The contraction of skeletal and cardiac muscle (striated muscle) is enabled when calcium ions bind to troponin, which causes a conformational change and pulls the tropomyosin off the myosin-binding sites on the actin filaments. The uncovering of the binding sites allows the myosin heads to bind the actin, forming a cross-bridge. Once ATP hydrolysis occurs, the power stroke needed for a muscle contraction pulls the actin and myosin filaments closer to the M line, shortening the sarcomere. Troponin is a trimeric complex of three proteins (I, T, and C), each with a different function that allows troponin to perform its role relating to muscle contraction.
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Function
Each of the protein subunits has an individualized function related to troponin’s role in muscle contraction. Troponin I (TnI) binds to the actin filament, inhibiting the ATPase activity from the actin-myosin binding.[3] Troponin T (TnT) attaches to tropomyosin, anchoring it to the actin and forming the Tn-tropomyosin complex.[3] Troponin C (TnC) binds to calcium ions, inducing the conformational changes in TnI and uncovering the myosin-binding sites blocked by the tropomyosin.[3] Through this process, cross-bridge cycling occurs so that a power stroke can activate the muscle contraction.
Coinciding with different types of muscle tissue in the body, the troponin subunits have various isoforms. TnI has three different isoforms: cardiac, slow skeletal, and fast skeletal muscle.[4] For the most part, each isoform is found exclusively in its respective muscle tissue (with one exception). During embryonic development, the slow skeletal muscle TnI isoform is expressed in the heart; however, following birth, that isoform is replaced by cardiac TnI.[4] Within the heart, the troponin complex controls cardiac output through its involuntary regulation of muscle contraction. Specifically, the diastolic relaxation and systolic contraction in the myocardium of the heart are controlled by the cardiac troponin complex and the interaction with Ca2+, which modulates the cardiac stroke volume.[5] When the heart increases the end-diastolic volume, the stroke volume also increases, meaning that more blood is ejected from the heart with every contraction. The increase in stroke volume is done by following the Frank-Starling law, which states that an increase in sarcomere length enhances the contractile force of the myocyte.[5]
Disease
Relevance
Structural highlights
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