The actomyosin ATPase: a two-state system.
Studies of the interaction between actin and myosin subfragment 1 (S1) in solution have shown that the association reaction takes place in at least two steps. Initially the association is relatively weak to form a complex called the A state which can then isomerize to the R state. The rate and equilibrium constants for the isomerization have been measured and are shown to depend upon the nucleotide bound to the S1 ATPase site; with ATP bound the A state is preferred but as ATP is hydrolysed and the products are sequentially released then the complex gradually shifts to the A state. An extensive series of experiments have characterized the A-to-R isomerization both in solution and in contracting muscle fibres and have shown it to be closely associated with the key events in the ATP-driven contraction cycle: the conformational change from the A to the R state can be monitored by fluorescent probes on either actin or the nucleotide; the isomerization can be perturbed by increases in hydrostatic pressure; the actin-induced acceleration of the rate of product release from myosin is coupled to the A-to-R isomerization; tropomyosin may control actin and myosin interaction by controlling the isomerization step and finally pressure perturbations of contracting muscle fibres shows there to be a close coupling between the isomerization of acto.S1 and the force generating event of muscle contraction.
Department of Biochemistry, School of Medical Sciences, University of Bristol, U.K.
M A
MA
Geeves
D000818:Animals; D001665:Binding Sites; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology; D009218:Myosins / Q000737:chemistry / Q000378:metabolism*; D014335:Tropomyosin / Q000378:metabolism
D014335:Tropomyosin; D009218:Myosins