Structure and functions of keratin proteins

In the past, the term 'keratin' encompassed all proteins derived from skin modifications such as horns, claws, and hooves. However, it was later discovered that this keratin actually consisted of a mixture of various proteins, including keratins, keratin filament-associated proteins, and enzymes. Keratins were then defined as specific filament-forming proteins with distinct physicochemical properties, extracted from the cornified layer of the epidermis. Filament-forming proteins extracted from the living layers of the epidermis were categorized as 'prekeratins' or 'cytokeratins'. Presently, the term 'keratin' encompasses all intermediate filament-forming proteins with specific physicochemical properties produced in any vertebrate epithelia.

Similarly, the nomenclature of epithelia as cornified, keratinized, or non-keratinized has historically been based on the belief that only the epidermis of skin modifications like horns, claws, and hooves is cornified, while the unmodified epidermis is a keratinized stratified epithelium. All other stratified and non-stratified epithelia were considered non-keratinized epithelia. However, the concepts of keratins, keratinized or cornified epithelia now require clarification and revision in terms of the structure and function of keratin and keratin filaments in different species' epithelia. This includes examining keratin genes, their modifications, and recent research findings such as keratin protein and gene sequencing, cell culture, transfection of epithelial cells, immunohistochemistry, and immunoblotting. Recent discoveries have unveiled new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport.

It is currently understood that all stratified epithelia are keratinized, and some of these keratinized stratified epithelia undergo cornification to form a Stratum corneum. The processes of keratinization and cornification in skin modifications differ, particularly in terms of the keratins produced. Future research on keratins will enhance our understanding of the keratinization and cornification processes in stratified epithelia, including those in skin modifications, as well as the adaptability of epithelia, skin diseases, and the structural and functional changes in epithelia throughout evolution. This review primarily focuses on keratins and keratin filaments in mammalian tissue, but it also considers keratins in the tissues of other vertebrates.

Types and Functions of Keratin

There are two main types of keratin: alpha-keratin and beta-keratin. Alpha-keratin is predominantly found in mammals, while beta-keratin is present in reptiles, birds, and some other species. Keratins exhibit tissue-specific expression patterns, giving rise to different forms with unique properties.

- Epithelial Keratins: Epithelial keratins, expressed in tissues such as skin, hair, nails, and glands, provide mechanical strength and protect against external stressors. For example, in the skin, keratin forms a waterproof barrier, preventing excessive water loss and shielding the underlying tissues from pathogens and physical damage.

- Hard Keratins: Hard keratins, found in structures like horns, hooves, feathers, and scales, provide rigidity and resilience. These keratins possess additional properties, such as high sulfur content and cross-linking, which enhance their strength and resistance to wear and tear.

Applications of Keratin

Keratin's unique characteristics have led to its application in various fields, bringing forth innovative products and technologies.

Cosmetics and Personal Care. Keratin is a popular ingredient in hair care products, such as shampoos, conditioners, and treatments, due to its ability to strengthen and repair damaged hair. It can help restore the hair's natural shine, reduce breakage, and provide heat protection. Additionally, keratin-based nail polishes and treatments improve the strength and appearance of nails.

Biomedical and Tissue Engineering. Keratin finds extensive use in biomedical applications, including wound healing, drug delivery systems, and tissue engineering. Keratin-based scaffolds provide a supportive framework for tissue regeneration, promoting cell adhesion and growth. These scaffolds can be tailored to mimic the specific properties of different tissues, aiding in the repair and regeneration of skin, bone, cartilage, and nerves.

Environmental Applications. Keratin has been explored for its potential in various environmental applications. For example, keratin-based materials have been used for water filtration due to their ability to adsorb heavy metals and dyes. Additionally, keratin waste from industries such as poultry processing can be upcycled into valuable materials like bioplastics and fertilizers, reducing environmental impact.

Materials Science and Textiles. Keratin fibers have exceptional mechanical properties, making them suitable for the production of high-performance materials. Researchers are exploring the use of keratin in developing bio-based composites, coatings, and textiles with enhanced strength, durability, and biodegradability. These applications have the potential to replace petroleum-based materials and contribute to sustainable manufacturing practices.

Hair Keratin Protein: Structure, Function, and Importance

Hair is an essential aspect of human appearance, and its health and vitality play a significant role in our self-esteem and overall well-being. One of the key components responsible for the structure and strength of hair is a protein known as keratin. Keratin is a fibrous protein that belongs to the family of scleroproteins, characterized by their tough and insoluble nature. Hair keratin, specifically, is a specialized form of keratin that provides the structural framework for hair fibers.

Structure of Hair Keratin. Hair keratin consists of long chains of amino acids, which are the building blocks of proteins. These chains are rich in sulfur-containing amino acids, such as cysteine, which play a crucial role in forming strong chemical bonds called disulfide bridges. These bridges create cross-links between adjacent keratin chains, contributing to the stability and resilience of hair.

The keratin protein in hair is organized into different hierarchical levels of structure. At the primary level, individual amino acids form peptide bonds, creating a linear chain. These chains then twist around each other to form a secondary structure known as an α-helix. Multiple α-helices further associate to form a higher-order structure called a protofibril. Bundles of protofibrils are arranged in a helical pattern, giving rise to macrofibrils. Finally, the macrofibrils combine to form the visible hair fiber.

Function of Hair Keratin. The primary function of hair keratin is to provide strength, resilience, and protection to the hair fiber. The intermolecular bonds, such as disulfide bridges, give hair its elasticity and structural integrity, allowing it to withstand physical stresses, such as brushing, combing, and styling. The intricate arrangement of keratin at different structural levels contributes to the flexibility and toughness of the hair strand.

Hair keratin also plays a role in moisture retention. The outermost layer of the hair, called the cuticle, consists of overlapping scales formed by keratinized cells. This protective layer helps prevent excessive water loss from the hair shaft while also safeguarding it against external damage, such as heat, UV radiation, and chemicals.

Importance of Hair Keratin. Maintaining the health and integrity of hair keratin is crucial for healthy-looking hair. However, various factors can impact the quality and condition of keratin in hair fibers. Environmental stressors, excessive heat styling, chemical treatments, exposure to ultraviolet radiation, and aging can all contribute to the degradation of hair keratin. As a result, hair may become brittle, dull, prone to breakage, and lose its overall vitality.

To support the health of hair keratin, it is important to adopt a hair care routine that includes gentle handling, minimizing the use of heat styling tools, protecting hair from excessive sun exposure, and using hair care products specifically formulated to nourish and strengthen the hair fibers. Additionally, maintaining a balanced diet rich in protein, vitamins, and minerals can provide the necessary nutrients for the synthesis of healthy keratin.


Keratin protein, with its remarkable structural properties and diverse applications, continues to captivate researchers and innovators in various fields. From its crucial role in providing strength and protection in biological systems to its applications in cosmetics, medicine, materials science, and more, keratin's versatility and functionality make it a valuable protein. As scientific understanding deepens and technological advancements continue, we can anticipate further discoveries and novel uses for keratin, contributing to advancements in multiple industries and benefiting both human and environmental well-being.