Explain the mechanism of action of contractile proteins like actin and myosin in muscle contraction. What structural features enable their function?

Mechanism of Action of Contractile Proteins: Actin and Myosin

Introduction

Muscle contraction is a vital biological process that allows movement and stability in the human body. This process is driven by the interaction of two main contractile proteins: actin and myosin. These proteins work together in a repeating cycle to produce contraction and relaxation in muscle fibers.

Structure of Actin and Myosin

Actin:

• Exists in two forms: G-actin (globular) and F-actin (filamentous).
• G-actin polymerizes to form F-actin, which is a double helical filament.
• Provides the track for myosin movement.

Myosin:

• Thick filament composed of heavy and light chains.
• Each myosin molecule has a head, neck, and tail region.
• The head binds to actin and has ATPase activity that powers movement.

Mechanism of Muscle Contraction (Sliding Filament Theory)

1. Resting State

• Actin binding sites are blocked by tropomyosin in relaxed muscle.
• Calcium ions (Ca²⁺) are stored in the sarcoplasmic reticulum.

2. Calcium Release

• Upon nerve stimulation, Ca²⁺ is released into the cytoplasm.
• Calcium binds to troponin, causing tropomyosin to move and expose actin binding sites.

3. Cross-Bridge Formation

• Myosin head binds to exposed actin site forming a cross-bridge.

4. Power Stroke

• Myosin head pivots, pulling actin filaments inward.
• This shortens the muscle fiber, leading to contraction.

5. Detachment

• ATP binds to myosin head, causing it to detach from actin.

6. Reactivation

• ATP is hydrolyzed into ADP + Pi, re-cocking the myosin head for another cycle.

Structural Features Enabling Function

Actin:

• Helical structure allows flexible binding sites for myosin.
• Polar filament structure ensures unidirectional movement.

Myosin:

• ATPase activity in the head provides energy for movement.
• Flexible neck region allows movement of the head.
• Tail regions align myosin molecules into thick filaments.

Conclusion

Actin and myosin work together in a cycle of attachment, movement, and detachment to generate muscle contraction. Their structural features and biochemical properties are finely tuned for this function. Understanding this mechanism is crucial in medicine, especially in muscle disorders and drug development.