Unformatted text preview:

TEST 4 I Myosins and Muscle contraction a There are over 20 different myosins known in a given type of myosin there may be different isoforms b Muscle organization Skeletal cardiac and smooth muscles Myosin II found in virtually all cells even non muscle cells it interacts with actin that produces contractual force i Striated muscle major focus of this topic 1 Accounts for skeletal muscles and cardiac muscles 2 In order to make muscles cells fuse together to form a syncytium one very large surrounded by one plasma membrane with more than one nuclei syncytium is the cellular equipment that creates a myofiber the basic unit of muscle cells 3 The myofiber cytoplasm is full of the structures that produce contractual force organized into long tubular cytoskeletal structures called myofibrils 4 Each myofibril is an end to end linkage of the contractual unit called the sarcomere sarcomeres line up end to end to form the myofibril this gives the muscle a striped appearance hence striated 5 Muscles organize into a bundle of myofibers 6 Force is produced in individual sarcomeres this force that is produced causes a shortening of the sarcomeres which gives in turn shortens contracts a muscle 7 Sarcomere structure a A cylindrical structure capped by Z discs on each end b Two linear structures extend to the inside of the sarcomere i Actin filaments attached to the Z discs but not attached to one another 1 All oriented the same way attached to the Z disc at their ends 2 Each actin in a given type of muscle is the same length but the actins in different muscles can have different lengths 3 This length is tightly controlled by the Z disc the reason the Z disc is called so is because it acts as a CapZ ceasing polymerization of actin at the end and tropmodulin at the end 4 A long protein called nebulin that extends all the way down an actin filament and it controls the length of the filament 5 Historically these actin filaments are referred to as thin filaments because they are thinner than the second set of structures found in this region ii Myosin filaments 1 Interdigitated between actin filaments are filaments of the motor protein myosin 2 These filaments thick filaments are in the center of the sarcomere and bridge the two actin filaments 3 Myosin filaments do not interact with the Z disc c The structure of sarcomeres creates two distinct regions 1 a region of actin that is composed of myosin filament associated with part of two actin filaments the A Band and 2 two regions of actin associated with Z discs but NOT associated with any myosin filament the I Bands flank the A band and are what contracts in the sarcomere d Z discs are shared by adjacent sarcomeres ii Smooth muscle Found primarily encircling tubular structures of the body esophagus arteries airway passages stomach etc produces contractual forces the same way striated muscles do c Myosin family of motor proteins i All share a very similar head domain composed of an actin filament binding site and an ATPase site that provides the energy for contraction ii One of the major differences between different myosins in the length of the neck region part of the head domain this region binds other proteins the peptides that bind to the neck are referred to as light chains myosin I has three light chains myosin II has two light chains and Myosin V has six light chains Light chains are proteins that are similar to calmodulin and are usually involved in the regulation of myosin activity iii The major distinction between the different classes of myosin is in the tail region of myosin each myosin isoform has a distinct length to their tails that are specific for cell function tail domains in myosin II and V form coiled coil dimer peptides of the tail domain are referred to as heavy chains myosin II and V have two heavy chains that form a dimer tails interact with other myosin II tails to form a bipolar filament composed of head domains near the ends of the filament and a core composed of tail domains In striated muscle there are about 150 myosins on each end of the thick filament the heads of the bipolar filaments interact with an actin filament which is associated with a Z disc see myosin II picture above iv d The sliding filament theory i Proposed by H Huxley ii It was known during the time that contraction of the sarcomere unit causes the Z discs to come closer to one another there was a formal possibility that filaments shorten OR it was possible that filaments slide over one another this latter proved to be true iii There is NO change in filament length during contraction of sarcomeres thin filaments actin slide over thick filaments myosin I bands shorten as result bringing thick filaments closer to the Z discs and thin filaments ends closer to the core of the sarcomere e Myosin crossbridge cycle i Myosin heads are attached to actin filaments at an angle cell ii At this point in the cycle the myosin head domain has nothing in the ATP binding site but ATP is all around the when ATP binds to the nucleotide binding site of the head domain of myosin the association of the head domain with the actin filament is destroyed myosin is attached to the actin in the absence of ATP and dissociates from actin in the presence of ATP iii Hydrolysis of ATP to ADP and Pi occurs the energy gained from this process is transformed into a conformational change in the neck region of the head domain this conformational change cocks the head toward the actin end of the actin filament NOTE the myosin produces force in one direction towards the end of the actin iv The head domain binds back to the actin filament upstream from original position v Loss of Pi from the ATPase site of the head domain of myosin initiates the powerstroke the powerstroke is a process in which the head domain takes its original conformation the conformation it had in the absence of ATP the powerstroke moves actin filament to the left in striated muscle vi The completion of the powerstroke causes the loss of ADP from the ATPase site on the head domain of the myosin filament the cycle continues again when ATP binds into the ATPase site on the head domain of myosin vii Each crossbridge cycle consumes one ATP viii If there is no regulation built into the myosin crossbridge cycle the process will go on continuously and uninhibited and muscles would continually contract in striated muscles the crossbridge cycle is highly regulated f Thin filament regulation of actin myosin force production in


View Full Document

FSU PCB 3134 - Myosins and Muscle contraction

Download Myosins and Muscle contraction
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Myosins and Muscle contraction and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Myosins and Muscle contraction 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?