The magnesium-ion-dependent adenosine triphosphatase of bovine cardiac Myosin and its subfragment-1

Publisher Website
Google Scholar
Abstract
The kinetics of the Mg2+-dependent ATPase activity of bovine cardiac myosin and its papain subfragment-1 were studied by using steady-state and pre-steady-state techniques, and results were compared with published values for the corresponding processes in the ATPase mechanism of rabbit skeletal-muscle myosin subfragment-1. The catalytic-center activity for cardiac subfragment-1 was 0.019 s-1, which was < 1/3 of that determined for the rabbit protein. The ATP-induced isomerization process, measured from enhancement of protein fluorescence on substrate binding, was similarly decreased in rate, as was the isomerization process associated with ADP release. However, the equilibrium constant for ATP cleavage, measured by quenched-flow by using [.gamma.-32P]ATP, showed little difference in the 2 species. Other experiments were carried out to investigate the rate of association of actin with subfragment-1 by light-scattering changes and also the rate of dissociation of the complex by ATP. The dissociation rate increased with increasing substrate concentration, to a maximum at high ATP concentrations, with a rate constant of about 2000 s-1. Isomerization processes which may involve conformational changes probably have substantially lower rate constants for the cardiac proteins, whereas equilibrium constants for substrate binding and cleavage are not significantly different. These differences may be related to the functional properties of these myosins in their different muscle types. Kinetic heterogeneity was detected in both steady-state and transient processes, and this is discussed in relation to the apparent chemical homogeneity of cardiac myosin.