Temperature dependence of the release of ATP hydrolysis products from the 10S conformation of smooth muscle myosin

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Abstract
The transition of smooth muscle myosin to the folded 10S monomeric conformation dramatically inhibits the release of the ATP hydrolysis products, ADP and Pi. In this work, we examined the influence of temperature on the time course of product release from the 10S conformer of chicken gizzard smooth muscle myosin. Release was monitored by single turnover assays, using either [γ-32P]ATP or the fluorescent ATP analog, formycin triphosphate (FTP). For all temperatures over the range 15–35° C, single exponential kinetics described the observed product release from 10S myosin. A 10° C increase in temperature resulted in a fourfold increase in the rate constant for the observed product release. Using single turnover analysis, we found a similar temperature dependence for the apparent rate constants for product release from the extended 6S monomeric conformation of myosin. However, at any given temperature, the rate constant for 6S myosin was approximately 1.5 orders of magnitude greater than that for the 10S. These results are consistent with a kinetic scheme in which 10S myosin must undergo transition to the 6S conformation prior to product release.