Molecular Biology of Myosin and ATP

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Questions and Answers

What structure allows the myosin head to bind to actin during muscle contraction?

Myosin tail

ATP binding site

Hinge on myosin (correct)

Actin filament structure

What happens when ATP is hydrolyzed during the crossbridge cycle?

Tropomyosin moves away from the binding sites

Myosin detaches from actin

Energy is transferred to the myosin head (correct)

Energy is released to the actin

Which protein is primarily responsible for covering the myosin-binding sites on actin at rest?

Tropomyosin (correct)

Troponin

Actin

Myosin

What does the term 'power stroke' refer to in muscle contraction?

Movement of the myosin head Signup and view all the answers

Which of the following statements about actin is true?

G-actin is polymerized to form F-actin. Signup and view all the answers

How does the myosin molecule resemble a golf club during its action?

It has a tail and two heads that provide leverage. Signup and view all the answers

Why is ATP essential for muscle contraction?

It provides energy for the myosin head movement. Signup and view all the answers

What role do calcium ions play in muscle function?

They expose myosin-binding sites on actin. Signup and view all the answers

What primarily composes the thick filaments in the sarcomere?

Myosin Signup and view all the answers

During the contraction of a muscle, which structure remains unchanged in length?

A band Signup and view all the answers

Which of the following molecules is NOT directly involved in the sliding filament theory of contraction?

Creatine phosphate Signup and view all the answers

What is the primary role of calcium ions in muscle contraction?

To activate troponin C Signup and view all the answers

How do the T tubules contribute to muscle contraction?

They carry depolarization signals to the interior of the muscle fiber Signup and view all the answers

What happens to the sarcomere during muscle contraction according to the sliding filament theory?

The thin filament slides past the thick filament Signup and view all the answers

Which protein is involved in the structural integrity of the thin filament?

Actin Signup and view all the answers

How does ATP contribute to muscle contraction?

It hydrolyzes to provide energy for myosin movement Signup and view all the answers

What is the primary role of tropomyosin in muscle contraction?

To cover the binding sites on actin when the muscle is unstimulated Signup and view all the answers

Which component of the troponin complex is specifically responsible for binding calcium ions?

Troponin C Signup and view all the answers

Which statement best describes the power stroke in muscle contraction?

The myosin head tilts backward, pulling the thin filament toward the sarcomere center Signup and view all the answers

What is the role of ATP in the process of muscle contraction?

To disconnect the myosin cross bridge from actin Signup and view all the answers

Which statement about the thin filaments in a sarcomere is correct?

Thin filaments consist of actin, tropomyosin, and troponin complex Signup and view all the answers

What triggers the conformational change in the troponin-tropomyosin complex?

Release of calcium ions into the cytosol Signup and view all the answers

What is the result of myosin binding to an exposed binding site on actin?

It marks the beginning of the power stroke Signup and view all the answers

Which of the following statements accurately describes the role of troponin I?

It inhibits muscle contraction by binding to actin Signup and view all the answers

Study Notes

Myosin Molecule

Myosin has a tail and two heads, also known as cross-bridges.
The shape of a myosin molecule resembles a golf spoon with two heads.
The myosin head has the ability to move back and forth, which provides the power stroke for muscle contraction.
The myosin tail has a hinge that allows for vertical movement enabling the cross-bridge to bind to actin.

ATP and Actin Binding Sites

The cross-bridge has two binding sites: one for ATP and one for actin.

Energized Cross-Bridge

ATP is a high energy molecule.
Binding ATP to the myosin molecule transfers energy to the myosin cross-bridge.
Hydrolysis of ATP into ADP and phosphate releases energy, which is transferred to the myosin head.
The myosin head is in a low-energy state before ATP binding and a high-energy state after ATP hydrolysis.

Thin Filaments of Sarcomere

Thin filaments are composed of three protein molecules: actin, tropomyosin, and the troponin complex.
Actin is the primary component of the thin filament.
- Actin subunits are twisted into a double helical chain.
- Each actin subunit has a specific binding site for the myosin cross-bridge.
- Actin is a globular protein in its G-actin form.
- G-actin is polymerized into two strands, forming an alpha-helical structure called F-actin.
- Actin has myosin-binding sites that are covered by tropomyosin when the muscle is at rest, preventing actin and myosin interaction..
Tropomyosin is a regulatory protein that twists around the actin filament.
- In unstimulated muscle, tropomyosin covers the binding sites on actin subunits, preventing myosin cross-bridge binding.
Troponin is a third molecule, called the troponin complex, that is attached periodically to the tropomyosin strand.
- Troponin facilitates the movement of tropomyosin to expose the binding sites on actin for myosin.

