Answer
The contractile proteins of muscle are myosin and actin. In muscle contraction the knoblike heads of myosin molecules bind to the myosin binding sites of actin molecules of thin filaments. The complex formed by this binding is called a cross-bridge. Myosin is a motor protein and it causes the sliding of the filaments and the contraction of the muscle. Actin, is the major protein of thin filaments; it has binding sites for myosin heads.
The regulatory proteins are tropomyosin and troponin; these molecules are parts of thin-- actin--filaments. When muscles are relaxed the tropomyosin-troponin complex is positioned in the thin filament in a way that blocks the myosin binding sites. When Calcium (Ca++) binds to troponin it causes a change in the shape of the troponin-tropomyosin complex that exposes the myosin binding sites; this facilitates the formation of cross-bridges between the thin and the thick filaments. Powered by ATP the motor protein myosin
heads pulls the thin filaments between the spaces ( between thin and thick filaments) and shortens the sarcomere.
Structural proteins
Some of the structural proteins are titin, nebulin, myomesin, desmin, dystrophin and alpha-actinin. Some of these proteins also have important physiological functions
Work Step by Step
Titin is a protein of very large molecular weight (MW > 3,700 kDa); it is also called connectin, and connects Z disks and M lines, It contributes passive elasticity to skeletal muscles. Nebulin (ca, 800 kDa )is also a large protein ; it binds actin molecules together in thin filaments. Myomesin is part of the M line; it binds to titin and connects thick filaments together. Desmin forms intermediate filaments.; it is found in sarcolemmas, nuclear membranes, and Z-disks. Dystrophin is a protein that connects the cytoskeleton of muscle fibers to extracellular matrix through the sarcolemma.
The genetic diseases of Duchenne and Becker's muscular dystrophy are due to metabolic dysfunctions which result insufficient dystrophin production--none, or too little.
