Sericin is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

created by ''

Bombyx mori The domestic silk moth (''Bombyx mori''), is an insect from the moth family Bombycidae. It is the closest relative of ''Bombyx mandarina'', the wild silk moth. The silkworm is the larva or caterpillar of a silk moth. It is an economically imp ...

'' (silkworms) in the production of

silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from th ...

. Silk is a fibre produced by the silkworm in production of its cocoon. It consists mainly of two proteins, fibroin and sericin. Silk consists of 70–80% fibroin and 20–30% sericin; fibroin being the structural center of the silk, and sericin being the gum coating the fibres and allowing them to stick to each other.

Structure

Sericin is composed of 18 different amino acids, of which 32% is serine. The secondary structure is usually a random coil, but it can also be easily converted into a β-sheet conformation, via repeated moisture absorption and mechanical stretching. The serine hydrogen bonds give its glue-like quality. The genes encoding sericin proteins have been sequenced. Its C-terminal part contains many serine-rich repeats. Using

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

examination, it was determined that sericin fibers are composed typically of three layers, all with fibers running in different patterns of directionality. The innermost layer, typically is composed of longitudinally running fibers, the middle layer is composed of cross fiber directional patterned fibers, and the outer layer consists of fiber directional fibers. The overall structure can also vary based on temperature, whereas the lower the temperature, there were typically more β-sheet conformations than random amorphous coils. There are also three different types of sericin, which make up the layers found on top of the fibroin. Sericin A, which is insoluble in water, is the outermost layer, and contains approximately 17% nitrogen, along with amino acids such as serine,

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO� ...

aspartic acid Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

, and

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...

. Sericin B, composed the middle layer and is nearly the same as sericin A, but also contains

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

. Sericin C is the innermost layer, the layer that comes closest to and is adjacent to fibroin. Also insoluble in water, sericin C can be separated from the fibroin via the addition of a hot,

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

. Sericin C also contains the amino acids present in B, along with the addition of

proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the p ...

Applications

Sericin has also been used in medicine and

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protec ...

. Due to its elasticity and tensile strength, along with a natural affinity for

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

, sericin is primarily used in medicine for wound suturing. It also has a natural infection resistance, and is used variably due to excellent biocompatibility, and thus is used commonly as a wound coagulant as well. When used in cosmetics, sericin has been found to improve skin elasticity and several anti-aging factors, including an anti-wrinkle property. This is done by minimizing water loss from the skin. To determine this, scientists ran several experimental procedures, including a

hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxyproline from hydrolyzed gelatin ...

assay, impedance measurements, water loss from the epidermis and scanning electron microscopy to analyze the rigidity and dryness of the skin. The presence of sericin increases hydroxyproline in the

stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compa ...

, which in turn, decreases skin impedance, thus increasing skin moisture. Adding in pluronic and

carbopol Poly(acrylic acid) (PAA; trade name Carbomer) is a polymer with the formula (CH2-CHCO2H)n. It is a derivative of acrylic acid (CH2=CHCO2H). In addition to the homopolymers, a variety of copolymers and crosslinked polymers, and partially deproto ...

, two other ingredients that can be included in sericin gels, performs the action of repairing natural moisture factors (NMF), along with minimizing water loss and in turn, improving skin moisture.

References

{{Reflist Sericulture Proteins Silk Silk production

Structure

Applications

See also

References