23 – Myosin & Muscle Flashcards
(21 cards)
Different actin networks = exist at front & back of cell
leading edge: branch actin networks nucleated by ARP 2-3 complex, profilin, cofillin, capping protein, etc
back of the cell: stress fibers made of contractile bundles of actin and non muscle myosin II
Extracellular signals =create
zones of different Rho activity
-Back: Rho activation
–Leading to myosin II activation
–Contraction of myosin II filament in both stress fibers & cell cortex
-Front: Rac activation – Cdc42 activation
–Leading to Arp2/3 activation
–Actin filament assembly & treadmilling in leading edge
Myosin
protein that produces force!
-Actin binding site – nucleotide binding site – lever arm
-Motor proteins convert chemical energy (ATP) into mechanical work
-Lever arm= Release of phosphate = large conformation changes = actin moves = force
Myosin II & Myosin-II bundles
Myosin II = form bundles that pull actin INWARD
-2 heads on each side of myosin II both pull inward -CONTRACT
Myosin-II bundles = contract actin arrays
-Muscle=relaxed to contracted
-Long skinny cell to short fat cell
- Muscles= made of
billions of contractile bundles
Muscles = bundle of muscle fibers = multinucleated muscle cell (nuclei) = Myofibril (Sarcomere)
Sarcomere
basic unit of muscle contraction
Contain Z disk on its both & filled with individual actin filament
= basic unit of sliding filament theory
-A band (MYOSIN) stay the same size
whether stretched or contracted
-I band decrease in size
-When it goes from stretched 120% to 90% contracted to 60% contracted
–Gaps between myosin decreases
Sarcomeres = tightly packed
= tightly packed arrays of actin filaments & myosin filaments
o Thick filaments = myosin
o Thin filament = actin
Sarcomeres undergo
rapid contracting upon muscle stimulation
Contraction: time required
FAST: only 50ms for type-II fibre
5-10ms = latent period between signal from & start of contraction
-Trigger muscle to contract
40-45ms = contraction phase – from fully elongated to fully contracted
-Crossbridge cycle
what is Crossbridge Cycle
couples 1 ATP hydrolysis to 1 power stroke
This is what happens in the 40-45ms
The sequences of biochemical events that convert signal from brain to actual contraction event
Crossbridge Cycle - step by step
- Binds ATP, head released from actin
-Myosin in low affinity to actin - ATP hydrolysis puts myosin into strained conformation – reversible reaction
-Hydrolysis of ATP to ADP + Pi
-Myosin head rotates to cocked state = ready to fire - Myosin binds actin filament in the ADP-Pi state
-Myosin head binds actin filament
Step 2-3 = to bind & power stroke in only 1 ms
-Little time in attached state to allow much myosin to do same work
- Phosphate release relaxes strain in myosin head
-Power stroke: release of P & elastic energy straightens myosin
-Moves actin filament left
-Release of free energy = force: moves actin filament ot left - New ATP binding – releases myosin from actin filament
-ADP released – ATP bound; head released from actin
All 5 steps = 10 ms
reason they spend so
little time in the attached state.
in order to have hundreds of thousands of myosins able to work all at the same time, each one can only grab and kick for a very short.
Then it needs to get off and get out of the way so that the other myosins have their chance to bind, exert their powerstroke and also then detach.
Rigor Mortis, how does it happen?
what happens to Muscles after death = frozen
= Failure of myosin to detach in the absence of ATP cause Rigor Mortis
-Muscle cell run out of ATP to release myosin from actin
-all of the actin filaments and the myosin filaments in muscle become stuck together in the absence of ATP
what happens:
in the nucleotide Free State, myosin head stays attached to the actin filament. (the binding of new ATP that causes the myosin head to release)
when you die, oxygen stops going in&the e- transport chain shuts down, ATP synthesis comes to a halt and muscle cells run out of ATP.
when the myosin heads become attached to the actin filament over time after death, no longer have ATP to release the myosins from actin filaments = myosins become irreversibly bound because there’s no ATP to release them = state of rigor mortis.
Lever arm
Neck domain of myosin – act as lever arm
Velocity:
-Longer lever arm = faster
-Longer = more # of repeat of domain
Magnitudes of cellular forces:
myosin & muscle myosin
Myosin produces ~5 pN of force – over a step size of ~10nm
-~60% efficient at converting energy of ATP into mechanical work
Muscle myosin produces ~50 pN nm of work
-Per ATP hydrolysis event
3 design challenges for muscles
- Prevent continuous contraction
-Nothing stops myosin hydrolysis cycle - Activate contraction
- Freeze the structure of sarcomere
-How to create & hold steady
Solutions to the 3 design challenges:
1. Prevent continuous contraction
Structure of sarcomeres – set by:
i. Nebulin
ii. Titin
iii. Capping proteins
actin filaments are wrapped around with a protein:
-Tropomyosin – blocks binding site for myosin on actin filament
So, myosin isn’t continuously contracting
Solutions to the 3 design challenges:
2. Activate contraction
Motor Neurons transmit signals from brain = activate muscle contraction
-Motor neurons stand on top of muscle fiber
-Stimulation by motor neurons = rapid spike in Ca2+ in muscle fibers
-Sarcoplasmic reticulum = calcium storing organelle
-Muscle contraction = stimulated by presence of calcium
i. Need to convert release of calcium by sarcoplasmic reticulum into changes in structure of thin filaments
Troponin
-calcium sensor
Troponin binds calcium & pulls tropomyosin out of the way & reveal myosin binding site (>10ms)
(convert release of calcium by sarcoplasmic reticulum into changes in structure of thin filaments)
What happens in the 10ms
- Brain signal
- action potential
- Motor neurons
- Hit synapse
- Acetylcholine
- Depolarize motor neuron
- Sarcoplasmic reticulum release Ca2+
- Ca2+ bind to troponin
- Pull tropomyosin out
- Bind to binding site
- Power stroke
Muscle contraction can
generate massive force & refined movement
