Contractile Proteins and the Molecular Mechanism of Muscle Contraction

Contractile Proteins and Muscle Contraction

Introduction to Contractile Proteins

Contractile proteins are essential for biological movement, including muscle contraction, organelle transport, and cellular motility. The primary contractile proteins in muscle are myosin and actin, which interact to convert chemical energy from ATP hydrolysis into mechanical work.

Key Concepts in Muscle Contraction

Overview of Muscle Contraction

• Biological movement requires the input of free energy, typically from ATP, to drive protein conformational changes that result in directional movement.

• Muscle contraction is mediated by two major proteins: myosin (thick filaments) and actin (thin filaments).

• The process is driven by the free energy of ATP hydrolysis, which is coupled to conformational changes in myosin and actin.

• These conformational changes alter the affinity of myosin for actin and ATP in an alternating fashion, enabling the "sliding filament mechanism." In this model, myosin thick filaments slide along actin thin filaments, shortening the muscle fiber.

• Contraction is initiated by a nerve impulse that triggers the release of Ca2+ from the sarcoplasmic reticulum into the cytosol of the muscle fiber.

Structure of Myosin

• Myosin is composed of two heavy chains and four light chains.

• Each heavy chain has a long, fibrous domain (amphipathic α-helix) for dimerization and a globular domain (the "S1 head") for actin binding and ATP hydrolysis.

• Conformational changes in the myosin head are coupled to stages in the ATP binding and hydrolysis cycle, causing myosin to bind and release successive G-actin monomers, resulting in a "ratcheting" motion along the actin filament.

Structure of Actin

• F-actin (filamentous actin) is a polymer of G-actin (globular actin) monomers arranged in twisted double strands.

• Each G-actin monomer contains a binding site for the myosin head.

Regulatory Proteins: Tropomyosin and Troponin Complex

• Tropomyosin is a coiled-coil homodimer that binds to F-actin, blocking myosin binding sites.

• The troponin complex consists of three subunits:

  • TnI: Inhibits myosin-actin interaction

  • TnT: Binds tropomyosin

  • TnC: Binds Ca2+ and acts as the calcium sensor

• Binding of Ca2+ to TnC triggers a conformational change in the troponin-tropomyosin complex, moving tropomyosin and exposing myosin binding sites on actin.

Role of Calcium in Muscle Contraction

• Upon nerve stimulation, Ca2+ is released from the sarcoplasmic reticulum (SR) into the cytosol via Ca2+ channels.

• Ca2+ binds to TnC, initiating the conformational change that allows myosin to bind actin and trigger contraction.

• When the nerve impulse ceases, Ca2+ is pumped back into the SR by an ATP-driven Ca2+ pump, allowing muscle relaxation.

Learning Objectives

• Describe the sliding filament model of muscle contraction.

• Identify the structures of thick and thin filaments within the sarcomere and their main protein components (actin, myosin, tropomyosin, troponin complex).

• Relate the structure/domains of myosin to its function.

• Explain the mechanism of free energy coupling in muscle contraction, including the roles of actin, myosin conformational changes, ATPase activity, ligand binding/dissociation, and the power stroke.

• Describe the roles of calcium, the troponin complex, and tropomyosin in muscle contraction, and identify the Ca2+ sensor.

• Explain how Ca2+ is released from and returned to the sarcoplasmic reticulum during contraction and relaxation.

Protein-Based Molecular Motors

General Features

• Protein-based molecular motors, such as myosin and kinesin, require chemical energy (usually from ATP) to drive conformational changes that produce directional force.

• These motors move along specific "tracks" (e.g., actin filaments for myosin, microtubules for kinesin) to guide their motion.

• Examples include:

  • Muscle myosin moving along F-actin

  • Kinesins transporting proteins, organelles, and vesicles along microtubules

  • DNA helicases moving along DNA and RNA tracks

• Motor proteins cycle between forms with high or low affinity for their tracks in response to ATP binding and hydrolysis, following a "bind, pull, and release" mechanism.

Structure of Skeletal Muscle

Organization

• Skeletal muscle is composed of bundles of muscle fibers (cells), each containing many myofibrils.

• Myofibrils are aligned, cylindrical bundles made primarily of myosin and actin filaments, along with accessory proteins.

• Muscle fibers are multinucleated and contain many mitochondria for aerobic ATP production.

Sarcomere Structure

• The sarcomere is the basic structural and functional unit of muscle contraction, defined as the region between two Z-disks.

• The sarcoplasmic reticulum is a specialized endoplasmic reticulum in muscle cells, storing high concentrations of Ca2+.

• Key regions:

  • I band: Contains thin filaments (mainly F-actin)

  • A band: Contains thick filaments (mainly myosin)

  • Z disk: Anchors thin filaments

  • M line: Center of the A band

• During contraction, thin and thick filaments slide past each other (sliding filament mechanism).

Molecular Mechanism of Muscle Contraction

ATP-Driven Cycle

1. ATP Binding: ATP binds to the myosin head, causing dissociation from actin.

2. ATP Hydrolysis: ATP is hydrolyzed to ADP and Pi, causing a conformational change that "cocks" the myosin head.

3. Weak Binding: Myosin-ADP-Pi binds weakly to a new position on the actin filament.

4. Power Stroke: Release of Pi triggers the power stroke, moving the actin filament relative to myosin. ADP is released, and myosin returns to its original conformation.

Equation for ATP Hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{HPO}_4^{2-} + \text{H}^+$

Structural Changes in Myosin

• The myosin head undergoes significant conformational changes during the ATPase cycle, particularly in the lever arm region, which rotates to produce the power stroke.

• Relay helices and switch regions in the myosin head adjust their conformation depending on whether ATP, ADP, or Pi is bound.

Coordination of Myosin Heads

• Each thick filament contains hundreds of myosin heads, which operate asynchronously.

• At any given time, about 3% of myosin heads are attached to actin, ensuring continuous tension and preventing backward sliding.

• Flexible regions in the myosin tail (S2 region) allow for segmental flexibility and asynchronous movement.

Regulation by Calcium, Troponin, and Tropomyosin

Initiation and Termination of Contraction

• A nerve impulse triggers the release of Ca2+ from the sarcoplasmic reticulum into the cytosol.

• Ca2+ binds to TnC, causing a conformational change in the troponin-tropomyosin complex, exposing myosin binding sites on actin.

• When stimulation ends, Ca2+ is actively transported back into the SR by ATP-driven Ca2+ pumps, leading to muscle relaxation.

Summary Table: Key Proteins in Muscle Contraction

Protein	Structure	Function
Myosin	2 heavy chains, 4 light chains; S1 head	ATPase activity, binds actin, generates force
Actin	G-actin monomers form F-actin filaments	Track for myosin movement
Tropomyosin	Coiled-coil dimer	Blocks myosin binding sites on actin
Troponin Complex	TnI, TnT, TnC subunits	Regulates tropomyosin position; TnC binds Ca2+

Example: Rigor Mortis

After death, ATP production ceases. Without ATP, myosin heads cannot dissociate from actin, resulting in a permanent cross-bridge and muscle stiffness known as rigor mortis.

Additional info:

• Myosin light chains (ELC and RLC) are homologous to calmodulin and may stabilize the myosin head structure.

• Accessory proteins and mitochondria are abundant in muscle fibers to support contraction and energy needs.