Molecular and Biochemical Mechanisms of Muscle Contraction скачать в хорошем качестве

Molecular and Biochemical Mechanisms of Muscle Contraction

• Sliding Filament Theory: Muscle contraction occurs as thick filaments (myosin) and thin filaments (actin) slide past one another. This movement causes the Z disks of the sarcomere to approach each other, effectively shortening the muscle fiber. • Myosin as a Molecular Motor: Myosin is a complex protein consisting of two heavy and four light chains. It functions as an ATPase enzyme, catalyzing the hydrolysis of ATP to drive mechanical movement. • The Four-Step Actomyosin Cycle: 1. Dissociation: ATP binds to the myosin head, causing it to release its grip on the actin filament. 2. Activation: ATP is hydrolyzed into ADP and inorganic phosphate (P i ​ ), shifting the myosin head into a "high-energy" conformation and changing its orientation. 3. Rebinding: The myosin head attaches to a new actin subunit farther along the filament, a process strengthened by the release of P i ​ . 4. Power Stroke: The release of P i ​ triggers a major conformational change—the "power stroke"—which pulls the myosin tail toward the Z disk. ADP is then released to complete the cycle. • Force and Displacement: A single cycle generates approximately 3 to 4 piconewtons (pN) of force and moves the thick filament 5 to 10 nanometers (nm) relative to the thin filament. • Calcium-Mediated Regulation: Contraction is triggered by a nerve impulse that releases Ca 2+ from the sarcoplasmic reticulum. The Ca 2+ binds to troponin, causing a conformational change in the tropomyosin-troponin complex that exposes the myosin-binding sites on actin. • Structural Proteins (Titin and Nebulin): Titin is a giant protein that links the Z disk to the M line, acting as a "molecular ruler" to regulate sarcomere length and prevent overextension. Nebulin similarly regulates the length of thin filaments. • Molecular Basis of Rigor Mortis: When a vertebrate dies, ATP levels are depleted. Because ATP is required for myosin to dissociate from actin, the myosin heads remain tightly bound to the thin filaments, causing the permanent stiffness known as rigor mortis. -------------------------------------------------------------------------------- Tags: #Biochemistry #MuscleContraction #Myosin #Actin #Sarcomere #ATPHydrolysis #Troponin #Titin #MolecularMotors #RigorMortis #CalciumSignaling #SlidingFilamentTheory