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Lecture 8

BIO270H1 Lecture Notes - Lecture 8: Duty Cycle, Myocyte, Hydrolysis

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Chris Garside

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Lecture 8 (November 11, 2015): Movement and Muscle Physiology
Myosin Continued
majority of movement that involves actin polymers uses myosin
most studied myosin
omyosin I and V
involved in transport
omyosin II
mainly muscle specific
hexamer of 2 heavy chains
myosin heads have ATPase and the actin binding site
myosin tails interact with other tails
myosin neck region is responsible for regulation of ATPase
Sliding Filament Model
actin = rope
myosin = arm
hand extends, grasps the rope, pulls, and releases
the direction of movement depends on which element is immobile
oif the rope is attached to the wall, you move in the direction of the wall
troponin, ATP, ADP + Pi, Ca2+, and Mg2+ are needed for this model
myosin takes chemical energy and turns it into a mechanical change
main processes
ochemical reaction: myosin binding to actin
creation of the cross-bridge
ostructural change: myosin bends
the power stroke
this process is a cycle
myosin is directed towards the + end
sliding filament model:
omyosin is bound to actin, no ATP is in the binding site, it's still polarized, cross-
bridge is still formed
oATP comes and reduces the affinity of myosin to actin, causing myosin to detach
odetachment triggers the hydrolysis of ATP, but the ADP + Pi still remains with the
binding site, which energizes the myosin, which allows for extensions and the
attachment of myosin to actin, creating the cross-bridge
othe release of Pi stimulates the power stroke which causes actin to move
othen ADP is released and cycle starts again with myosin being bound to actin in
the absence of ATP
no ATP = rigor mortis = myosin remains bound to actin
without ATP, myosin cannot release actin
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factors affecting movement
ounitary displacement
distance myosin extends during the cross-bridge cycle
dependent on structure of myosin, neck length, and location of the
binding site on actin
actin is helical
1 turn = 36 nm
myosin V (dimer) steps 36 nm every time
oduty cycle
proportion of time that myosin is actually bound to actin
around 0.5
myosin I is a monomer, so it's harder to grab the next point, so it uses
other monomers to keep a itself on the cytoskeletal tract, so it won't fall
Muscle Types
vesicle transport, microvilli, amoeboid movement all use actin/myosin function
oall are in eukaryotes
two main types of muscle are distinguished by arrangement of actin and myosin
skeletal and cardiac
actin and myosin are arranged in parallel
not striped
actin and myosin are less organized
skeletal muscle is the only muscle that is voluntary
striated muscle
othick filament are made of myosin II
polymers of myosin
tails are all webbed together
othin filaments
polymers are made of alpha actin
microfilaments have beta actin so they assemble and disassemble
to maintain the length of thin filaments, the ends are capped by
tropomodulin (- end) and CapZ (+ end)
oassociated proteins mediate interactions
bind Ca2+
dimer that covers 7 actin monomers, covering myosin binding sites
on actin
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