Myosin is the molecular motor in muscle that generates torque and transiently reacts with actin. The mechanical work performed by the motor occurs by successive decrements in the free energy of the myosin-nucleotide system. The seat of these transitions is the globular "head" domain of the myosin molecule (subfragment 1 or S1). A very useful (hitherto empirical) signal of these transitions has been optical, namely, detection of state-dependent changes in absorbance or fluorescence of S1. This effect has now been found to arise in a particular myosin residue (Trp510 in rabbit skeletal muscle), enabling the study of its intimate mechanism. In this work, based on measuring time-dependent signals, we find that the signal change upon nucleotide binding is adequately explained by assuming that nucleotide binding to a remote site causes a transition from a situation in which Trp510 is strongly statically quenched to a situation in which it is weakly statically quenched. The Trp510-static quencher interaction is also responsible, in part, for the changing tryptophan optical density in S1 upon nucleotide binding. Using crystallographically based geometry, calculation of the Trp510 electronic wave function indicates that Tyr503 is the static quencher.
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