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A Literature Review on the Myosin Organelle (Essay Sample)

This is a literature Review of Myosin ,a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. The literature review follows a logical structure from the basic description of what myosin is, its structure, and function. The review also explores myosin modulators which refer to the set of all stimuli that affect the working of myosin filaments. source..
Literature Review Myosin Myosin encompasses a superfamily of “molecular motors” concerned with the transport of specific vesicles, organelles and biomolecules in eukaryotic cells CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). The process entails converting ATP (chemical energy) to mechanical energy. The discovery of myosin is usually credited to Pollard and Korn (1973). Their work with the Acanthamoeba castellanii unearthed enzymes that display functions commonly observed in myosin. Other sources credit discovery of myosin to Willy Kuhne in 1864 owing to the identification of motor protein M2 from muscle extracts in Heidelberg. Over the years, research into myosin has helped identify a wide range of myosin genes in eukaryotes. Syamaladevi, et al. (2012) claimed that myosin is usually associated with muscles, however, years of research indicate that myosin represents a large group of proteins. The protein products are classified into one superfamily due to similar properties in their ATPase Enzyme activities, force transduction and Actin binding CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). Moreover, the myosin superfamily can be subdivided into more than twenty-four classes based on the structural organization of myosin CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). Structure of myosin Myosin molecules are made up of motor protein. In the sarcomere, Myosin filaments are comprised of a “head, neck, and tail” domain. Each of the domains has a specific function in myosin filaments CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). Past research indicates that except Myosin VI, all other classes of myosin use the head domain to bind the filamentous actin and move along the filament towards the positively charged end of myosin CITATION Har12 \l 1033 (Hartman, & Spudich, 2012). In contrast to other classes of the myosin superfamily, myosin VI movers towards the negatively charged end CITATION Har12 \l 1033 (Hartman, & Spudich, 2012). Consequently, the neck domain is usually viewed as lever and connector that transmits force produced by the catalytic motor domain. In addition to its functions as a linker, the neck domain is also a site for binding of light chains that are essential for regulatory purposes in myosin molecule CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). The tail domain is designed for facilitating interactions with other myosin sub-molecules and or with cargo molecules CITATION Har12 \l 1033 (Hartman, & Spudich, 2012). In some cases, the tail domain is used to regulate movement in myosin molecules CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). In the sarcomere, there are two head domains in one myosin filament. Each of the heads has two binding sites. One of the sites is an ATP binding site while the other is an actin-binding site. There are also two neck domains in every myosin filament; because of this, the tail domain is structured like a coil. In addition to the structural features of myosin, an elastic protein known as Titin supports the thick myosin filaments. Biological images show Titin running through the thick filament. The function of myosin in muscle contraction Conventional myosin is responsible for muscle contraction in eukaryotes. Researchers and scholars in the field also refer to conventional myosin using its class as myosin II CITATION Cha10 \l 1033 (Chantler, et al., 2010). Muscle cells can generate force due to innate processes at the cellular level. Fundamentally, the movement of myosin on the actin filaments generates force CITATION Kou14 \l 1033 (Koubassova & Tsaturyan, 2014). The basic function of myosin molecules is similar in almost all classes and variations of myosin CITATION Har12 \l 1033 (Hartman, & Spudich, 2012). Human skeletal muscles contract when the filaments glide against each other. The process of muscles contraction begins with a nervous stimulation. Myosin plays an important role in the entire muscle contraction process; however, elaborating the process without prior knowledge on the entire cycle may lead to ambiguous descriptions. In this review, the function of myosin in muscle contraction is elaborated in three critical phases. The first phase is concerned with the nervous simulation sent to muscle fiber. An electrical charge triggers the