usually put on (-) end i) myosin i) most actin-based movements involve the motor protein myosin (1) sliding filament model ii) myosin is an ATPase
(1) converts energy from ATP to mechanical energy iii) 17 classes of myosin (1) multiple isoforms In each class (2) all isoforms have similar structure (a) head ATPase activity (b) tail can bind to subcellular components (c) neck regulation of ATPase j) sliding filament model: actin + mysosin i) analogous to pulling yourself along a rope (1) actin – rope (2) myosin – your arm ii) alternating cycle of grasp, pull, and release (1) your hand grasps the rope (2) your muscles contract to pull rope (3) your hands releases, extends, and grabs further along the rope iii) two processes (1) chemical reaction (a) myosin binds to actin (cross-bridge) (2) structural change (a) myosin bends (power stroke) (3) cross-bridge cycle (a) formation of cross-bridge, power stroke, release, and extension (4) need ATP to release and reattach to actin (a) absence of ATP causes rigor mortis (i) myosin cannot release actin iv) two factors affect movement (1) unitary displacement (a) distance myosin steps during each cross-bridge cycle (b) depends on (i) myosin neck length (ii) location of binding sites on actin 1. helical structure of actin (2) duty cycle (a) cross-bridge time/cross-bridge cycle time (i) typically, ~0.5 (only 50% bound) (b) use of multiple myosin dimers to maintain contact, vesicles would be release if only 1 myosin was used. k) Muscle cells (myocytes) i) Myocytes (muscle cells) (1) Contractile cell unique to animals ii) Contractile elements within myocytes (1) Thick filaments (a) Polymers of myosin (b) ~300 myosin II hexamers
(2) thin filaments (a) polymers of alpha actin (b) ends capped by tropomodulin (-) and capZ (+) to stabilize (c) proteins troponin and tropomyosin on outer surface – regulate contractions. (i) Prevent cross-bridges from forming, regulates contractions iii) Two main types of muscle cells are based on the arrangement of actin and myosin (1) Striated (striped) (a) Skeletal and cardiac muscle (b) Actin and myosin arranged in parallel (2) Smooth (a) Actin and myosin are not arranged in any particular way (b) Blood vessels, viscera iv) Striated muscle cell structure (1) Thick and thin filaments arranged into sarcomeres (2) Repeated in parallel and In series (a) Side-by-side across myocyte (i) Causes striated appearance (b) End to end along myocyte (3) Structural features of sarcomeres (a) Z-disk (i) Forms border (end) of each sarcomere (ii) Thin filaments are attacked to Z-disk and extend from it towards the middle of the sarcomere (b) A-band (i) Middle region of sarcomere occupied by thick filaments (c) I-band (i) Located on either side of Z-disk (ii) occupied by thin filament v) Sarcomeres (1) Thin and thick filaments overlap in two regions of each sarcomere (2) Each thick filament is surrounded by 6 filaments (3) Nebulin (a) Determines length of thin filament (b) Along length of thin filament (4) Titin (a) Keeps thick filament centered in sarcomere (b) Attaches thick filament to Z-disk 7)
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- Spring '14