Stopped-flow calorimetry of myosin ATP hydrolysis: an implication of chemomechanical energy transduction
Kodama, T.
Advances in Experimental Medicine and Biology 226: 671-676
1988
ISSN/ISBN: 0065-2598 PMID: 2970212 Document Number: 320764
Measurements were made of heat production in the pre-steady state phase of ATP hydrolysis by myosin subfragment 1 using an improved stopped-flow calorimeter. This instrument can measure temperature changes as small as 3 X 10(-4) K with a thermal response time of less than 10 ms. When SF-1 was mixed with ATP at 0 degrees C, there was a fairly rapid heat absorbing phase (rate constant, 20-25 s-1 at pH 8) after a burst of heat production was complete within the dead time of the instrument (10 ms). Then followed a phase of very slow heat production with a heat rate of 7 kJ/mol/s. This result indicates that the ATP binding to SF-1 is an exothermic process whereas the subsequent isoenergetic splitting of ATP on the myosin site is endothermic. Thus, a decrease in the internal energy drives the ATP binding which may be regarded as the chemical energy trapping process. By contrast, the myosin intermediate with hydrolysis products (ADP + Pi) is stabilized by a large increase in entropy. In energy transduction by myosin/actin system of muscle, therefore, switching of chemical energy into a form available for mechanical work would be a transition from a low entropy to a high entropy state of weakly actin binding conformations of myosin.