Practice question

Massive protein titin, encoded by TTN gene 363 exons, 38,138 residues in canonical cardiac isoform N2B 3.8 megadalton, spans half sarcomere from Z disc to M line, single molecule. N terminal Z disc anchor binds alpha actinin via Z repeats 1 to 7 and telethonin T cap forming antiparallel complex, I band extensible region contains tandem immunoglobulin like domains Ig 80 repeats that unfold at low force providing entropic elasticity, PEVK region rich in proline glutamate valine lysine behaving as worm like chain, and cardiac specific N2B element with spring properties. A band region with FN3 and Ig super repeats binds myosin thick filament and myosin binding protein C regulating assembly spacing, C terminal M line segment interacts with myomesin maintaining thick filament centrality. Passive tension generated upon stretch restores resting length, contributes to diastolic filling and Frank Starling length dependent activation. Titin does not cap actin, not motor hydrolyzing ATP, not activate myosin, function elastic scaffold binding myosin and providing reversible extensibility central to muscle mechanics and sarcomere stability.