Molecular Biology

Structure and Function of Myosin

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Myosin is one of the proteins known to scientists as an ATP-dependant motor protein and is recognized as one of the most abundant proteins in the human body. Its structure and function allows myosin to perform a characteristic function in the eukaryotic cell, which is to support the cells motility processes, in combination with another protein known as ‘actin’. However, in order to accommodate various motility needs in various organs, myosin proteins have adapted themsevles by having different structures.

With the use of modern technology, scientists have uncovered almost 18 types of myosin and, among them, the commonest type is considered to be the type II. Type II myosin is also considered a ‘conventional type’ and is present in the muscle tissues of the animal. It is the myosin which gives the muscles their ability to contract. However, in order to achieve this function, the tiny myosin proteins require taking complex configurations along with the other muscle proteins.

The macrostructure of skeletal muscles

When considering the macrostructure of a skeletal muscle, it is composed of bundles of muscle fibers. These muscle fibers are in turn composed of many myofibrils, which consist of both thin and thick myofilaments. While the thin myofilaments are composed primarily of actin protein, the thick myofilaments are made of multiple overlapping myosin II proteins.

The microstructure of myosin filaments

According to scientists, each myosin molecule will have three distinguishable sections. The head domain will bind the filamentous actin protein using energy generated through ATP hydrolysis at the time of movement. The other two domains, the neck and the tail, will act as regulatory and supplementary domains for the main function. However, while the neck domain may act as a binding site for myosin light chains the tail domain can interact with the cargo molecules in the process of regulating the motor activity.

When digging further into the myosin structure, scientists consider it to have a dimeric shape. Being a hexamer, which contains two heavy chains and four light chains, the myosin filaments are able to maintain a stable coiled structure. In order to do this, the tail region of the myosin is periodically interspersed with hydrophobic residues to give the coiled coil type of structure. To assist this structure, the tail region also contains non-helical amino acids as well as inter-myosin ionic bonds.

The structure in relation to its function

According to scientists, the contraction of muscle fibers is considered to take place according to a ‘sliding filament’ model. As the head of the myosin filament lies next to the actin filament, it is able to bind each temporarily. Following binding, the myosin filaments are thought to rotate to a different position, which is amply supported by its coiled structure and the support given by the levering neck region. Throughout this process, ATP hydrolysis will provide the binding and the movements the required energy.

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