Troponin Complex

Troponin is a heterotrimer consisting of:
- Troponin T: binds to a single molecule of tropomyosin.
- Troponin I: binds to actin and inhibits contraction, facilitating the inhibition of myosin binding to actin by tropomyosin.
- Troponin C: binds calcium ions (Ca2+) and promotes the movement of tropomyosin.

Six Steps of Single Cross-Bridge Cycling

Step 1: Exposure of binding sites on actin:
- An action potential in the muscle cell membrane leads to the release of calcium ions from the terminal cisternae.
- Calcium ions bind to troponin, causing a conformational change in the troponin-tropomyosin complex.
- Tropomyosin moves away from the myosin binding sites on actin, exposing them.
Step 2: Binding of myosin to actin:
- Once the binding site on actin is exposed, the energized cross-bridge binds to actin.
Step 3: Power stroke of the cross-bridge:
- ADP and inorganic phosphate (Pi) are released from the myosin head.
- The myosin head tilts backward, pulling the thin filament towards the center of the sarcomere.
Step 4: Disconnecting the cross-bridge:
- An ATP molecule binds to the myosin cross-bridge, disconnecting it from actin.
Step 5: Re-energizing the cross-bridge:
- ATP is hydrolyzed into ADP and phosphate, providing energy for the myosin head to return to its original position.
Step 6: Preparation for the next cycle:
- The cycle repeats as long as calcium ions are present and ATP is available.

Skeletal Muscle Physiology of Contraction

The process of muscle contraction involves three stages:
- Synaptic transmission at the neuromuscular junction: Nerve impulses trigger the release of neurotransmitters at the neuromuscular junction.
- Excitation-contraction coupling: The nerve impulse triggers a series of events that lead to the release of calcium ions from the sarcoplasmic reticulum.
- Contraction-relaxation cycle: Calcium ions bind to troponin, initiating the sliding filament mechanism.

Transverse Tubules

The transverse (T) tubules are an extensive network of muscle cell membrane that invaginates deep into the muscle fiber.
T tubules carry depolarization from action potentials at the muscle cell surface to the interior of the fiber.
T tubules make contact with the terminal cisternae of the sarcoplasmic reticulum and contain a voltage-sensitive protein called the dihydropyridine receptor (DHP).

Calcium Release

Membrane depolarization opens the L-type Ca2+ channel, also known as the DHP receptor.
The DHP receptor is mechanically coupled to the Ca2+ release channel (ryanodine receptor; RyR), causing it to open.
Ca2+ exits the sarcoplasmic reticulum (SR) via the RyR channel and activates troponin C, leading to muscle contraction.

Sarcomere

The sarcomere is the functional unit of a muscle fiber that contains contractile units called myofilaments.
Thick Filaments: primarily composed of myosin.
Thin Filaments: contain actin, troponin, and tropomyosin.

Sliding Filament Theory of Contraction

The Sliding Filament Theory describes how muscle cells contract.
During contraction, the sarcomere shortens, and thin and thick filaments overlap to a greater degree.
The thick and thin filaments do not change length during contraction.
Actin filaments slide over the myosin filaments, moving toward the M line in the center of the sarcomere.
The A band does not change length, but the H zone and I band get shorter during filament overlap.

Molecular Participants of Sliding Theory

The Sliding Filament Theory involves the activities of five different molecules:
- Actin: The thin filament protein that binds to myosin.
- Myosin: The thick filament protein that binds to actin and generates the power stroke.
- Tropomyosin: A regulatory protein that covers the myosin binding sites on actin when the muscle is at rest.
- Troponin: A complex of proteins that regulate the interaction of tropomyosin with actin.
- ATP: The energy source for muscle contraction.
Calcium ions are also involved in the process of muscle contraction.

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Description

Explore the structure and function of myosin molecules, including their role in muscle contraction and interaction with ATP and actin. This quiz covers the key concepts of the myosin cross-bridge cycle and the structure of thin filaments in the sarcomere. Test your knowledge on how energy transfer occurs during muscle movements.