Studies of the Chemo-Mechanical Conversion in Artificially Produced Streamings

II. An Order-Disorder Phase Transition in the Chemo-Mechanical Conversion

Abstract

The dependence of the acto-heavy meromyosin (acto-HMM) ATPase activity on the velocity of streaming of the HMM solution was measured in our stream systems constructed from rabbit skeletal muscle. The acto-HMM ATPase activity was an increasing function of the streaming velocity. It was verified that this increase of the ATPase activity is caused by a mechanism such that the rate of decomposition of AM^*P into A+M+P is increased by the streaming. Both the streaming velocity and the ATPase activity showed biphasic behavior with respect to the inverse of the temperature: below the critical temperature T_c, the temperature dependence of the acto-HMM ATPase activity was essentially the same as that in homogeneous systems randomly dispersed in a test tube, while the streaming was observed as a linear function of l/T only in the phase appearing above T_c, and the acto-HMM ATPase activity there shows a remarkable increase against -1/T. The critical temperature T_c is not attributable to a transition of the molecular state inside acto-HMM but to the dynamic state of the stream system, which is controlled by changing the amount of F-actin fixed onto the stream cell. It can be concluded that the phase appearing below T_c is a disordered phase where the molecular dynamics of acto-HMM are essentially those of random and independent motion, while the phase appearing above T_c is an ordered one and the molecular dynamics of acto-HMM show some cooperativity (the dynamic cooperativity).