MyosinMyosin

• • Myosin is a protein molecule Myosin is a protein molecule found in the thick filaments.found in the thick filaments.

MyosinMyosin

►• • Myosin has a tail and two heads Myosin has a tail and two heads (called cross bridges) which will move (called cross bridges) which will move back and forth, providing the power back and forth, providing the power stroke for muscle contraction.stroke for muscle contraction.

Myosin Molecule with Myosin Molecule with Hinged Head and TailHinged Head and Tail

►• • The tail of myosin has a hinge which The tail of myosin has a hinge which allows vertical movement so that the allows vertical movement so that the cross bridge can bind to actin.cross bridge can bind to actin.

Myosin ATP Binding SiteMyosin ATP Binding Site

►• • The cross bridge (head) of myosin The cross bridge (head) of myosin has a binding site for  ATP.has a binding site for  ATP.

►• • Myosin is in its low energy Myosin is in its low energy conformation when the cross bridge is conformation when the cross bridge is in this position:in this position:

Energized Cross BridgeEnergized Cross Bridge

►• • ATP is a molecule with a high ATP is a molecule with a high chemical energy.  ATP binds to myosin chemical energy.  ATP binds to myosin heads when they are tilted back in heads when they are tilted back in their low energy position.  When ATP is their low energy position.  When ATP is hydrolyzed into ADP and phosphate, hydrolyzed into ADP and phosphate, the energy is released and transferred the energy is released and transferred to the myosin head.to the myosin head.

Energized Cross BridgeEnergized Cross Bridge

► • • Note that the ATP is glowing yellow Note that the ATP is glowing yellow indicating that it's in a high energy indicating that it's in a high energy state.  After the ATP has been hydrolyzed to state.  After the ATP has been hydrolyzed to ADP and phosphate, the energy is ADP and phosphate, the energy is transferred to the myosin head.  Now the transferred to the myosin head.  Now the head is glowing to show that it's high head is glowing to show that it's high energy.  When the myosin head is pointing energy.  When the myosin head is pointing up, it is in a high energy state.up, it is in a high energy state.

4 steps for myosin 4 steps for myosin functioningfunctioning

Actin Binding Site on Actin Binding Site on MyosinMyosin

►• • There are two binding sites on each There are two binding sites on each myosin head, one for ATP and one for myosin head, one for ATP and one for actin.actin.

Thin Filaments of the Thin Filaments of the SarcomereSarcomere

►• • Thin filaments are made of these Thin filaments are made of these three protein molecules:three protein molecules:

►1. actin                                1. actin                                ►2.tropomyosin  2.tropomyosin  ►3. troponin3. troponin

TropomyosinTropomyosin

►• • The regulatory protein, tropomyosin, The regulatory protein, tropomyosin, is also part of the thin is also part of the thin filament.  Tropomyosin twists around filament.  Tropomyosin twists around the actin.  When the sarcomere is not the actin.  When the sarcomere is not shortening, the position of the shortening, the position of the tropomyosin covers the binding sites tropomyosin covers the binding sites on the actin subunits and prevents on the actin subunits and prevents myosin cross bridge binding.myosin cross bridge binding.

TroponinTroponin

►• • Troponin, which is found periodically Troponin, which is found periodically along the tropomyosin strand, along the tropomyosin strand, functions to move the tropomyosin functions to move the tropomyosin aside, exposing the myosin binding aside, exposing the myosin binding sites.sites.

Calcium IonsCalcium Ions

► • • Role of Calcium in Muscle Contraction:Role of Calcium in Muscle Contraction:

► Action potential occurs.Action potential occurs.↓↓

► Calcium ions are released from the terminal Calcium ions are released from the terminal cisternae from the Sarcoplasmic Reticulum (SR).cisternae from the Sarcoplasmic Reticulum (SR).

↓↓► Calcium ions then bind to troponin.Calcium ions then bind to troponin.

↓↓► Tropomyosin moves away from the myosin binding Tropomyosin moves away from the myosin binding

sites on actin.sites on actin.

SARCOMERE ARRANGEMENTSARCOMERE ARRANGEMENT

Sequence of Events in a Single Sequence of Events in a Single Cross Bridge Cycle Cross Bridge Cycle

► 1. An influx of calcium, triggering the exposure of 1. An influx of calcium, triggering the exposure of binding sites on actin.binding sites on actin.

► 2. The binding of myosin to actin.2. The binding of myosin to actin.► 3. The power stroke of the cross bridge that causes 3. The power stroke of the cross bridge that causes

the sliding of the thin filaments.the sliding of the thin filaments.► 4. The binding of ATP to the cross bridge, which 4. The binding of ATP to the cross bridge, which

results in the cross bridge disconnecting from actin.results in the cross bridge disconnecting from actin.► 5. The hydrolysis of ATP, which leads to the re-5. The hydrolysis of ATP, which leads to the re-

energizing and repositioning of the cross bridge.energizing and repositioning of the cross bridge.► 6. The transport of calcium ions back into the 6. The transport of calcium ions back into the

sarcoplasmic reticulum.sarcoplasmic reticulum.

Summary of the Role of ATP Summary of the Role of ATP

►1. ATP transfers its energy to the 1. ATP transfers its energy to the myosin cross bridge, which in turn myosin cross bridge, which in turn energizes the power stroke.energizes the power stroke.

►2. ATP disconnects the myosin cross 2. ATP disconnects the myosin cross bridge from the binding site on actin.bridge from the binding site on actin.

►3. ATP fuels the pump that actively 3. ATP fuels the pump that actively transports calcium ions back into the transports calcium ions back into the sarcoplasmic reticulum.sarcoplasmic reticulum.

Sequence of Events in a Single Sequence of Events in a Single Cross Bridge Cycle Cross Bridge Cycle

► 1. An influx of calcium, triggering the exposure of 1. An influx of calcium, triggering the exposure of binding sites on actin.binding sites on actin.

► 2. The binding of myosin to actin.2. The binding of myosin to actin.► 3. The power stroke of the cross bridge that causes 3. The power stroke of the cross bridge that causes

the sliding of the thin filaments.the sliding of the thin filaments.► 4. The binding of ATP to the cross bridge, which 4. The binding of ATP to the cross bridge, which

results in the cross bridge disconnecting from actin.results in the cross bridge disconnecting from actin.► 5. The hydrolysis of ATP, which leads to the re-5. The hydrolysis of ATP, which leads to the re-

energizing and repositioning of the cross bridge.energizing and repositioning of the cross bridge.► 6. The transport of calcium ions back into the 6. The transport of calcium ions back into the

sarcoplasmic reticulum.sarcoplasmic reticulum.