|MadSci Network: Biochemistry|
In mammals, hair, horns, nails, claws, hooves, and even skin are all made up of the same material: keratin.
There are three classes of filamentous (long and stringy) proteins inside the cell: microfilaments, which are the thinnest, and are made up of actin; microtubules, which are the thickest, and are hollow tubes made up of tubulin; and intermediate filaments, which (as the name implies) have a thickness in between the other two. Each type of intermediate filament is named for the type of protein which is used to make it. Each type of intermediate filament has a specific job in the cell: desmin helps some cells maintain their connection to other cells; neurofilament is important for nerves to communicate; and keratin makes skin cells tough and durable enough to protect the rest of the body from the environment.
Early in the evolution of fish, keratin in the skin was used to make specialized patches of hardened skin called scales. Later, these scales have been adapted to form hair, nails, and feathers in mammals, reptiles, and birds. Some paleontologists have even speculated as to whether some dinosaurs had hair or feathers instead of scales. Keratin gives each of these structures its strength and durability.
Keratin was first described by Linus Pauling, who won two Nobel Prizes. Keratin in hair is made of a coiled coil of three (as in human hair) or seven (as in sheep wool) long polypeptide chains (proteins), meaning that each polypeptide chain forms its own alpha-helical coil, and then the coiled chains coil around each other like twisted telephone cords. These coiled coil units are then arranged end to end and bundled together to form a long, thick strand of protein, which is a keratin filament. These filaments are then arranged in the cell to form a lattice (as in skin) or a fiber (as in hair). As the cells mature, they build their keratin structures, and die when they are done. Hair and skin are made up of layers upon layers of these "cornified" cells which are essentially keratin for strength and lipid (from the cells' membranes) to keep in moisture.
Some animals' (and peoples') keratin contains more of the amino acid, cysteine, which allows keratin filaments to cross-link to each other to such a degree, that it invokes a slight curve to each fiber, resulting in curly hair.
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