Download L 9 Myosin

Cytoskeltal Motors
Network of long protein strands located in the cytosol not
surrounded by membranes
Consist of microtubules and microfilaments
Microfilaments
protein threads of actin
cell movement and muscle contraction
Microtubules : help in movement of organelle around
made up of tubulin
longest strands of cytoskeleton
make up spindle fibers (role in mitosis and meiosis)
Cytoskeletal motors
Motor proteins utilizing the cytoskeleton for
movement fall into two categories based on
their substrates:
•Actin motors such as myosin move
long microfilaments through interaction
with actin.
•Microtubule motors such as dynein and
kinesin move along microtubules through
interaction with tubulin.
Cytoskeletal motors
– Myosin is responsible for muscle contraction
– Kinesin moves cargo inside cells away from the nucleus
along microtubules track.
– Dynein transports cargo along microtubules towards the
cell nucleus.
MYOSIN MOTORS
Structure of a Muscle Cell
• Vertebrate muscle that is under voluntary control
has a banded (striated) appearance when
examined under a microscope.
• It consists of multinucleated cells that are
bounded by plasma membrane.
• A muscle cell contains many parallel myofibrils,
each about 1 mm in diameter.
• The functional unit, called a sarcomere, typically
repeats every 2.3 mm (23,000 Å) along the fibril
axis in relaxed muscle
Structure of Muscle Cell
Structure of a Muscle Cell
Skeletal muscle myofibril showing a
single sarcomere
• A dark A band and a light I band alternate regularly.
The central region of the A band, termed the H zone,
is less dense that the rest of the band. The I band is
bisected by a very dense, narrow Z line.
Sarcomere
• A sarcomere of a myofibril consists of two
kinds of interacting protein filaments.
• The thick filaments have diameters of about
15 nm (150 Å) and consist primarily of myosin.
• The thin filaments have diameters of
approximately 9 nm (90 Å) and consist of actin
as well as tropomyosin and the troponin
complex.
Skeletal muscle myofibril showing a
single sarcomere
Sliding-Filament Model
• Muscle contraction is achieved through the
sliding of the thin filaments along the length
of the thick filaments, driven by the hydrolysis
of ATP .
Sliding-Filament Model
• Tropomyosin and the troponin complex regulate
this sliding in response to nerve impulses.
• Under resting conditions, tropomyosin blocks the
intimate interaction between mysosin and actin.
• A nerve impulse leads to an increase in calcium
ion concentration within the muscle cell.
• A component of the troponin complex senses the
increase in calcium and, in response, relieves the
inhibition of myosin - actin interactions by
tropomyosin.
Thick Filament: Myosin head domains at each end
and a relatively narrow central region
Interaction of thick and thin filaments in skeletalmuscle contraction
Actin Is a Polar, Dynamic Polymer
• Actin
monomers
(often
called G-actin for globular)
come together to form actin
filaments (F-actin filament).
• The structure is polar, with
discernibly different ends.
One end is called the barbed
(plus) end, and the other is
called the pointed (minus)
end.
Myosin Power Stroke
• Individual myosin heads bind the actin
filament and undergo a conformational
change (the power stroke) that pulls the actin
filament.
• After a period of time, the myosin head
releases the actin, which then snaps back into
place.
• The complete cycle of ATP-binding,
hydrolysis, and phosphate release is called
the "power stroke" cycle
• All myosins are composed of a globular catalytic
head domain, a converter and a lever arm.
• Conformational changes in the catalytic head are
amplified by the lever arm during the ATPase
cycle through an ~ 70° rotation of the lever arm.
Myosin motors are plus end motors
The 'powerstroke' cycle of (+)-end-directed myosins.
(Myosin II, Myosin V motor)
Step 1: At the end of the previous round of movement and the start of the next
cycle, the myosin head lacks a bound ATP and it is attached to the actin filament
in a very short-lived conformation known as the 'rigor conformation'.
Step 2: ATP-binding to the myosin head domain induces a small conformational
shift in the actin-binding site that reduces its affinity for actin and causes the
myosin
head
to
release
the
actin
filament.
Step 3: ATP-binding also causes a large conformational shift in the 'lever arm' of
myosin that 'cocks' the head into a position further along the filament. ATP is
then hydrolysed, but the inorganic phosphate and ADP remain bound to myosin.
Step 4: The myosin head makes weak contact with the actin filament and a slight
conformational change occurs on myosin that promotes the release of inorganic
phosphate.
Step 5: The release of inorganic phosphate reinforces the binding interaction
between myosin and actin and subsequently triggers the 'power stroke'.
Step 6: As myosin regains its original conformation, the ADP is released, but the
myosin head remains tightly bound to the filament at a new position from where
it started, thereby bringing the cycle back to the beginning.
The complete cycle of ATP-binding, hydrolysis, and phosphate release is called
the "power stroke" cycle
Refer to following link for
animation:
http://highered.mcgrawhill.com/sites/0072495855/student_
view0/chapter10/animation__breakd
own_of_atp_and_crossbridge_movement_during_muscle_c
ontraction.html