Question

Myosin cross-bridge head detaches from actin active sites when the cross bridge head binds with

• A. Creatine phosphate
• B. A.T.P.
• C. Calcium
• D. Calmodulin

Hint:

Remember that ATP has two distinct roles in muscle contraction:
1. Its binding is required to cause detachment of the cross-bridge.
2. Its hydrolysis is required to provide the energy for the next power stroke.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Skeletal muscle contraction occurs via the sliding filament mechanism, which is driven by the cyclic attachment, rotation, and detachment of myosin cross-bridges interacting with actin filaments. This cycle relies on the binding and hydrolysis of adenosine triphosphate (ATP).

Step 2: Detailed Explanation:
Let us trace the biochemical steps of the cross-bridge cycle:

1. Attachment (Power Stroke): In the presence of Calcium (\(\text{Ca}^{2+}\)), troponin exposes the active binding sites on actin. The myosin head, bound to ADP and inorganic phosphate (\(\text{P}_i\)), binds to actin.
The release of ADP and \(\text{P}_i\) triggers the "power stroke," where the myosin head rotates, sliding the actin filament toward the center of the sarcomere.

2. Rigor State: At the end of the power stroke, the myosin head remains tightly bound to actin in a low-energy state (rigor state).

3. Detachment (The Release Step): To break this actin-myosin bond, a new molecule ofATP must bind to the nucleotide-binding pocket on the myosin cross-bridge head.
The binding of ATP induces a conformational change in the myosin head that dramatically reduces its affinity for actin, causing it to detach from the active site.

4. Cocking: Once detached, myosin hydrolyzes ATP into ADP and \(\text{P}_i\), storing the released energy to "cock" the head back into its high-energy position, ready for another cycle.
If ATP is absent (as occurs after death), the cross-bridges cannot detach, leaving the muscles locked in a rigid state known as rigor mortis.

Step 3: Final Answer:
The correct option is (B), representing A.T.P.