lecture28 - Myosin interaction. myosin binds to actin...

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Unformatted text preview: Myosin interaction. myosin binds to actin filament it then changes its configuration so that it "pulls on the filament" myosin releases bound ADP, Pi ATP enters the site on the myosin Hydrolysis of ATP releases energy that is used to free the myosin from the actin. The ADP + Pi remains in the myosin head Then the "head" swings forward and binds again to the actin. 11/5/2007 12:49 PM In the transition from g Actin to F actin, ATP is required and is cleaved during the process That means that Actin is an ATPase. Cells contain relatively unorganized actin and myosin. Nov 5­12:49 PM 1 1. How do you get a muscle to contract 2. How do you get a muscle to relax? 3. What's with rigor mortis? Tropomyosin ­ thin filament on the actin filament Troponin Complex Troponin c (Calcium) Troponin i (inhibition) troponin a 11/5/2007 12:10 PM myosin can only interact with actin in the groove that is covered by tropomyosin Calcium ions interact with troponin c, causing a change in troponin i and troponin t such that the tropomyosin sinks deeper into the groove and myosin can contact the actin 11/5/2007 12:13 PM 2 T tubule (tranverse) sarcoplasmic reticulum 11/5/2007 12:13 PM 11/5/2007 12:13 PM 3 Sarcoplasmic Reticulum: Stores and releases Ca++ for storage, it contains two proteins 1. Ca++ pump (requires ATP, Ca++)(Mg++ too)­ pumps Ca++ from the cytoplasm into the SR 2. Calsequestrin­ inside the SR space, binds Ca++ Release: 1. Calcium++ channel that is sensitive to voltage. Ryanodine sensitive channels. 11/5/2007 12:14 PM When the Ca in the sarcoplasm is low, the access of myosin to actin in the myofibril is limited because of the position of tropomyosin. On stimulation, a nerve impulse sets up a current in the membrane. the current travels down the T tubules past the SR membranes Stimulates the Ca++ channels to open, releasing Ca++ The Ca++ interacts with troponin C >>shift of position of tropomyosin, myosin heads can bind to actin, cause contraction. At the same time, the Ca++ pump removes Ca++ from the sarcoplasm, and when the level is sufficiently low, reverts to the starting condition. ATP is needed: release phase of muscle contraction activity of the Ca++pump 11/5/2007 1:10 PM 4 when the Ca in the sarcoplasm is low, troponin causes the tropomyosin to block access of myosin heads to the actin. >> no contraction To cause contraction, A nerve impulse sets up a current on the muscle membrane. The current travels down the transverse tubules past the SR membranes. Activates the Ca channels (ryanodine sensitive channels) in the SR membranes, causing them to open. That releases Ca ions into the sarcoplasm. Those ions interact with Troponin C to cause a change in T­I and T­T so that the tropomyosin moves out of the way and allows Myosin to interact with actin. The Ca pump continues to remove the Ca from the sarcoplasm, and the muscle relaxes. 11/5/2007 12:14 PM 5 ...
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