Keratin

Representative structure of keratin, represented by two polypeptide chains and indicating some types of amino acids involved in intra and intermolecule links.

Keratin (from the Greek κερατίνη, horny) is a protein with a fibrous structure, very rich in sulfur, which constitutes the main component that forms the outermost layers. external to the epidermis of vertebrates and other organs derived from the ectoderm, faneras such as hair, nails, feathers, horns, ruffs and hooves. One of the biomolecules whose hardness is close to that of keratin is chitin, with which it should not be confused, as the latter is a polysaccharide.

Keratin - Image of fluorescence microscope.

Types and composition

Keratins are usually classified into two families, based on the abundance of their proportion in α-helices or β-sheets type structures:

Keratin-α

Alpha-keratin has cysteine residues (monomers) in its amino acid chains, which constitute disulfide bridges, called the cystine group. Disulfide bridges provide the rigidity and strength to alpha keratin. Thus, there is a greater amount of cystine (disulfide) bonds in structural regions of an animal's horns and in the nails or hair. Alpha keratin is found in hair, horns, nails, and faneras.

Keratin-β

Beta-keratin does not present cysteine, or does so in a very low proportion, therefore it contains few intermacromolecular crosslinks through disulfide (cystine) bridges. However, beta-type keratin presents a higher proportion of β-sheet folds. The high cohesion given by the large number of associations by hydrogen bonds of the β-sheets, makes the presence of β-keratin result in highly resistant materials such as spider silk. α-keratins may also present β-sheets, although to a lesser extent than β-keratins. Likewise, β-keratins also present α-helices. This makes this classification of keratins controversial.

Structural function

Keratin is a protein with a secondary structure. That is, the macromolecular chain with a certain primary structure (or sequence) folds on itself, acquiring three dimensions. This structure can be of the helical type, thus being called α-helix protein, or of a laminar or β-sheet form. Intermolecular interactions such as hydrogen bonds, hydrophobic forces, or salt bonds, such as those between the amino acids glutamic acid and lysine, hold the amino acids of different strands or macromolecular segments of the protein together. The ability to transfer charge through intermolecular bonds makes keratins good structural biomaterials, with good mechanical properties.

Hair keratin

Keratin process in the hair

Hair keratin is classified within fibrous proteins; Its characteristics are long chains of secondary structure, insoluble in water, and low salinity solutions, making it ideal for performing skeletal functions and high physical resistance with structural functions.

Hair is constructed of keratin macrofibrils packed on the outside; these are made up of microfibrils, which are twisted in a counterclockwise coil. The interactions between the strands occur through disulfide bridges. Hair keratin has a high proportion of alpha keratin, with the possibility of transforming it into beta keratin if, for example, we apply heat plus humidity. The hair can even double its length. This happens because the hydrogen bonds of the helix are broken and the polypeptide chains adopt an extended conformation, in which the bulkiness of the -R side groups makes the -beta conformation unstable, and therefore, after a short time, they become unstable. adopt the helix conformation again, with which the hair recovers its original length. It is incorrect to say that hair is made up of cells, there is only sulfur-rich keratin and an amorphous matrix that keeps the microfibrils packed on the outside in the way the cell that synthesized them once had; The hair follicle is the one that has active cells that are responsible for synthesizing the above elements.

The cuticle, formed by cells composed of keratin, is responsible for protecting the interior of the hair, while influencing its shine and color. There are several factors that affect its good quality: mechanical treatments (bad brushing, etc.), environmental conditions (contamination, etc.), cause its deterioration and special mention to chemical works (uncurling, etc.); these cause the cuticle to swell and open, something that, with continuous use, modifies the structure of the hair, making it dry, fragile, porous and even brittle.

Keratin treatments

Keratin treatments are believed to be used to straighten hair. And, although to some extent it does help to make it look a little smoother, its main function is to repair the hair fiber, remove frizz from the hair and provide shine and vitality. All this makes the hair look hydrated and uniform.

Brazilian keratin treatment adds amino acids from keratin to the protein fiber in our hair. Thus, a cross-linking reagent is used to glue the added amino acids to the keratin strands in the hair. After this, the hair is stretched and heated with a flat iron. Critics of this technique point to the fact that it uses formaldehyde as one of the cross-linking reagents, and formaldehyde is considered a carcinogen when inhaled. formaldehyde is naturally hardened and settles as a cuticle