Proteins are macromolecular substances formed by the polymerization of thousands of amino acids. The structural complexity of proteins can be easier analyzed by dividing the level of organization into other four levels. Firstly, the primary structure is represented by the amino acid sequence, secondary structure is represented by the regular disposition of amino acids from the primary structure, tertiary structure is the three-dimensional alignment of polypeptide chains and quaternary structure represents the final conformation of the protein. After their final structure, proteins can be classified into globular proteins and fibril proteins. The class of fibril proteins includes two well-known proteins: collagen and elastin.
Collagen has a high rate in the human body. The molecule is elongated, rigid. The structure of the collagen is represented by three polypeptide chains, also named alpha-chains; twisted one around the other forming a triple-helix. The organization of the molecules depends on the type of tissue the molecules form. In some tissues, collagen has a jelly like aspect and has the role to sustain the structure of the tissue (for example we have the vitreous body, which fills the space between the lens and the retina, and the extra cellular matrix). In other tissues, the collagen is arranged in bundles of parallel fibrin which gives a high resistance to the tissue. Therefore, collagen can be easily classified after its function and it's localization in the body into more types:
- Fibril collagen which is organized in fibrils. It enters into the consistence of skin, bones, tendons, blood vessels, cornea, cartilages, intervertebral discs and vitreous body.
- Reticular collagen which forms networks. It enters into the consistence of basal membrane and structures found beneath the epithelium.
Genetic diseases of the collagen molecule are manifested by the incapacity of the collagen to form normal fibers. This fact leads to the loss of flexible resistance of diverse tissues. Ehlers-Danlos syndrome (named after two doctors: Edvard Ehlers from Denmark and Henri-Alexandre Danlos from France) is an example of this type of genetic mutation which affects blood vessels, skin and joints. People with EDS have a very high flexibility, vulnerability to chest infections, low muscle tone, clearly visible veins, and are predisposed to mouth ulcers.
Unlike collagen which forms strong fibers, elastin is a protein with similar properties to the pneumatic tire. Elastic fibers, formed by elastin microfilaments and glycoprotein enter into the structure of the lungs, the great blood vessels wall and elastic ligaments. These fibers can be stretched but always return to their normal length. Elastin is an insoluble protean polymer synthesized from tropoelastin, which is a linear polypeptide formed by about seven hundred amino acids with small and nonpolar molecule (like glycine, alanine and valine). Elastin is also rich in proline and lisine. It contains a small amount of hydroxyproline. Tropoelastin is secreted by cells form extra cellular space. Here, in the extra cellular space, elastin interacts with specific glycoproteins, like fibrin. Glycoproteins form a supportive frame for elastin molecules.
Genetic mutations of fibril may cause the apparition of the Marfan syndrome (name given by Antoine Marfan, a French pediatrician), a disease of the conjunctive tissue, which causes structural abnormalities of the skeletal system, ocular abnormalities and cardiovascular abnormalities. People with Marfan syndrome present very long fingers, high waist and long upper limbs.