Aggregation-linked kinetic heterogeneity in bovine cardiac myosin subfragment

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Abstract
Studies of the cardiac myosin fragment 1 concentration dependence of the rate constants for ATP binding and steady-state hydrolysis reveal that the observed rate constants are remarkably dependent on the protein concentration. The kinetics for ATP binding are biphasic and both the fast- and slow-phase rate constants and the respective fractions of fast and slow material vary as a function of protein concentration. Two different types of kinetic experiments were conducted, 1 in which the ATP concentration was fixed but the subfragment 1 concentration was varied and another for which the ATP/subfragment 1 ratio was fixed but both concentrations were varied. The results of these 2 experiments on cardiac subfragment 1 are consistent with an ATP-dependent reATP-dependent reversible aggregation. Light-scattering experiments confirm the presence of this aggregation and the ATP dependence. Similar studies on rabbit skeletal subfragment 1 give monophasic, protein-independent kinetics consistent with a monomeric species in solution. A simple monomer-dimer mechanism can account for the cardiac subfragment 1 kinetic results when changes in tryptophan fluorescence are used. Light-scattering results show that cardiac myosin subfragment 1 undergoes multiple reversible MW changes in solution and may be tetrameric at high concentrations.
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