The body has specialized organs, such as the heart, liver and kidneys. The organs are made up of tissues, which are made up of trillions of cells. Each cell has a copy of DNA that codes for the creation of proteins. It does so by specifying the sequence of amino acids, which are smaller molecules that make up the proteins. Proteins play many roles in the body. They may carry chemicals through the blood, act as hormones, attack invading organisms, support cells and tissues, and break down or build up chemicals, among many other functions. Muscle cells, called myocytes, use proteins for muscle contraction in order to perform work.
Protein Structure
Proteins are conceptualized on four structural levels: primary, secondary, tertiary and quaternary. The primary structure is its sequence of amino acids. The secondary structure are architectural formations of amino acids such as spirals or sheets. The tertiary structure is the three dimensional structure of the protein. The quaternary structure applies if multiple protein subunits come together to make a larger protein.
Muscle Proteins
The entire sequence of muscle contraction, from the release of neurotransmitters from the nerve to the actual contraction of muscle involves very many proteins. The main proteins involved in the actual contraction are actin, myosin, troponin and tropomyosin.
Muscle Structure
A myocyte is a long cell. The cell is packed with protein fibers called myofibrils. Thick filaments are made of myosin, which have a protruding head on one end. Thin filaments are made of actin, arranged in a double helix. Tropomyosin and troponin lie along the length of the double helix. The thick and thin filaments lie side by side.
Contraction
When the muscle receives a signal to contract, usually from the neurotransmitter acetylcholine, special protein channels open in the muscle and allow calcium to rush in to the cell. The calcium binds to troponin, which changes its tertiary structure, which in turn changes the tertiary structure of tropomyosin. Tropomyosin moves off of its binding sites on actin. The myosin heads bind to actin, and using the energy contained in adenosine triphosphate, or ATP, the main energy carrier of the cell, myosin pulls actin filaments and shortens the muscle. This sequence of events is called the sliding filament model.
References
- "Molecular Biology of the Cell"; Bruce Alberts et al.; 4th Ed 2002
- "Lehninger Principles of Biochemistry"; David L. Nelson, et al.; 4th Ed 2004
- "Physiology"; Linda S.Costanzo; Fourth Ed 2008
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