For many
proteins, especially for molecular motors and other
enzymes, the functional mechanisms remain unsolved due to a gap between static structural data and kinetics. We have filled this gap by detecting structure and kinetics simultaneously. This structural kinetics experiment is made possible by a new technique, (
TR)(2)FRET (transient time-resolved FRET), which resolves
protein structural states on the submillisecond timescale during the transient phase of a biochemical reaction. (
TR)(2)FRET is accomplished with a fluorescence instrument that uses a
pulsed laser and direct waveform recording to acquire an accurate subnanosecond time-resolved fluorescence decay every 0.1 ms after stopped flow. To apply this method to
myosin, we labeled the force-generating region site specifically with two probes, mixed rapidly with
ATP to initiate the recovery
stroke, and measured the interprobe distance by (
TR)(2)FRET with high resolution in both space and time. We found that the relay helix
bends during the recovery
stroke, most of which occurs before
ATP is hydrolyzed, and two structural states (relay helix straight and bent) are resolved in each
nucleotide-bound biochemical state. Thus the structural transition of the force-generating region of
myosin is only loosely coupled to the
ATPase reaction, with conformational selection driving the motor mechanism.