The heads project outward to attach to actin during contraction. Each myosin molecule contains an actin-binding site as well as an ATP binding site.
2. Regulatory Proteins
Tropomyosin : Is part of the thin filament. To be discussed later but in a relaxed muscle, myosin is blocked from binding to actin because of strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands are held in place by troponin molecules
Troponin : Is part of the thin filament and as mentioned above hold the tropomyosin strands in place.
3. Structural Proteins
About a dozen types which contribute to the alignment, stability, elasticity, and extensibility of myofibrils. Titin : meaning “gigantic” (50X larger than an averaged size protein) is the 3 rd most plentiful protein in skeletal muscle (after actin and myosin). It spans ½ of a sarcomere from Z-disc to M- line. Each titin protein connects a Z-disc to an M-line stabilizing the position of the thick filament. Titin molecules display great elasticity which accounts for much of the extensibility and elasticity of the myofibril.
α (Alpha) – Actinin : binds to actin molecules of the thin filament and to titin.
Myomesin : These molecules form the M-line, bind to titin and connect adjacent thick filaments to one another. Nebulin : a long nonelastic protein wrapped around the entire length of each thin filament helping to anchor the thin filaments to the Z-disc and regulate the length of the thin filaments during development. Dystrophin : Links thin filaments of the sarcomere to integral proteins of the sarcolemma, which are attached in turn to proteins of the CT ECM that surrounds muscle fibres. This protein is thought to reinforce the sarcolemma and is implicated in Muscular Dystrophy – most common
96
Made with FlippingBook Publishing Software