Sericin: The Unsung Hero of Silk Proteins

When we speak of silk, we often praise the luxurious feel, the shimmer, and the durability. These qualities, however, are mostly attributed to fibroin, one of the two primary proteins found in silk. But, there’s another silk protein that deserves its share of the limelight – Sericin, often deemed as the unsung hero of silk proteins. Let’s dive deeper into the world of Sericin, its unique properties, benefits, and future applications in research and industry.

A Deeper Look into Sericin

Sericin, also known as silk gum, is a protein produced by Bombyx mori (silkworm) during the production of silk. It serves as a protective “glue” that holds the fibroin fibers together in the cocoon. Once viewed as a waste product in the silk manufacturing process, Sericin is now recognized as a valuable component with remarkable properties. Despite making up just 20-30% of the total silk protein content, it has significant potential in various fields like cosmetics, biomedicine, and textiles, thanks to its bioactivity and excellent biocompatibility.

Types of Silk Sericin

Silk sericin is classified into two types based on the feeding source of the silkworms: mulberry and non-mulberry sericin. 

• Mulberry silkworms, specifically the Bombyx mori species, are domesticated and feed on mulberry leaves. They are primarily used for commercial silk production. 
• Non-mulberry silkworms, on the other hand, are usually wild and feed on other types of leaves like oak and castor oil leaves. They can’t be fully reared indoors. Examples of non-mulberry silkworms include Antheraea, Samia ricini, and Cricula trifenestrata. These silkworms produce different types of silk, such as “tasar” silk from Antheraea and “eri” silk from S. ricini. The type of silkworm and its diet can influence the characteristics of the sericin it produces.

Localization of Silk Sericin

Sericin is located at several sites of silkworms and cocoons. In the mulberry silkworm, B. mori, it has been reported that sericin is present in three components including silk gland, cocoon, and floss (Gamo et al. 1977; Kikkawa 1953; Yamada 1978). For non-mul- berry silkworms, sericin is also secreted in the cocoon peduncle (Dash et al. 2006). The silk gland is the site that produces sericin. In a histological study, sericin was found to be mainly synthesized in the middle and posterior of the silk gland. Sericin protein is then sent to anterior silk glands via the lumen for secretion and cocoon construction (Consortium 2008; Kikkawa 1953; Yamanouchi 1922). 

Sericin Properties

1. Biophysical Properties

• Water Solubility: Sericin can dissolve in hot water and precipitate in cold water. This property is attributed to its amino acid content, particularly serine, and threonine, which make sericin hydrophilic. The structure of sericin, specifically its β-sheet conformation, also contributes to its water solubility.
• Gelation: Sericin can form gels under certain conditions, such as high concentration, temperature, and specific pH levels. This property is useful for biomaterial applications.
• Thermal Stability: Sericin’s stability varies with temperature. Non-mulberry sericins are reported to be more stable than mulberry sericin.
• Ultraviolet (UV) Protection: Sericin can protect cells from UV radiation, reducing skin oxidative stress and inhibiting UVB-induced apoptosis.
• Adhesion Properties and Electrostatic Interaction: Sericin has adhesive properties, which are important for silkworms during their cocoon stage. This property is facilitated by the high contents of charged amino acids in sericin proteins.

2. Biochemical Activity

• Anti-tyrosinase Activity: Sericin has been found to inhibit the activity of tyrosinase, an enzyme involved in melanin synthesis.
• Anti-elastase Activity: Sericin has shown the ability to inhibit elastase, a proteolytic enzyme that degrades elastin, thereby contributing to skin elasticity.
• Antioxidant Activity: Sericin exhibits antioxidant properties, reducing oxidative stress in cells.
• Anti-lipid Peroxidation Activity: Sericin has been found to inhibit lipid peroxidation, a process that can lead to cell damage.

3. Biological Activity

• Anti-inflammatory Activity: Sericin has shown anti-inflammatory properties, reducing inflammation in various in vivo models.
• Anti-tumor Activity: Sericin has been found to inhibit the growth of various tumor cells, including colon and skin tumors. It reduces colon tumors by decreasing oxidative stress and inhibiting cell proliferation. In skin cancer, sericin has strong anti-tyrosinase activity, which is a key enzyme in melanogenesis. It reduces skin tumors from UV radiation by reducing oxidative stress, decreasing cyclooxygenase-2, and lowering cell proliferation on the skin. Non-mulberry sericins have also shown anti-tumor activity, and in vitro, testing has shown that they can destroy human squamous carcinoma and human tongue carcinoma cells.

• Inducing Collagen Production: Sericin has been reported to induce fibroblast cell proliferation and collagen production. The effectiveness of collagen production is related to the amino acid composition of sericin. Non-mulberry sericins have been reported to have a protective effect from collagen degradation induced by UV radiation.
• Antibacterial Activity: Sericin has been tested for its antibacterial activity against Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. The purity and extraction method of sericin affected its antibacterial properties. Sericin has been found to inhibit biofilm formation, with urea-extracted sericin showing the highest potential anti-biofilm activity for Streptococcus mutans.

These properties make sericin a promising substance for various medical and cosmetic applications, including skin treatment and the enhancement of biomaterial properties. However, the biological properties of sericin can vary depending on the type of silkworm and the extraction method used, so these factors are important to consider when selecting sericin for a particular application.

Current Research and Future Applications

While traditionally, the primary use of Sericin was in the textile industry as a sizing agent, the understanding and appreciation of this protein’s unique properties have grown exponentially in recent years. 

In the cosmetics industry, Sericin is already used in skincare and haircare products for its moisturizing and protective properties. 

In the realm of biomedicine, ongoing research explores Sericin’s potential as a biomaterial in drug delivery systems, wound dressings, and tissue engineering. Its non-toxic, biodegradable nature makes it a promising candidate for such applications. 

Meanwhile, in the textile industry, there’s an interest in exploiting Sericin’s UV-resistant and antioxidant properties to create fabrics with built-in sun protection and durability. 

Looking ahead, as researchers continue to explore Sericin’s properties and potential uses, it is poised to make a significant.

Stay tuned with us at Serione, as we explore, innovate, and bring to light the fascinating capabilities of silk proteins and their derivatives.

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