Cellular Basis of Muscle Contraction Claudia Stanescu, Ph.D. Office Hours in Gittings 108 Tue 10-11am and Thurs 1-2pm or by appointmentObjectives: 1. Diagram and label the thin and thick filaments including all of the proteins that make up each myofilament. 2. Give a general description of the sliding filament mechanism 3. Describe the contraction cycle. 4. Describe rigor mortis.Structural Organization of Skeletal Muscle -muscle -> muscle fiber -> myofibril -> sarcomereSarcomeres: Functional Unit of Contraction Elements of Sarcomere Structure 1. Thin Filament Attached to Z line (Z disc) Z ‘myosin binding site’ on actin covered by regulatory proteins composed of two helically arranged strands of actin two twisted strands of pearls -actin, troponin, tropomyosin: determine when contraction occures-actin has region//brown dot -> myosin binding site-tropomyosin covers binding sites-troponin controls tropomyosin-troponin + Ca = shifts tropomyosin (begins contraction)Sarcomeres: Functional Unit of Contraction Elements of Sarcomere Structure 2. Thick Filament ‘myosin head’ has ATPase activity when bound to actin imagine two golf clubs with their handles twisted about each other spans the distance and overlaps thin filaments Z composed of a highly organized array of myosin molecules -highly organized-3D structure-6 thin around each thick-myosin heads stick out for interaction with thin (all 6)-head interacts -cross bridges generatesforce-energy from ATP-ATPase = enzyme which breaks down ATP-energy to change position of myosin head in contraction!! actin and myosin interaction !!(structural)-helps w/ relaxationContractile Proteins 1) Actin • found in thin filaments • has myosin binding sites for crossbridge formation with myosin 2) Myosin • found in thick filaments • has myosin head that binds to the myosin binding site on actin and forms crossbridge during muscle contraction -ATPase activity during cross bridge cyclingRegulatory Proteins 1) Tropomyosin • found in thin filaments • covers the myosin binding sites on actin when muscle is relaxed 2) 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 on actin  exposes the binding sites to allow crossbridge formation -where Ca binds-causes confirmational (shape) change which shifts tropomyosin which exposesbinding sitesSliding Filament Model of Muscle Contraction 1. Myosin ‘heads’ bind to actin to form a ‘crossbridge’ 2. Conformational change, energized by ATP hydrolysis, causes thin filaments to slide along thick filaments 3. Myosin head groups release, form new crossbridges, and the sliding cycle repeats... Result: Z-lines move toward one another Sarcomere length decreases Myofibril shortens Muscle fiber shortens... (stay in place) -grab thin and pullinto sarcomere!!! as long as Ca and ATP aroundAs Thick/Thin filament overlap increases... I band length decreases A band length remains constant H zone length decreases Zone of overlap increases only one that increases(shortens sarcomere, myrofibril, muscle fiber)I band almost disapearsDavidson College Biology 111 Home Page http://www.bio.davidson.edu/misc/movies/musclcp.movMuscle contraction animation Cross-Bridge Cycle animation (captioned): http://www.youtube.com/watch?v=Tuzr5N0TInMCrossbridge Cycling Cycle is entered following exposure of myosin binding sites on the actin thin filament (regulatory role for Ca2+) Prior to entering the contraction cycle – at the end of the last muscle contraction: 1) ATP binds to the myosin head 2) This ATP is hydrolyzed by the unbound head, and the released energy results in a conformational ‘cocking’ of the head group (the ADP and Pi remain bound to the cocked head) http://www.bms.ed.ac.uk starts when Ca is available in muscle - in order to have Ca there, electrical signal must happen in brainfloods inside of muscle fiber with intercellular fluidonce done contracting, resetting of myosin heads happens (to preparefor next contraction) -at end of contractionbroken down into ADP and phosphate-leads to conformational position (ready to bind) adp and phosphate bound to head(at end of each contraction)Crossbridge Cycling myosin headactinThe contraction cycle begins when Ca2+ is released from the SR, binds to troponin and the myosin binding sites on actin are exposed Starting point muscle is ready to contract because myosin heads are ‘cocked’ and binding sites are exposed 4 1 2 3 (-z disc)(-m line)(rigor complex)-energy released from phosphate bondmoves head back to cocked position-causes head tochange positionso it can bindagainhappens repeatedly as long as ATP present(whole processreversible)(imagine like an arm moving filament above)Contraction cycle steps: 1) Myosin binding sites on actin become exposed when Ca2+ binds to troponin 2) Myosin heads bind to actin forming crossbridges 3) Myosin heads pivot toward the center of the sarcomere (power stroke) 4) ATP binds to the myosin head  detachment of myosin head from actin (crossbridges break) 5) ATP is hydrolyzed and the energy released is used to re-energize the myosin head back to its start position (‘cocked’) so a new crossbridge can form 6) The contraction cycle repeats until the myosin binding sites on actin are no longer available -at rest, head in cockedposition ready to bindRigor complex The attached head group after the power stroke is called a rigor complex Rigor mortis – the rigor of death because of lack of ATP to detach the crossbridge 2 -like rigor-mortis (rigor of death) -Ca leaks out of membranes and causes head to attach and power stroke and then they get stuckbecause ATP doesn’t happento detach cross bridge(stays contracted) -4 hours after death up to 24 hours-can cause movement in deadpeopleATP is needed to detach the cross bridge 2 3San Diego State University College of Sciences Biology 590 - Human Physiology Actin Myosin Crossbridge 3D Animation