Problem 6 - Entrance Test

During skeletal muscle contraction, which of the following events correctly describes the sequence of molecular interactions following the binding of calcium ions to troponin?

A. Myosin heads bind to actin -> Tropomyosin moves to expose binding sites -> ATP hydrolysis -> Power stroke.B. Tropomyosin moves to expose actin binding sites -> Myosin heads bind to actin -> Power stroke -> ATP binds to myosin, causing detachment.C. ATP hydrolysis on myosin heads -> Myosin heads bind to actin -> Power stroke -> Tropomyosin moves to expose binding sites.D. Myosin heads detach from actin -> ATP binds to myosin -> Tropomyosin moves to expose binding sites -> Power stroke.

Correct: B

The sequence of events in the sliding filament model of muscle contraction, starting from calcium binding: 1. Calcium binds to troponin: When Ca2+ is released from the sarcoplasmic reticulum, it binds to troponin. 2. Tropomyosin moves: This binding causes a conformational change in troponin, which in turn pulls tropomyosin away from the active (myosin-binding) sites on the actin filaments. This exposes the binding sites. 3. Myosin heads bind to actin: With the binding sites exposed, the energized myosin heads (which have already hydrolyzed ATP into ADP and Pi, storing energy) can now form cross-bridges with actin. 4. Power stroke: The release of ADP and Pi from the myosin head triggers the power stroke, where the myosin head pivots and pulls the actin filament towards the M-line (center of the sarcomere). 5. ATP binds to myosin, causing detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin. 6. ATP hydrolysis: The bound ATP is then hydrolyzed to ADP and Pi, re-energizing the myosin head and returning it to its cocked position, ready for another cycle. Let's evaluate the options based on this sequence: A. Incorrect. Tropomyosin moves before myosin heads bind to actin. ATP hydrolysis happens before binding, to energize the head. B. Correct. Tropomyosin moves to expose actin binding sites, then myosin heads bind to actin, then the power stroke occurs, and finally ATP binds to myosin causing detachment. This precisely follows the known mechanism. C. Incorrect. ATP hydrolysis occurs before myosin heads bind to actin (to cock the head). Tropomyosin moves before myosin binding. D. Incorrect. Myosin heads detach after ATP binds, and tropomyosin moves before binding.