Transient kinetic and isotopic tracer studies of the myosin adenosine triphosphatase reaction

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Abstract
From transient kinetic studies of the Mg2+‐dependent adenosine triphosphatase of myosin subfragment 1, prepared from rabbit skeletal muscle, a seven‐step mechanism has been proposed. Features of this mechanism include two‐step processes for ATP and ADP binding in which the binary complex isomerizes in addition to a rapid nucleotide association step. In the case of ATP a large negative standard free energy change is associated with the isomerization. An overall rate‐limiting isomerization of the myosin‐product complex prior to product release has been identified. Studies on the mechanism of cleavage of ATP bound to the active site indicate the process is readily reversible and can account for the observation that more than one oxygen of the product phosphate arises from water. This proposal has been substantiated by the finding that the oxygen atoms of the γ‐phosphoryl group of bound ATP also undergo extensive exchange with water.