Apart from dynein and kinesin, myosin is one of the motor proteins responsible for the processes of cell movement and transport within the cell. Unlike the other two motor proteins, myosin only works together with actin. Actin is in turn a component of the cytoskeleton of the eukaryotic cell. This is responsible for the structure and stability of the cell.
Furthermore, actin with myosin and two other structural proteins forms the actual contractile structural unit of the muscle. Two-thirds of the muscle's contractile proteins are myosin and one-third actin. However, myosins are present not only in muscle cells but also in all other eukaryotic cells. This applies to unicellular eukaryotes as well as to plant and animal cells. The microfilaments (actin filaments) are involved in the construction of the cytoskeleton in all cells and, together with myosin, control the protoplasmic flows.
Myosins can be divided into different classes and subclasses. Currently, over 18 different classes are known, with classes I, II and V being the most significant. The muscle fiber myosin is called conventional myosin and belongs to class II. The structure of all myosins is similar. They all consist of a head part (myosin head), a neck part and a tail part.
The myosin filaments of the skeletal muscle consist of approximately 200 myosin II molecules, each with a molecular weight of 500 kDa. The headboard is genetically very conservative. The classification into the structural classes is mainly determined by the genetic variability of the tail part. The head part binds to the actin molecule while the neck part acts as a hinge. The tail parts of several myosin molecules assemble to form filaments (bundles). The myosin II molecule consists of two heavy and four light chains.
The two heavy chains form a so-called dimer. The longer of the two chains has an alpha-helical structure and is composed of 1300 amino acids. The shorter chain consists of 800 amino acids and represents the so-called motor domain. It forms the head of the molecule, which is responsible for the movements and transport processes. The four light chains are connected to the head and neck of the heavy chains. The headlong remote Light Chains are referred to as regulatory and the head-near Light Chains as essential chains. They are very affine to calcium and can thus control the mobility of the neck.
The most important function of all myosins is to transport cell organelles in eukaryotic cells and to perform shifts within the cytoskeleton. The conventional myosin II molecules together with actin and the proteins tropomyosin and troponin are responsible for muscle contraction. For this purpose, myosin is first integrated into the Z-disks of the sacro with the help of the protein titin. Six titin filaments fix a myosin filament.
In the sacomer, a myosin filament forms about 100 transverse connections to the sides. Depending on the structure of the myosin molecules and the content of myoglobin, several forms of muscle fibers can be distinguished. Within the sacomere muscle contraction occurs through the movement of myosin in the transverse bridge cycle. First, the myosin head is firmly attached to the actin molecule. Then ATP is cleaved to ADP, with the energy released leading to the strain of the myosin head. At the same time, the Light Chains increase the calcium ions. As a result, the myosin plug attaches to an adjacent Aktinmolekül as a result of a conformational change.
Under solution of the old connection, the tension is now converted by a so-called force blow into mechanical energy. The movement is similar to a rowing stroke. The Myosinkopf tilts from 90 degrees to between 40 and 50 degrees. The result is a muscle movement. In muscle contraction, only the length of the sacomer is shortened, while the lengths of actin and myosin filaments remain the same. The ATP supply in the muscle only lasts for about three seconds. By breaking down glucose and fat, ADP rebuilds ATP, allowing chemical conversion to mechanical energy.
Structural changes of myosin by mutations can lead to muscle diseases. An example of such a disorder is familial hypertrophic cardiomyopathy. Familial hypertrophic cardiomyopathy is a hereditary disease that is inherited as an autosomal dominant disorder. The disease is characterized by thickening of the left ventricle without dilatation.
With a prevalence of 0.2% in the general population, it is a relatively common heart disease. This disease is caused by mutations that lead to structural changes of betamyosin and alphatropomyosin. This is not one, but multiple point mutations of the proteins involved in the assembly of the sacomere. Most of the mutations are located on chromosome 14. Pathologically, the disease is characterized by a thickening of the muscles in the left ventricle.
This asymmetry of cardiac muscle thickness can cause cardiovascular problems with cardiac arrhythmias, dyspnoea, dizziness, loss of consciousness and angina pectoris. Although many patients have little or no cardiac impairment, progressive heart failure may develop.Tags: