Keratin

Keratin is a vital family of structural fibrous proteins. These proteins are also known as scleroproteins. They serve as the primary building material for many physical structures in the animal kingdom. You can find keratin in hair, nails, claws, hooves, and feathers. It also forms the scales of reptiles and the outer layer of skin in tetrapod vertebrates.

Beyond just providing shape, keratin protects epithelial cells from physical stress and damage. One of its most remarkable properties is its chemical stability. Keratin is extremely insoluble in water and organic solvents. This makes it an ideal shield for organisms living in various environments.

The strength of keratin comes from how its tiny parts organize themselves. It begins with small units called monomers. These monomers assemble into bundles to create intermediate filaments. These filaments are very tough and strong. They form unmineralized epidermal appendages in mammals, birds, reptiles, and amphibians.

In some species, a process called excessive keratinization occurs. This builds up very thick tissues. For example, cattle and rhinos have very strong horns due to this buildup. Armadillos also use this to create protective osteoderms. In terms of pure toughness, only one other biological substance, chitin, is known to approach the strength of keratinized tissue.

Scientists categorize keratin into two distinct types based on their structure and where they are found. The first type is alpha-keratin, or α-keratins. These are considered the primitive and softer forms. They are found in all vertebrates. In mammals, α-keratins make up hair, wool, nails, and the outer skin layer. They also form the claws, hooves, and the slime threads of hagfish. Even whales use α-keratins to build their baleen plates.

The second type is beta-keratin, or β-keratins. These are the harder, derived forms. They are found only among sauropsids, which include all living birds and reptiles. You can find β-keratins in bird beaks, feathers, and claws, as well as reptile scales and shells. Because these are structurally different from α-keratins, some researchers use the new term corneous beta protein (CBP) to avoid confusion.

Our understanding of these proteins grew significantly through molecular research. In 1982 and 1983, researchers Israel Hanukoglu and Elaine Fuchs determined the first keratin sequences. Their work revealed that there are two distinct but homologous keratin families. They named these type I and type II keratins. Through their analysis, they suggested a specific structural model for these proteins. They found that keratins contain a central domain of about 310 residues. This domain has four segments in an α-helical conformation. These segments are separated by three short linker segments in a beta-turn conformation.

In humans, the instructions for building keratin are stored in our DNA. The human genome encodes 54 functional keratin genes. These genes are not scattered randomly; they are organized into two specific clusters. One cluster is located on chromosome 12, and the other is on chromosome 17.

This specific arrangement suggests that these genes originated from a series of gene duplications over time. Because there are so many genes, each one can assist with a specific biological process. For instance, KRT1 is involved in establishing the skin barrier and responding to oxidative stress. Other genes, like KRT14, are essential for the hair cycle and stem cell differentiation.

Many different keratin genes perform specialized roles in the body. Some genes help with the development of specific structures. For example, KRT10 is vital for the development of the epidermis and epithelial cell differentiation. KRT17 helps with the morphogenesis of hair follicles. Other keratins are involved in much more complex tasks, such as KRT18, which relates to cell-cell adhesion and the cell cycle. Even KRT76 plays a role in pigmentation and the development of sebaceous glands. This variety allows the body to create many different types of protective tissues.

Keratin is also a key part of how scientists study biology today. In laboratory settings, researchers use specific antibodies to find certain keratins. For example, they use antibodies for keratin 5, 8, and 14 to study mouse thymic epithelial cells. These antibodies act as fluorescent markers. This allows scientists to distinguish different subsets of cells during genetic studies of the thymus. This connection between protein structure and cellular function helps us understand how complex organisms grow and stay healthy.