Changes in the 31P NMR spectrum of rabbit muscle myosin subfragment 1. MgADP with temperature

2,3-Butandione 2-monoxime inhibits skeletal myosin II by accelerating ATP cleavage

2,3-Butandione 2-monoxime (BDM) is a widely used myosin inhibitor with an unclear mode of action. In this report, we investigated the mechanism of BDM oxime group nucleophilic reactivity on the phosphoester bond of ATP. BDM increased the ATPase activity of skeletal myosin subfragment 1 (S1) under conditions in which ATP cleavage is the rate-limiting step (K⁺, EDTA-ATPase activity of native S1 and Mg²⁺-ATPase activity of trinitrophenylated S1 and partially unfolded S1). Furthermore, the effect of BDM on the S1-bound adenosine 5'-(β,γ-imido) triphosphate (AMPPNP) ³¹P NMR spectrum suggests that BDM changes the microenvironment around the phosphorus atoms of myosin-bound nucleotide. A computational search for the BDM-binding site in the adenosine 5'-[γ-thio] triphosphate (myosin-ATPγS) complex predicted that BDM is located adjacent to the nucleotide on myosin. Therefore, we propose that the BDM oxime group catalytically assists in ATP cleavage, thereby enhancing the ATPase activity of myosin in a manner analogous to pralidoxime-mediated reactivation of organophosphate-inactivated acetylcholinesterase. This is the first study suggesting that oxime provides catalytic assistance for ATP cleavage by an ATP-hydrolyzing enzyme.

Myosin-catalyzed ATP hydrolysis elucidated by 31P NMR kinetic studies and 1H PFG-diffusion measurements

We conducted (31)P NMR kinetic studies and (1)H-diffusion measurements on myosin-catalyzed hydrolysis of adenosine triphosphate (ATP) under varied conditions. The data elucidate well the overall hydrolysis rate and various factors that significantly impact the reaction. We found that the enzymatic hydrolysis of ATP to adenosine diphosphate (ADP) was followed by ADP hydrolysis, and different nucleotides such as ADP and guanosine triphosphate acted as competitors of ATP. Increasing ATP or Mg(2+) concentration resulted in decreased hydrolysis rate, and such effect can be related to the decrease of ATP diffusion constants. Below 50 degrees C, the hydrolysis was accelerated by increasing temperature following the Arrhenius' law, but the hydrolysis rate was significantly lowered at higher temperature (approximately 60 degrees C), due to the thermal-denaturation of myosin. The optimal pH range was around pH 6-8. These results are important for characterization of myosin-catalyzed ATP hydrolysis, and the method is also applicable to other enzymatic nucleotide reactions.