Actin are the collective protein molecules from which thin bands are formed. On the other hand, the myosin is the group of proteins by which thick bands are formed. Actin and myosin are responsible for several types of cell movement, the most striking being muscle contraction, which provides the best model for understanding the role of actin and myosin.
Now, to understand the role of actin-myosin, it is necessary to gather some information about muscle contraction. Cellular and molecular movements in the body depend on muscle cells. Vertebrates have three cell types muscle: smooth muscle, cardiac muscle and skeletal muscle.
Smooth muscles are known for carrying out involuntary movement in the body; cardiac muscles are known for pumping our blood. heart The skeletal muscles play their role in all kinds of voluntary movements.
Skeletal muscles contain quantities of muscle fibres, these are a group of numerous cells, which fused together to give rise to individual large cells at the time of development. The muscle cells contain numerous nuclei, and their cytoplasm contains myofibrils, which consist of cylindrical bundles of thick and thin filaments.
The thin filament is made up of a protein known as actin, and the thick filament is made up of a protein known as myosin, and these are organised as repeating chain units known as sarcomeres. Sarcomeres are forced to give the striated appearance to cardiac and striated muscles.
Therefore, myosin and actin are said to work together at the time of muscle contractions, where myosin is the precursor protein that plays a key role in converting chemical energy (ATP) into mechanical energy. In this article, we will provide vital differences and the points at which actin and myosin vary with their similarities.
Comparative graph between actin and myosin
| BASIS FOR COMPARISON | ACTIN | MIOSIN |
|---|---|---|
| Sense | Actin is the protein, known to form the thin bands in myofibrils. | Myosin is the protein, known to form the thick bands in myofibrils. |
| It consists of | 1. Actin forms a short filament of 2-2.6 um and is thin down to 0.005 um. 2. Actin contains troponin and tropomyosin (protein). |
1. Myosin forms a long filament of 4.5 um, which is 0.01 um thick. 2. Myosin contains meromyosin (protein). |
| Found in | Actin is present in the A and I bands. | Myosin is present in the A-bands of the sarcomere. |
| Cross bridges | Actin does not form cross-bridges. | Myosin forms cross-bridges. |
| Surface | The actin surface is smooth. | The surface of myosin is rough. |
| Number | Actin is numerous in number. | Myosin is smaller in number, and there are one for every six actin filaments. |
| Slide in | Actin sliding in the H-zone at the moment of contraction. | Myosin does not slide at the moment of contraction. |
Definition of actin
As discussed above, the two main protein filaments found in muscles are actin and myosin. It is one of the essential components of cytoskeleton cellular, especially in eukaryotes. It is a highly conserved protein with a molecular weight of 42 kDa.
Actin is present in monomeric form as G-actin or in polymeric form as F-actin.where ‘G’ stands for globular actin protein, while ‘F’ stands for filamentous actin protein or polymeric fibrous protein. These have different cellular functions, such as muscle contraction, the contraction of muscles, the cytokinesis and cell migration.
Since actin filaments play a major role in the formation of the dynamic cytoskeleton of the cellThe cytoskeleton, therefore, also provides movement and shape to the cell. The cytoskeleton also provides communications with neighbouring cells, as well as supporting the internal environment within the cell.
Definition of myosin
Myosin is another type of protein filament that functions in the presence of calcium ions. Myosin is known to generate the force required during muscle contraction. Therefore, it is also known as a motor protein.
Skeletal muscles are known for voluntary action in the body, where actin and myosin are present as repetitive units. The thick filament of myosin is surrounded by the thin filament of actins. On the other hand, the thin actin filament is surrounded by the thick myosin filament. So, this continuous and repetitive result in the formation of the filament bundle in muscles.
Myosin has three parts: headThe head, neck and tail are composed of numerous light chains and two heavy chains. The globular head region has the binding site for ATP and actin, and the neck part has an alpha-helix region where the tail has other binding sites. The head region converts ATP to ADP, by the enzyme ATPase.
As soon as the nerve sends a signal to the muscle cell For muscle contraction, myosin and actin are activated. After that, myosin starts to work on releasing energy (ATP) and, in addition, myosin together with actin filaments slide past each other.
Tropomyosin and troponin are two other muscle proteins that temporarily fuse with actin and myosin for muscle contraction. This function can be understood by the ‘sliding filament theory’.
Actin and myosin also play a fundamental role in non-muscle cells. There are two actions performed consecutively by muscles, which are contraction and relaxation. Contraction results in the shortening of the muscles and results in movement, while relaxation returns the muscle to its normal state. muscle to its original length.
Key differences between actin and myosin
The following are the few, but essential, differences between actin and myosin:
- Actin and myosin are the protein filaments found in muscle cells, and actin is known to form the thin bands in myofibrils, while myosin forms the thick bands in myofibrils.
- Actin forms a short filament of 2-2.6 umand is thin down to 0.005 µmbut myosin forms a long filament of 4.5 µm, which is 0.01 um thick.which means that actin is thinner than myosin.
- Actin contains troponin and tropomyosin (proteins), and myosin contains meromyosin (protein), which (myosin) has an ATP binding site, through which it releases energy for muscle contraction.
- Actin is present in A and I bandswhile myosin is present in the A-bands of the sarcomere.
- The surface of actin is smooth, and they are more numerous compared to myosin, the ratio is one for every six actin molecules. Myosin has a rough surface.
- Actin slides in the H-zone at the moment of contraction, whereas myosin does not slide at the moment of contraction.
Similarities
- Actin and myosin are the protein filaments present in muscles.
- Both types of proteins are needed during muscle contraction.
- Calcium ions are necessary for muscle contraction.
Conclusion
So we can say that in addition to muscle contraction, actin and myosin play a vital role in cellular biology by participating in the cell divisionin non-muscle cell functions, etc. Myosin is thicker than actin and has darker striations. The functioning of muscle contraction can be understood by means of the sliding filament theory.