What does Ca2+ bind to during muscle contraction?

During muscle contraction, Ca2+ binds to the troponin complex, which is a crucial step in the regulation of muscle contraction in skeletal and cardiac muscle fibers. The troponin complex is made up of three proteins: troponin C, troponin I, and troponin T. Specifically, calcium ions bind to troponin C. This binding induces a conformational change in the troponin complex, which leads to the movement of tropomyosin, a protein that covers the active sites on actin filaments.

When tropomyosin shifts, it uncovers the binding sites on actin for the myosin heads to attach. This engagement between actin and myosin facilitates the power stroke that ultimately shortens the muscle fiber and results in contraction. Thus, the role of Ca2+ in binding to the troponin complex is essential for initiating and regulating the process of muscle contraction. Understanding this mechanism is critical for grasping how muscle contraction is controlled at a molecular level.