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Biophys J, November 1999, p. 2657-2664, Vol. 77, No. 5

Mechanochemical Coupling in Spin-Labeled, Active, Isometric Muscle

Josh E. Baker, Leslie E. W. LaConte, Ingrid Brust-Mascher, and David D. Thomas

Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota Medical School, Minneapolis, Minnesota 55455 USA

Observed effects of inorganic phosphate (Pi) on active isometric muscle may provide the answer to one of the fundamental questions in muscle biophysics: how are the free energies of the chemical species in the myosin-catalyzed ATP hydrolysis (ATPase) reaction coupled to muscle force? Pate and Cooke (1989. Pflugers Arch. 414:73-81) showed that active, isometric muscle force varies logarithmically with [Pi]. Here, by simultaneously measuring electron paramagnetic resonance and the force of spin-labeled muscle fibers, we show that, in active, isometric muscle, the fraction of myosin heads in any given biochemical state is independent of both [Pi] and force. These direct observations of mechanochemical coupling in muscle are immediately described by a muscle equation of state containing muscle force as a state variable. These results challenge the conventional assumption mechanochemical coupling is localized to individual myosin heads in muscle.

Biophys J, November 1999, p. 2657-2664, Vol. 77, No. 5
© 1999 by the Biophysical Society   0006-3495/99/11/2657/08  $2.00


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