Unit 2b notes - September 28, 2010 a. Thick filaments a.i....

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September 28, 2010 a. Thick filaments a.i. Composed of just one protein: myosin a.ii. Head and arm region are usually referred to as the cross-bridge a.iii. Myosin, at the top, are sites that bind to actin a.iv. Sites on head are ATP binding sites a.v. Head can act as an ATPase, which means it can break down ATP into ADP plus a phosphate a.v.1. A cell would do this to get energy for muscle contraction a.vi. Cross bridge itself binds to actin, can swivel directed by the energy of ATP (back and forth motion), brings actin toward M-line a.vii. Thick filaments are made up of lots of myosin in different directions (pull toward M-line in center of myosin) a.viii. Crossbridge Cycle a.viii.1. Fig. 12.12 a.viii.1.a. Myosin ATPase converts ATP to ADP + a phosphate (an inorganic phosphate) a.viii.1.a.i. ADP and phosphate still bound to crossbridge a.viii.1.a.ii. ATP hydrolyzed causing myosin to go back to normal position a.viii.1.a.iii. Energize the cross bridge (in the “cocked” position)
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a.viii.1.a.iv. When the myosin head as ADP and phosphate bound to it, it has very high affinity for actin (binds very well if it can if tropomyosin is in way, will not bind) a.viii.1.b. Myosin binds to actin (aka “attachment”) a.viii.1.c. Phosphate gets released from the cross bridge a.viii.1.c.i. Cross bridge undergoes conformational change ”Power Stroke” a.viii.1.d. Powerstroke pulls actin toward the center of the sarcomere (M- line); ADP is released a.viii.1.e. New ATP binds to the cross-bridge a.viii.1.e.i. Cross-bridge has much lower affinity for actin a.viii.1.e.i.1. If decrease affinity, crossbridge releases actin and cycle starts all over again (start at step “a” again) a.viii.2. If ran out of ATP, when mitochondria quits making ATP (when in rigor mortis) a.viii.3. Rigor Mortis a.viii.3.a. Muscles in deceased contract a.viii.3.a.i. Cross-bridge cycles start but if no new ATP is stuck in the contracted state a.viii.3.b. If you’re alive= physiological contracture a.viii.3.b.i. Very hard to get this a.viii.3.b.ii. Very difficult to work muscle hard enough to run out of ATP a.viii.3.c. Get stuck in cross-bridge cycle a.viii.3.d. Doesn’t last forever because the proteins (actin and myosin) start to deteriorate and muscles become flaccid again a.ix. Fig 12.10 a.ix.1. Bottom pic had released its ADP lets go of myosin a.x. Fig 12.11 a.xi. Fig 12.14
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a.xi.1. Remember tropomyosin’s job is to block binding sites a.xi.2. If want cross bridge cycle to stop, must get rid of Ca a.xi.3. If want to start cross bridge cycle: must have an increase in Ca 2+, which binds to troponin, which moves tropomyosin, which undercovers the myosin binding sites on actin (the active sites on actin) a.xi.4. If want muscle to relax, opposite must happen a.xii. End cross bridge cycle—reverse the process a.xii.1. Fig 12.15 a.xii.1.a. (transverse) T-tubules are indentations in the membrane which goes down into the cell a.xii.1.a.i. Inside the indentations is extracellular material
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This note was uploaded on 02/12/2012 for the course BI 202 taught by Professor Winn during the Fall '10 term at Northern Michigan University.

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Unit 2b notes - September 28, 2010 a. Thick filaments a.i....

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