release of calcium ions stored in the sarcoplasmic reticulumCITATION Cal75 \l 1033 (Szent-Györgyi, 1975); CITATION Wak15 \l 1033 (Wakabayashi, 2015). The calcium ions pass through the T-tube of the muscle myofibril and enter the sarcomere which is usually placed adjacent to each other. The role of calcium ions in muscle contraction is to trigger an enzyme reaction that activates the binding sites of actin filamentsCITATION Cal75 \l 1033 (Szent-Györgyi, 1975). When muscles are relaxed, an enzyme known as tropomyosin inhibits the binding sites of actin filaments. Calcium ions bind with another hormone known as troponin when muscle contraction is triggered CITATION Wak15 \l 1033 (Wakabayashi, 2015). The latter action displaces the tropomyosin enzyme, thus activating the actin binding sites. In this review, this triggers the second phases of processes where myosin is involved. Once, the binding sites are active the contraction of the sarcomere begins. The adjacent sarcomeres are comprised of interchanging thick and thin filaments. Muscle contraction occurs when the filaments slide against each other through a mechanism known as “the sliding filament mechanism of muscle contraction” CITATION Squ16 \l 1033 (Squire, 2016); CITATION Har12 \l 1033 (Hartman, & Spudich, 2012); CITATION Rya18 \l 1033 (Ryan & Nebenfuhr, 2018). The thick filaments are myosin while the thin filaments are actin CITATION Squ16 \l 1033 (Squire, 2016). At the organelle level, myosin filaments attach to actin binding sites pulling the actin filament along its length. The process of attaching to the binding sites by myosin filaments occurs when ATP is converted to inorganic phosphate and ADP through hydrolysis CITATION Squ16 \l 1033 (Squire, 2016). The binding site where actin and myosin filaments attach to each other forms a cross-bridge. For purpose of elaboration, this review attempts to describe the structure of the sarcomere. The ends of actin filaments are anchored to Z-lines. The myosin filament is anchored to the M-line in the middle of the filament CITATION Bha15 \l 1033 (Bhagavan & Ha, 2015). During muscle contraction, the Z-lines at each end of the sarcomere is pulled closer to the M-line. Once a cross-bridge is formed, the myosin filaments pull actin filaments along its length CITATION Har12 \l 1033 (Hartman, & Spudich, 2012); CITATION Bha15 \l 1033 (Bhagavan & Ha, 2015). The latter described movement is referred to as the power stroke in the scientific literature CITATION Bha15 \l 1033 (Bhagavan & Ha, 2015). During the power stroke, myosin filaments are pulled towards the M-line while actin filaments are pulled towards the Z-line CITATION Bha15 \l 1033 (Bhagavan & Ha, 2015); this, is typically described as “sliding”. Notably, during the entire process, the myosin filaments do not change in length CITATION Sya12 \l 1033 (Syamaladevi, et al., 2012). Additionally, once the power stroke occurs, the myosin filaments do not unbind from the cross-bridge until another ATP molecule undergoes hydrolysis to release ADP and phosphate CITATION Kou14 \l 1033 (Koubassova & Tsaturyan, 2014); CITATION Squ16 \l 1033 (Squire, 2016); CITATION Bha15 \l 1033 (Bhagavan & Ha, 2015). Once, the myosin filament detaches from the binding site, the actin filaments are pulled back by the Z-lines to their original positions. The process repeats itself upon nervous simulation. In this study, the retreat of actin ends the third phase. Overall, the function of myosin in muscle contraction is to extend to the binding site of actin filaments to form a crossbridge and pull the actin filaments along its length. From a wider perspective, myosin is involved in the contraction of sarcomere by pulling the Z-line to the M-line. The unison contraction of muscle fiber results in the contraction of the entire muscle. Myosin modulators How myosin modulators work Myosin modulators refer to the set of all stimuli that affect the working of myosin filaments. Myosin modulators range from environmental conditions such as temperature and pH to chemical inhibitors like phosphates CITATION Kar17 \l 1033 (Karatzaferi, et al., 2017). Other studies express myosin modulators as protein products found at the neck domain of myosin filaments that have a role in force generation CITATION Mam18 \l 1033 (Mamidi, et al., 2018). In the latter described study Mamidi et al. (2018) investigated the effect of a myosin modulator known as mavacamten on cross-bridge function and force generation. The researchers based their study on the “murine model of hypercontractility”. Mavacamten is also known as MYK-461 and Mamidi et al. (2018) asserted that the molecule used in myosin binding has a negative effect on hypercontractility. In their study, CITA...
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