Structural proteins primarily serve as tensile scaffolds in cells and tissues. They usually have no enzymatic function, so they do not normally interfere with metabolic processes. Structural proteins usually form long fibers and give, for example, ligaments, tendons and bones their strength and motility, their mobility. Several different types of structural proteins account for about 30% of all occurring proteins in humans.
What is structural protein?
Proteins that mainly give tissues their structure and tensile strength are collectively known 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 amino acids strung together, each linked by 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 helices, which leads to special mechanical strength. Important and well-known structural proteins include keratin, collagen and elastin. Keratin is one of the fiber-forming structural proteins that gives structure to the epidermis, hair and nails. Collagens form the largest group of structural proteins, accounting for over 24% of all proteins found in the human body. A striking feature of collagens is 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). In contrast to collagens, which are hardly stretchable, elastin gives certain tissues stretchability. Elastin is therefore an important component in the lungs, in the walls of blood vessels and in the skin, among other things.
Function, effect and tasks
Various classes of proteins are subsumed under the term structural protein. All structural proteins have in common that their main function is to provide structure and strength to the tissues in which they are found. This requires a wide range of the necessary structural properties. Collagens, which form the structural protein in ligaments and tendons, among other things, are extremely tear-resistant, as ligaments and tendons are subjected to high stresses in terms of tear strength. As a component in bones and teeth, collagens must also be able to form fracture-resistant structures. Other body tissues require special elasticity in addition to tear resistance in order to be able to adapt to the respective conditions. This task is performed by structural proteins that belong to the elastins. They can be stretched and are comparable to a certain extent with elastic fibers in fabric tissues. Elastins enable rapid volume adjustments in blood vessels, lungs and various skins and membranes that encase organs and have to cope with changing organ volumes. Collagens and elastins also complement each other in human skin to provide both strength and the ability for the skin to shift. While collagens in ligaments and tendons mainly guarantee tear resistance in a specific direction, keratins, which are part of fingernails and toenails, must provide planar (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 in response to a specific neuronal stimulus, causing the muscle to temporarily shorten while expending energy.
Formation, occurrence, and properties
Structural proteins, like other proteins, are synthesized in cells. The prerequisite is that the supply of the appropriate amino acids is ensured. First, several amino acids are linked to form peptides and polypeptides. These fragments of a protein are assembled at the rough endoplasmic reticulum to form larger fragments and then the complete protein molecule. Structural proteins that must perform functions outside the cells in the extracellular matrix receive a label and are transported into the extracellular space by exocytosis using secretory vesicles. The required properties of the structural proteins cover a wide spectrum between tensile strength and elasticity.Structural proteins normally occur only as a component of tissues, so their concentration cannot be readily measured directly. Therefore, an optimal concentration cannot be specified.
Diseases and disorders
The multifaceted tasks that the various structural proteins must perform suggest that malfunctions can also occur, leading to disorders and symptoms. Similarly, dysfunction can occur within the synthesis chain because a variety of enzymes and vitamins are required for synthesis. The most noticeable disorders occur when, due to an undersupply of amino acids, the corresponding proteins cannot be synthesized. The majority of the required amino acids can be synthesized by the body itself, but not the essential amino acids, which must be supplied externally in the form of food or dietary supplements. Even with an adequate supply of essential amino acids, absorption in the small intestine may be impaired and cause deficiency due to disease or because of ingested toxins or as a side effect of certain medications. A well-known, although rare, disease in this context is Duchenne muscular dystrophy. The disease is caused by a genetic defect on the x chromosome, so only males are directly affected. The gene defect leads to the fact that the structural protein dystrophin, which is responsible for the anchoring of muscle fibers of the skeletal muscles, cannot be synthesized. This results in muscular dystrophy with a severe course. Another – also rare – hereditary disease leads to mitochondriopathy. Several known gene defects within DNA and mitochondrial DNA can cause mitochondriopathy. Altered composition of certain mitochondrial structural proteins cause decreased energy supply to the entire organism.
