Magnesium ion dependent adenosine triphosphatase activity of heavy meromyosin as a function of temperature between +20 and -15.degree.C

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Abstract
The hydrolysis of Mg2+-ATP by [rabbit muscle] heavy meromyosin was studied between +20 and -15.degree. C, especially in the low-temperature range, in a medium containing 30% (vol/vol) ethylene glycol by fluorometric, spectrophotometric, and potentiometric measurements. The time course of the fluorescence changes of the enzyme during the reaction depends markedly on the temperature in consequence of large differences between the activation energies of the various steps. The observed kinetics were analyzed according to the simplified scheme of Bagshaw and Trentham: M + ATP .fwdarw. M*.cntdot.ATP .dblarw. M**.cntdot.ADP.cntdot.Pi .fwdarw. M*.cntdot.ADP .dblarw. M + ADP + Pi + H+. The following results were obtained. The rate-limiting step of the reaction changes in this temperature range; at 20.degree. C, M**.cntdot.ADP.cntdot.Pi is the predominant steady-state complex, and M*.cntdot.ADP predominates at -15.degree. C, with a half-life of .apprx. 10 min. As expected, on the basis that it is the dissociation of the M*.cntdot.ADP complex which becomes rate limiting at low temperature, in the pre-steady-state below 0.degree. C, both a proton burst and a lag phase in ADP release are observed. At low temperature, the equilibrium M*.cntdot.ATP .dblarw. M**.cntdot.ADP.cntdot.Pi is displaced to the left. The kinetic data obtained are compatible with a simple pathway for the Mg2+-ATP hydrolysis by myosin and with sequential release of the reaction products.