Athena Vale
Muscles contain contractile proteins, such as actin and myosin, which are responsible for muscle contraction and, ultimately, movement. Actin is the primary component of thin filaments, while myosin forms thick filaments. These filaments slide over each other during contraction, with myosin pulling actin towards the centre of the sarcomere, the functional unit of contraction within a muscle fibre. This process is facilitated by calcium ions, which bind to troponin on actin filaments, exposing the active sites for myosin to bind to. The energy required for this process is provided by the oxidation of carbohydrates or lipids, and in the form of ATP, which is produced by mitochondria.
| Characteristics | Values |
|---|---|
| What are contractile proteins | Actin (thin filament) and myosin (thick filament) |
| Actin | A globular contractile protein |
| Myosin | An elongated, probably double-stranded peptide chain |
| Myosin's molecular weight | Close to 500,000 |
| Myosin's amino acids with positively charged side chains | 18% of the total number of amino acids |
| Myosin's amino acids with negatively charged side chains | 16% of the total number of amino acids |
| Actin's molecular weight | About 50,000 |
| Actin's composition of muscle proteins | 12 to 15% |
| Myosin's composition of muscle proteins | 20% |
| Muscle composition of body weight | About 40% |
| Muscle protein composition of body weight | About 5 to 6 kilograms or 11 to 13 pounds |
| Muscle contraction | Thin filaments slide over thick filaments |
| Calcium | Binds to troponin on actin filaments and exposes the active sites for myosin |
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What You'll Learn
Actin and myosin are contractile proteins
Skeletal muscle is composed of muscle fibres that have smaller units called myofibrils. There are three types of proteins that make up each myofibril: contractile, regulatory, and structural proteins. The contractile proteins are actin (thin filament) and myosin (thick filament).
Actin and myosin are the two contractile proteins that enable muscle contraction. Actin exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin). The form involved in muscle contraction is F-actin, which is composed of two helical "F" actin and each 'F' actin is made up of multiple units of 'G' actin. In muscle, two long strands of bead-like actin molecules are twisted together to form a thin filament.
Myosin is the principal component of thick myofilaments and is the most abundant protein found in muscle. It constitutes approximately one half of the total myofibrillar protein. Myosin combines easily with another muscle protein called actin, which forms about 12 to 15 percent of muscle proteins. Myosin works as a motor, hydrolysing adenosine triphosphate (ATP) to release energy in such a way that a myosin filament moves along an actin filament, causing the two filaments to slide past each other.
The cyclical association and dissociation of actin and myosin during contraction result in conformational changes in the cross-bridge linkages between the thick and thin myofilaments. This sliding movement of the actin-myosin protein conjugate responsible for contracting muscles can be inhibited using a dendrimer that glues them together.
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Actin is a thin filament
Muscles contain contractile proteins to enable muscle contraction and relaxation, which is how we are able to perform daily activities such as walking, running, writing, and typing. Actin, a thin filament, is one of these contractile proteins.
The thin filament is said to be in a blocked state in the absence of calcium ions (Ca2+). When a neural signal reaches the neuromuscular junction, acetylcholine is released, and action potential is generated. This leads to the release of calcium ions in the sarcoplasm, which then bind to troponin on actin filaments. This movement of calcium ions triggers a conformational change in troponin, shifting the azimuthal position of tropomyosin on F-actin to allow for actin–myosin interactions. The thin filament then transitions from the blocked to the closed state, and the myosin heads can interact with actin and undergo a conformational change to produce force, resulting in muscle contraction.
The length of thin filaments varies depending on the type of muscle, with in vivo thin filaments estimated to be 1.38 μm long, skeletal muscle thin filaments measuring 1.05 μm, and mammalian cardiac muscles having thin filaments ranging from 0.6 to >1.1 μm. The diameter of thin filaments is 6-10 nm, and they are composed of actin, tropomyosin, and troponin in a 7:1:1 stoichiometry. The structural core of the thin filament is the helical F-actin polymer, which repeats once every 14 monomers and has an average axial repeat size of 36.5 nm. One complete turn of the helix occurs every 74 nm, corresponding to approximately 13 actin monomers.
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Myosin is a thick filament
Muscles in the human body enable us to perform a variety of activities, such as walking, running, writing, and typing. Muscle contraction and relaxation are made possible by contractile proteins, which are one of three types of proteins that make up the smaller units of muscle fibres, called myofibrils. Actin (thin filament) and myosin (thick filament) are the contractile proteins.
Myosin is the main component of the thick filament and plays a crucial role in its formation and dynamics in skeletal muscle cells. A single thick filament is made up of approximately 180 to 300 myosin molecules and their associated proteins. The thick filament interacts with the thin filaments, composed of actin, to cause muscle contraction. During contraction, the thin filaments slide over the thick filaments, and myosin binds to the exposed active site on actin, pulling the actin filament towards the centre, resulting in contraction.
The bipolar structure of the thick filament, formed by the myosin molecules, enables actin to interact along its entire length, contributing to the greater shortening ability of smooth muscles compared to skeletal muscles. Myosin is the molecular motor of the sarcomere, hydrolysing adenosine triphosphate (ATP) to interact with the thin filament actin. However, only about 10% of myosin molecules are utilised for each contraction, optimising energy usage.
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Contractile proteins are essential for muscle contraction
There are three types of proteins that make up each myofibril: contractile, regulatory, and structural proteins. The contractile proteins actin (thin filament) and myosin (thick filament) are responsible for muscle contraction. The thin and thick filaments have different compositions and locations. The thick filaments occur only in the A band of a myofibril, while the thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band.
During contraction, the thin filaments slide over the thick filaments. This sliding is made possible by the interaction between actin and myosin. Myosin binds to the active site on actin, pulling the actin filament towards the centre and causing the Z lines to be pulled closer together, resulting in contraction. This process requires energy, which is provided by the hydrolysis of ATP.
The striated appearance of skeletal muscle tissue is caused by the regular arrangement of contractile proteins actin and myosin. The alternating bands of actin and myosin allow sarcomeres to contract, giving muscle its banded appearance.
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Contractile proteins are soluble in salt solutions
Contractile proteins are essential elements in the skeletal muscle's contractile system. The contractile proteins actin (thin filament) and myosin (thick filament) make up each myofibril, which are smaller units of muscle fibres. The thin filaments slide over the thick filaments during contraction, with myosin binding to actin, resulting in muscle contraction.
The solubility of proteins in salt solutions depends on the type of salt and its concentration. For instance, protein interactions in sodium chloride show little salt dependence even at very high salt concentrations, while proteins in ammonium sulfate show a sharp drop in the second osmotic virial coefficient with increasing salt concentration. At low salt concentrations, protein-protein interactions can be attractive or repulsive, potentially due to the anisotropy of the protein charge distribution.
The solubility of salt also plays a role. For example, sodium sulfate, which is only soluble up to 1.3 M at 20°C, cannot salt out proteins like ovalbumin or BSA, while ammonium sulfate, soluble up to 4.03 M at 20°C, can. The efficiency of salts as crystallization agents is influenced by their solubility.
Additionally, ion binding can impact protein solubility in salt solutions. While not all ions strongly bind to proteins, weak salting-out agents tend to bind proteins more readily, affecting protein interactions. For instance, potassium thiocyanate ions binding to ovalbumin increase with concentration.
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