PSIO 201 5th Edition Lecture 18Outline of Last Lecture Muscle IntroductionOutline of Current Lecture I. Sarcomere II. Thin FilamentsIII. Thick FilamentsIV. ProteinsV. Sliding ModelVI. Crossbridge Cycle VII. Rigor ComplexCurrent Lecture - Sarcomere – functional unit of contraction ; contains different regions…1. Thin filaments – attached to Z line, two twisted strands of pearls composed of two helically arranged strands of actin ; Myosin binding site on actin covered by regulatory proteins ( troponin and tropomyosin) - Tropomyosin is controlled by troponin which is controlled by calcium; regulate contraction by regulating binding sites2. Thick Filament – spans the distance and overlaps thin filaments; composed of a highly organized array of myosin molecules. Contains myosin heads that interact with actin; both together form crossbridges. Myosin head also has ATPase activity when bound to actin (releases ATP to power contraction)Protein Overview a) Actin - found in thin filaments- has myosin binding sites for crossbridges formation with myosinb) Myosin- found in thick filaments- has myosin head that binds to the actin and forms cross bridge during contractionRegulatory Proteinsa) Tropomyosin - found in thin filaments- covers the myosin binding sites on actin when muscle is relaxedb) Troponin- found in thin filaments- holds tropomyosin in place when muscle is relaxed- during contraction, calcium binds to troponin and causes a conformational change that shifts tropomyosin away from the myosin binding sites of actin, exposing the binding sites for crossbridge formation-Sliding Filament Model of Muscle Contraction :1. Myosin “heads” bind to actin to form a crossbridge2. Conformational change, energized by ATP hydrolysis, causes think filaments to sline along thick filaments3. Myosin head groups release, form new crossbridges, and the sliding cycle repeats….Result : Z- line move toward one another, Sacromere length decreases, Myofibril shortens, Muscle fiber shortens As Think/Thin filaments overlap increases….1. I Band length decreases2. A band length remains constant3. H Zone length decreases4. Zone of overlap increases Crossbridge Cycling - cycle is entered following exposure of myosin binding sites on actin filaments (regulatory role for CA2-)Prior to entering cycle – at the end of the contraction ; myosin heads prepares in the cocked positions to bind to actin. 1) ATP has to bind to myosin head2) ATP is hydrolyzed by the unbound head and releases energy resulting in the “cocked” Position of the head groupsOnce Cocked the Crossbridge Cycling can occur….1. Crossbridge formation – 2. Power stroke – ADP is released, and head pivots toward center3. Crossbrige detachment – link between head and actin detaches4. Reactivation of Myosin head – ATP to ADP and inorganic phosphate* Ends with calcium ions are transported back to Reticulum Detailed Version in Lecture 1. Myosin heads bind to actin forming crossbridges; only when Ca2+ is released from SR binding to troponin, exposing the binding sites. ( crossbridge formation)2. Myosin heads pivot towards center of sarcomere (power stroke)3. As myosin heads bind to ATP, the crossbridges detach from actin (crossbridge detachment)4. Myosin heads hydrolyze ATP and become reoriented and re- energized ( re-cocked) Rigor Complex – the attached head group after the power stroke- Rigor Mortis – the rigor of death because of lack of ATP to detach the crossbridge 1. Myosin heads rotate back towards center of sarcomere2. Myosin heads bind to ATP, and crossbridges
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