Proteins are the building blocks of muscles. The protein molecules are the actin and myosin found in muscles that are primarily responsible for muscular contraction in both people and other species. Actin and myosin are polypeptide filaments that operate when calcium ions are present. The sliding filament theory best explains the activities of actin and myosin in muscle contraction. Ralph Niedergerke, Jean Hanson, and Andrew Huxley introduced this hypothesis in 1954.
Summary
Together with the governing proteins troponin, tropomyosin, and meromyosin, actin and myosin govern voluntary muscle movements inside the body. Whereas, the myofibril filaments are formed by actin and myosin proteins, which are organized longitudinally in the myofibrils.
Actin vs Myosin
The main difference between actin and myosin is that within muscle cells, actin forms thin contractual filaments, whereas myosin produces thick contractile fibers. Actin filaments are a type of light striated muscle. Actin myofilaments are thinner than myosin filaments. Actin and myosin are involved in both intracellular and non-cellular motions.
Actin is a family of globular proteins found in the bulk of eukaryotic cells that helps the body maintain its shape, structure, and movement. Actin is a protein that has a lot of similarities to other proteins. Monomeric actin (G-actin) and filamentous actin (F-actin) are the two types of actin. Under physiological circumstances, G-actin polymerizes quickly to create F-actin with the help of ATP energy. The actin filaments determine the cell’s structure and mobility. Actin filaments are responsible for forming a cell’s dynamic cytoskeleton. The cytoskeleton provides structural support and connects the inside of the cell to its environment.
Myosin belonged to motor protein group that is essential for muscle fiber contraction, together with actin proteins. One or two heavy chains and multiple light chains make up each molecule of myosin. Head, neck, and tail are the three domains found in this protein. Actin and ATP binding sites can be found in the globular head domain. A -helical is located in the neck area. The binding sites for various compounds are located at the tail site.
Comparison Table Between Actin and Myosin
| Parameters of comparison | Actin | Myosin |
| Definition | A protein called actin is found in muscle cells and forms a thin muscular filament. | Myosin is a protein that helps muscle cells generate thick contractile filaments. |
| Size | Actin produces a short (2–2.6 m) and thin (0.005 m) filament. | Myosin creates a long (4.5 m) and thick (0.01 m) filament. |
| Regulatory Proteins | Tropomyosin and troponin make up actin filaments. | Meromyosin is found in myosin filaments. |
| Location | A and I bands include actin filaments. | A sarcomere’s A bands include myosin filaments. |
| Surface | The actin muscle have a smooth surface. | The myosin muscle have a rough surface. |
| Number of filaments | The quantity of actin filaments is enormous. | There are six actin filaments for every one myosin filament. |
| Slide | During contraction, actin filaments slip into the H zone. | During contraction, myosin filaments do not move. |
What is Actin?
In muscle cells, actin is a protein molecule a thin contractile filament. In eukaryotic cells, this is the most protein found. Although actin filament polymerization begins at both endpoints of the filament, the quantity of polymerization at each end is not equal. As a result, the filament has an inherent polarity. The barbed (+) end is the fast polymerizing end, whereas the pointed (-) end is the slow polymerizing end. The actin filament is stabilized by the interaction of tropomyosin and troponin.
The actin filaments determine the cell’s structure and mobility. Actin filaments are responsible for forming a cell’s dynamic cytoskeleton. The cytoskeleton provides structural support and connects the inside of the cell to its environment. During mitosis, actin filaments help in the transfer of organelles to the new cells. Muscular cells have a series of thin filaments that create forces that promote muscle contraction.
What is Myosin?
Myosin is a protein that helps muscle cells generate thick contractile filaments. One or two heavy bands and multiple light chains make up each myosin molecule. This protein has three distinct domains: head, neck, and tail. The globular head domain includes actin and ATP binding sites. Actin and myosin proteins generate filaments that are organized longitudinally in myofibrils.
Myosin is divided into thirteen different classes, such as myosin I, II, III, IV, and so on. The myosin I protein is involved in vesicle transport. Myosin II is the protein that causes muscle contraction. It is the conversion of chemical energy is converted into mechanical by changing the structure of the myosin, causing ATP to attach to the actin. The sliding filament theory describes the contraction of muscles. Tension is created in the muscle when thin actin filaments glide across a bulky myosin filament.
Main Differences Between Actin and Myosin
- Muscle contraction is aided by the proteins actin and myosin. Actin has fewer striations and is narrower than myosin. Myosins are darkly striated and thick.
- Actin and myosin are involved in both cellular and non-cellular motions.
- Actin filaments are the light striations. They’re also known as the I band. The thicker myosin filaments, on the other hand, are thicker than actin myofilaments.
- The sarcolemma receives action potentials, which activate the sarcoplasmic reticulum to transfer calcium ions into the cytoplasm. The binding of myosin to actin, which initiates filament sliding, is triggered by calcium ions.
- Actin creates a thin filament while myosin makes a thick one. Muscle contraction is caused by the sliding of the two filaments over one another in a sequence of repeating actions.
Conclusion
Muscle contraction is accomplished by both of these mechanisms. When the nervous system stimulates muscle fibres, the heads of myosin bind to the binding sites on the slender filaments, and the sliding begins. Each cross bridge connects at the same moment and detaches continually for numerous times during contraction in the presence of adenosine triphosphate (ATP), the energy source.
Actin and myosin are involved in both cellular and non-cellular motions. As a result, muscular contraction causes muscle shortening and motion. Muscle relaxation, on the other hand, causes the muscle to revert to its original length.