KERATIN: TYPES, STRUCTURE, LOCATION AND FUNCTIONS - BIOLOGY - 2025

The Keratin is an insoluble fibrous protein that forms the structural part of the cells and the integuments of many organisms, especially vertebrates. It has very varied forms and is not very reactive, chemically speaking.

Its structure was first described by scientists Linus Pauling and Robert Corey in 1951, while analyzing the structure of animal hair. These researchers also gave insights into the structure of myosin in muscle tissue.

Alpha-keratin organization scheme (Source: Mlpatton via Wikimedia Commons)

After collagen, it is one of the most important proteins in animals and represents most of the dry weight of hair, wool, nails, claws and hooves, feathers, horns and a substantial part of the outer layer of skin.

The elements or "keratinized" parts of animals can have very different morphologies that depend, to a large extent, on the function they perform in each particular organism.

Keratin is a protein that has characteristics that give it great mechanical efficiency in terms of tension and compression. It is produced by a special type of cell called "keratinocytes," which usually die after it is produced.

Some authors affirm that keratins are expressed in a tissue and stage-specific manner. In humans there are more than 30 genes that encode these proteins and they belong to a family that evolved through several rounds of genetic duplication.

Types of keratins and their structure

There are essentially two types of keratins: α and β. These are distinguished by having a basic structure that is composed primarily of polypeptide chains that can be wound as alpha helices (α-keratins) or joined in parallel as the β-folded sheets (β-keratins).

α-Keratins

This type of keratin is the most studied and it is known that mammals have at least 30 different variants of this type of keratin. In these animals, α-keratins are part of nails, hair, horns, hooves, quills and epidermis.

Like collagen, these proteins contain in their structure an abundant proportion of small amino acids such as glycine and alanine, which are what make the establishment of alpha helices possible. The molecular structure of an α-keratin is made up of three different regions: (1) the crystalline fibrils or helices, (2) the terminal domains of the filaments, and (3) the matrix.

The helices are two and form a dimer that resembles a coiled spiral that is held together thanks to the presence of bonds or disulfide bridges (SS). Each of the helices has approximately 3.6 amino acid residues in each turn it makes and is made up of about 310 amino acids.

These coiled coils can then be associated to form a structure known as a protofilament or protofibril, which has the ability to assemble with others of the same type.

Protofilaments possess non-helical N- and C-termini which are rich in cysteine ​​residues and which are attached to the core or matrix region. These molecules polymerize to form intermediate filaments that have a diameter close to 7nm.

Two types of intermediate filaments composed of keratin are distinguished: acidic intermediate filaments (type I) and basic (type II). These are embedded in a protein matrix and the way in which these filaments are arranged directly influences the mechanical properties of the structure they make up.

In type I filaments, the helices are connected to each other by means of three "helical connectors" known as L1, L12 and L2 and which are thought to provide flexibility to the helical domain. In type II filaments, there are also two subdomains that lie between the helical domains.

Example of a structure with α-keratins: hair

If the structure of a typical hair is analyzed, it is approximately 20 microns in diameter and is composed of dead cells that contain packed macrofibrils that are oriented in parallel (side by side).

The hair of mammals, like this cow, is made of keratin (Source: Frank Winkler via pixabay.com)

Macrofibrils are made up of microfibrils, which are smaller in diameter and are linked to each other through an amorphous protein substance with high sulfur content.

These microfibrils are groups of smaller protofibrils with a 9 + 2 organization pattern, which means that nine protofibrils surround two central protofibrils; all of these structures are essentially composed of α-keratin.

Soft keratins and hard keratins

Depending on their sulfur content, α-keratins can be classified as soft keratins or hard keratins. This has to do with the mechanical resistance force imposed by the disulfide bonds in the protein structure.

The group of hard keratins includes those that are part of the hair, horns and nails, while soft keratins are represented by the filaments found in the skin and corns.

Disulfide bonds can be removed by applying a reducing agent, so that the structures composed of keratin are not easily digestible by animals, unless they have intestines rich in mercaptans, as is the case with some insects.

