Vanadate(Vi) and ADP induced domain motions in myosin head by DSC and EPR

Thermal stability and internal dynamics of myosin head in psoas muscle fibres of rabbit in the intermediate state AM.ADP.P-i-mimicked by AM.ADP.V-i-of the ATP hydrolysis cycle was studied by differential scanning calorimetry and spin label electron paramagnetic resonance spectroscopy. Three overlapping endotherms were detected in rigor, in strongly binding ADP and weakly binding AM.ADP.V-i state of myosin to actin. The transition at 54.0degreesC can be assigned to the 50 k actin-binding domain. The transition at highest temperature (67.3degreesC) represents the unfolding of actin and the contributions arising from the nucleotide-myosin head interaction. The transition at 58.4degreesC reflects the melting of the large rod part of myosin. Nucleotide binding (ADP, ATP plus orthovanadate) induced shifts of the melting temperatures and produced changes in the calorimetric enthalpies. The changes of the EPR parameters indicated local rearrangements of the internal structure in myosin heads in agreement with DSC findings.