How do proteins actin and myosin interact to make a muscle contract?

The interaction between the proteins actin and myosin is crucial for muscle contraction. Here's an overview of how actin and myosin work together to bring about muscle contraction:

1. Actin and Myosin Structure:

Actin filaments are thin, thread-like protein structures present in large numbers within muscle cells. Myosin molecules, on the other hand, are thick, elongated proteins with globular heads. Each myosin molecule has two heads that can bind to actin filaments.

2. Formation of Cross-Bridges:

During muscle contraction, myosin heads extend and bind to specific sites on the actin filaments, forming cross-bridges. These cross-bridges are essential for generating the force needed for muscle contraction.

3. ATP Hydrolysis:

Myosin heads contain binding sites for adenosine triphosphate (ATP), the energy currency of cells. When ATP binds to myosin, it undergoes hydrolysis, breaking down into adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis reaction releases energy that drives the conformational changes in the myosin head.

4. Power Stroke:

The energy released from ATP hydrolysis causes a conformational change in the myosin head, known as the power stroke. This power stroke pulls the actin filaments toward the center of the sarcomere, the basic unit of muscle contraction. As a result, the thin actin filaments slide past the thick myosin filaments, causing the muscle fiber to shorten and generate force.

5. Calcium Regulation:

The interaction between actin and myosin is regulated by calcium ions (Ca2+). When a muscle fiber receives a signal from the nervous system, it triggers the release of Ca2+ from the sarcoplasmic reticulum, the cell's internal calcium store. Ca2+ binds to a protein called troponin, which is associated with the actin filaments.

6. Tropomyosin and Troponin:

In the absence of Ca2+, another protein called tropomyosin blocks the binding sites on actin filaments, preventing the formation of cross-bridges. However, when Ca2+ binds to troponin, it causes a conformational change that moves tropomyosin away from the binding sites, allowing myosin heads to bind to actin and initiate contraction.

The coordinated interplay of actin and myosin, regulated by ATP and calcium ions, enables muscle fibers to undergo contraction and relaxation, resulting in movement and various physiological functions in the body.