	Acts as a sticky, protective gum coating
Solubility	Insoluble in water	Soluble in hot water
Properties	Strong, durable, high tensile strength	Adhesive, moisturizing, protective, antioxidant
Primary Role	Main structural component of the fiber	Binder that holds fibroin strands together

Unveiling the Microscopic World of Silk

Beyond just the two proteins, the overall properties of silk also depend on the precise arrangement and interaction of fibroin and sericin. The semi-crystalline structure of the fibroin core, with its alternating rigid and amorphous domains, is key to its mechanical performance. The processing of silk plays a significant role in determining the final product's characteristics. For instance, the complete removal of sericin creates the smooth, lustrous silk fabric prized in textiles, whereas sericin-retaining applications leverage its moisture-retaining and protective properties. Scientists continue to study the intricate biochemistry of these proteins to replicate their properties in biomedical and materials science applications, a field where silk's biocompatibility is highly valued. The natural, renewable source of silk and its biodegradable nature also make it a sustainable material of interest for modern applications.

Conclusion: The Chemistry Behind the Luxury

In conclusion, what is silk protein made of is a question best answered by describing the synergistic relationship between its two main protein components: the strong, crystalline fibroin core and the protective, hydrophilic sericin gum. The specific amino acid sequences of these proteins, predominantly glycine and alanine in fibroin and serine in sericin, dictate their respective roles and unique properties. Together, they form the composite material that has captivated humanity for centuries, balancing exceptional strength with a soft, lustrous feel. From luxurious garments to advanced biomaterials, the compositional chemistry of silk is the key to its enduring appeal and remarkable versatility. Further exploration into this topic can be found in scientific literature like this review: PMC article on silk as a biomaterial.

Frequently Asked Questions

Fibroin is the core, fibrous protein that provides silk with its strength and durability. Sericin is the outer, glue-like protein that holds the fibroin strands together and is typically removed during the manufacturing of finished silk fabrics.

The main amino acids in silk protein vary between its two components. Fibroin is primarily composed of glycine, alanine, and serine, while sericin is notably high in serine and aspartic acid.

In fibroin, the high proportion of small, non-bulky amino acids like glycine and alanine allows protein chains to pack tightly into a beta-sheet structure. This tight packing is stabilized by hydrogen bonds, which gives silk its impressive tensile strength.

Yes, both fibroin and sericin are used in cosmetics. Hydrolyzed silk protein (from fibroin) is valued for its moisturizing and smoothing effects on skin and hair, while sericin's antioxidant and moisturizing properties make it beneficial in skincare products.

Sericin is removed through a process called degumming. This involves boiling the raw silk cocoons in hot water or an alkaline solution, which dissolves the water-soluble sericin, leaving the pure fibroin fibers behind.

While both are fibrous proteins, spider silk has a different amino acid sequence and does not contain sericin. This difference results in unique mechanical properties, with spider silk being known for its exceptional toughness.

Yes, the composition and properties of silk can vary depending on the insect species. While Bombyx mori silk is the most common, other species of silkworms and insects produce silks with slightly different amino acid sequences and characteristics.