Kinetic mechanism of Ca²⁺-controlled changes of skeletal troponin I in psoas myofibrils.

Conformational changes in the skeletal troponin complex (sTn) induced by rapidly increasing or decreasing the [Ca(2+)] were probed by 5-iodoacetamidofluorescein covalently bound to Cys-133 of skeletal troponin I (sTnI). Kinetics of conformational changes was determined for the isolated complex and after incorporating the complex into rabbit psoas myofibrils. Isolated and incorporated sTn exhibited biphasic Ca(2+)-activation kinetics. Whereas the fast phase (k(obs)∼1000 s(-1)) is only observed in this study, where kinetics were induced by Ca(2+), the slower phase resembles the monophasic kinetics of sTnI switching observed in another study (Brenner and Chalovich. 1999. Biophys. J. 77:2692-2708) that investigated the sTnI switching induced by releasing the feedback of force-generating cross-bridges on thin filament activation. Therefore, the slower conformational change likely reflects the sTnI switch that regulates force development. Modeling reveals that the fast conformational change can occur after the first Ca(2+) ion binds to skeletal troponin C (sTnC), whereas the slower change requires Ca(2+) binding to both regulatory sites of sTnC. Incorporating sTn into myofibrils increased the off-rate and lowered the Ca(2+) sensitivity of sTnI switching. Comparison of switch-off kinetics with myofibril force relaxation kinetics measured in a mechanical setup indicates that sTnI switching might limit the rate of fast skeletal muscle relaxation.