Energetics of Myosin ATP Hydrolysis by Calorimetry

Abstract

For over ten years from the mid-1970s, ATP hydrolysis into ADP and inorganic phosphate (P _i ) by myosin were investigated by reaction calorimetry using own-designed instruments. The purpose was to estimate enthalpy changes for intermediate steps of the hydrolysis cycle. The results indicated that the steps are accompanied by large enthalpy changes alternating between negative and positive values. By combining this enthalpic profile with Gibbs energy changes for the corresponding steps estimated by kinetics and equilibrium analysis, the overall energetic profile of the ATP hydrolysis cycle has been revealed. The most characteristic feature is that all the intermediates are near the same Gibbs energy levels (isoenergetic); this occurs because large enthalpy and entropy changes compensate each other in the bound ATP-hydrolyzing and subsequent P _i -releasing steps. Possible sources of the large entropy changes were investigated in the late 1990s using dielectric spectroscopy to measure changes in hydration level of myosin during the ATP hydrolysis cycle. The result indicated dehydration during the hydrolysis step and rehydration during the P _i -releasing step. The extent of these hydration changes on the myosin molecular surface was just sufficient to account for their observed entropy changes. Taken together, once myosin traps ATP, the system is brought into a low-entropy state that is maintained until hydrolysis products, P _i and ADP are released from myosin. Thus, ATP plays the role of mediator to bring negative entropy into the function of the myosin.