Lecture 13 – Muscle Physiology I

Titin goes from one myosin to the z disc then to the

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Titin goes from one myosin to the z disc then to the other myosin Actin Chain Troponyosin is a long thin chain that wraps around actin beads Myosin Complex protein Has a long tail  ATP binding site and an actin binding site Two heavy chains and four light chains Essential and regularoet light chains Two on the motor domains Head of the myosin molecule has two binding sites Myosin ATP ase site  Also splits ATP into ADP and P
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Myosin has heads called cross-bridges Aligned in oppositre direction When they bind to actin, they change configuration and pull actin molecules from  opposite ends towards each other ATPase site overlap with thin filaments Usually configured with troponin/tropomyosin complex so that cross bridges cannot form Opposite aligned myosin heads will drag actin molecules together when the muscle  contracts The z discs are being pulled closer together Cross Bridge Cycle Four steps 1. Attachment of the cross bridge to a thin filament formation of a cross bridge when the calcium level in the cytosol rises between the myosin binding sites and the actin binding sites this is an energized cross bridge with a specific configuration has ADP and P bound to it OVERVIEW: Actin binding – in a resting muscle fiber, the calcium concentration is  low and the myosin cross bridges cannot bind to actin. This is because TROP  complex is covering the actin molecules. The cross bridges are in an energized  state produced by splitting ATP and the hydrolysis products (ADP + P are still  bound to myosin.) Cross-bridge formation is initiated when Ca enters the 
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