β-Keratins

Β-keratins are much stronger than α-keratins and are found in reptiles and birds as part of claws, scales, feathers, and beaks. In geckos, the microvilli found on their legs (mushrooms) are also made up of this protein.

Its molecular structure is composed of β-folded sheets formed by antiparallel polypeptide chains that are joined together through bonds or hydrogen bonds. These chains, one next to the other, form small rigid and flat surfaces, slightly folded.

Where is it and what are its functions?

The functions of keratin are related, above all, to the type of structure it builds and where it is found in the animal's body.

Like other fibrous proteins, it confers stability and structural rigidity to cells, since it belongs to the large family of proteins known as the family of intermediate filaments, which are proteins of the cytoskeleton.

In protection and coverage

The upper layer of the skin of higher animals has a large network of intermediate filaments formed by keratin. This layer is called the epidermis and is between 30 microns and 1 nm thick in humans.

The epidermis functions as a protective barrier against different types of mechanical and chemical stress and is synthesized by a special type of cells called "keratinocytes."

In addition to the epidermis, there is an even more external layer that is constantly shedding and is known as the stratum corneum, which performs similar functions.

Thorns and quills are also used by various animals for their own protection against predators and other aggressors.

The "armor" of the Pangolins, small insectivorous mammals that inhabit Asia and Africa, is also composed of keratin "scales" that protect them.

In defense and other functions

The horns are observed in animals of the Bovidae family, that is, in cows, sheep and goats. They are very strong and resistant structures and the animals that have them use them as organs of defense and courtship.

The horns are formed by a bony center composed of “spongy” bone that is covered by skin that projects from the posterior area of ​​the skull.

Nails are another example of body parts made up of keratin (Source: Adobe Stock via pixabay.com)

The claws and nails, in addition to their functions in feeding and restraining, also serve animals as "weapons" of defense against attackers and predators.

The beaks of the birds serve several purposes, among which are food, defense, courtship, heat exchange, and grooming, among others. Multiple varieties of beaks are found in nature in birds, especially in terms of shape, color, size and strength of the associated jaws.

The beaks are composed, like the horns, of a bony center projecting from the skull and covered with strong sheets of β-keratin.

The teeth of non-mandibulated animals ("ancestral" vertebrates) are composed of keratin and, like the teeth of "higher" vertebrates, have multiple functions in feeding and defense.

On the move

The hooves of many ruminant and ungulate animals (horses, donkeys, elk, etc.) are made of keratin, are very resistant and are designed to protect the legs and aid in movement.

Feathers, which are also used by birds to move around, are made of β-keratin. These structures also have functions in camouflage, courtship, thermal insulation and impermeability.

The feathers and beak of birds are also composed of keratin (Source: Couleur, via pixabay.com)

In the industry

The textile industry is one of the main exploiters of keratinized structures, anthropocentrically speaking. The wool and hair of many animals is important at an industrial level, since with them various garments are manufactured that are useful to men from different points of view.

References

1. Koolman, J., & Roehm, K. (2005). Color Atlas of Biochemistry (2nd ed.). New York, USA: Thieme.
2. Mathews, C., van Holde, K., & Ahern, K. (2000). Biochemistry (3rd ed.). San Francisco, California: Pearson.
3. Nelson, DL, & Cox, MM (2009). Lehninger Principles of Biochemistry. Omega Editions (5th ed.).
4. Pauling, L., & Corey, R. (1951). The Structure of Hair, Muscle, and Related Proteins. Chemistry, 37, 261-271.
5. Phillips, D., Korge, B., & James, W. (1994). Keratin and keratinization. Journal of the American Academy of Dermatology, 30 (1), 85–102.
6. Rouse, JG, & Dyke, ME Van. (2010). A Review of Keratin-Based Biomaterials for Biomedical Applications. Materials, 3, 999-1014.
7. Smith, FJD (2003). The Molecular Genetics of Keratin Disorders. Am J Clin Dermatol, 4 (5), 347–364.
8. Voet, D., & Voet, J. (2006). Biochemistry (3rd ed.). Editorial Médica Panamericana.
9. Wang, B., Yang, W., McKittrick, J., & Meyers, MA (2016). Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration. Progress in Materials Science.