Energetics and kinetics of interconversion of two myosin subfragment-1.adenosine 5'-diphosphate complexes as viewed by phosphorus-31 nuclear magnetic resonance.

The 31P NMR spectrum of MgADP bound to myosin subfragment-1 (S-1) at 0 degrees C contains two resolved beta-phosphate resonances corresponding to two interconvertible conformations of the S-1 . ADP complex [Shriver, J. W., & Sykes, B. D. (1981) Biochemistry 20, 2004]. The two conformations, MTADP and MRADP, are in slow exchange on the NMR time scale, and the rates of interconversion are less than 20 s-1. This is consistent with transient kinetic experiments reported in the literature and allows a determination of the rate constants of interconversion: k+ approximately equal to k- approximately equal to 7 s-1 at 0 degrees C. The relative population of the two conformations is highly temperature dependent, and only one form is significantly populated at 25 degrees C. Simulations of the 31P NMR spectra are used to evaluate an equilibrium constant at various temperatures from 0 to 25 degrees C. The standard enthalpy and entropy differences for the R leads to T transition are determined from the variation of the relative free energies of the two states as a function of temperature: delta H degree = 15 (+/- 2) kcal/mol and delta S degree = 55 (+/- 5) cal/(deg mol) (K = 1 at 271 K). This suggests that a significant conformational change occurs in the R leads to T transition with MgADP bound in the active site. However, the entropy and enthalpy differences are nearly compensatory at physiological temperatures. At 25 degrees C the endothermic R leads to T transition is entropy driven, and delta G degree = 1.4 kcal/mol.