Proteins that mainly confer tissue structure and tear resistance to tissue are grouped together as structural proteins. Structural proteins are characterized by the fact that they are generally not involved in enzymatic-catalytic metabolic processes.
Scleroproteins, which are counted among the structural proteins, usually form long chain molecules in the form of juxtaposed amino acids, which are each linked together via peptide bonds. Structural proteins often have recurring amino acid sequences that allow the molecules to have special secondary and tertiary structures, such as double or triple helixes, resulting in particular mechanical strength. Important and well-known structural proteins are, for. As keratin, collagen and elastin. Keratin belongs to the fiber-forming structural proteins that give structure to the epidermis, hair and nails.
Collagens make up the largest group of structural proteins with over 24% of all proteins found in the human body. It is striking in collagens that every third amino acid is glycine and there is an accumulation of the sequence glycine-proline-hydroxyproline. The tear-resistant collagens are the most important components of bones, teeth, ligaments and tendons (connective tissue). Unlike collagens, which are barely elastic, elastin gives elasticity to certain tissues. Among other things, elastin is therefore an important component in the lungs, in the walls of blood vessels and in the skin.
The term structural protein subsumes various classes of proteins. All structural proteins have in common that their main function is to give structure and strength to the tissue in which they occur. This requires a wide range of the necessary structural properties. Collagens, which form the structural protein among other things in ligaments and tendons, are extremely tear-resistant, since the ligaments and tendons are exposed to high loads in terms of tear strength.
As part of bones and teeth, the collagens must also be able to form fracture-resistant structures. Other body tissues require, in addition to the tear strength, a special elasticity in order to be able to adapt to the respective conditions. This task is fulfilled by structural proteins belonging to the elastins. They can be stretched and are conditionally comparable to elastic fibers in fabric fabrics. Elastins enable rapid volume adjustments to blood vessels, lungs, and various skins and membranes that envelop organs and cope with changing organ circumferences. Collagen and elastin complement each other in the skin of the human to ensure both firmness and mobility of the skin.
While collagen in ligaments and tendons is primarily responsible for ensuring tear strength in a particular direction, keratins, which are part of fingernails and toenails, must provide surface (two-dimensional) strength. Another class of structural proteins is formed by so-called motor proteins, which are the main component of muscle cells. Myosin and other motor proteins have the ability to contract due to a particular neuronal stimulus, so that the muscle shortens temporarily with energy expenditure.
Structural proteins, like other proteins, are synthesized in the cells. The prerequisite is that the supply of the corresponding amino acids is guaranteed. First, several amino acids are linked to peptides and polypeptides. These fragments of a protein are assembled on the rough endoplasmic reticulum into larger sections and then to the complete protein molecule.
Structural proteins which have to fulfill functions outside the cells in the extracellular matrix are labeled and are transported by exocytosis into the extracellular space by means of secretory vesicles. The required properties of the structural proteins cover a wide spectrum between tensile strength and elasticity. Structural proteins normally only occur as part of tissues, so that their concentration can not be readily measured directly. An optimum concentration can therefore not be specified.
The multi-faceted tasks that must be undertaken by the different structural proteins can be expected to cause malfunctions that lead to disorders and symptoms. Likewise, it can lead to malfunction within the synthesis chain, because the synthesis of a variety of enzymes and vitamins is required.
The most noticeable disturbances arise when due to a lack of amino acids, the corresponding proteins can not be synthesized. The majority of amino acids needed can be synthesized by the body itself, but not the essential amino acids that need to be supplied externally in the form of food or dietary supplements. Even with sufficient supply of essential amino acids, absorption in the small intestine may be disturbed due to illness or due to ingested toxins or as a side effect of certain medications, causing a deficiency. A well-known, although rare, disease in this context is Duchenne muscular dystrophy.
The disease is triggered by a gene defect on the x chromosome, so that only men are directly affected. The genetic defect means that the structural protein dystrophin, which is responsible for the anchoring of muscle fibers in skeletal muscle, can not be synthesized. This leads to a muscular dystrophy with a serious course. Another - also rare - hereditary disease leads to mitochondriopathy. Several known genetic defects within the DNA and mitochondrial DNA can cause mitochondria. An altered composition of certain mitochondrial structural proteins causes a reduced energy supply of the entire organism.