Abstract
The change in orientation of myosin crossbridges in contracting muscle during sudden length changes was examined by fluorescence polarization. This study used a fluorescent ATP analogue, 1,N6-etheno-2-aza-ATP(epsilon-2-aza-ATP) as a probe. Its fluorescence is considerably enhanced upon binding with myosin and is dependent on the chemical state of the myosin-nucleotide complex in muscle. The results showed that nucleotides bound to crossbridges in the intermediate attached state (presumably AM-epsilon-2-aza-ADP-Pi) during isometric contraction are highly oriented at the same angle as that of AM in rigor with bound epsilon-2-aza-ADP. Furthermore the orientation of nucleotides bound to crossbridges in the attached state is not altered during sudden changes in length of isometrically contracting muscle. The results of this time-resolved measurement support the conclusion obtained from a previous steady-state experiment that change in axial orientation of the active site of the myosin head is not involved in force generation.
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