In this article results are reviewed from different experimental approaches to determine the size of the power
stroke generated by
myosin molecules during their
ATPase cycle. While data from fiber studies and
protein crystallography predict a
stroke size of about 10 nm for
skeletal muscle myosins, single molecule studies imply a
stroke size for these
myosins of only about 5 nm. Single molecule studies also showed the
stroke size to be proportional to the length of the light chain binding domain, acting like a lever arm. At the same lever arm length, however, the
stroke size of smooth muscle
myosin II is found about twice as large and a
stroke size of about 14 nm was reported for class-I
myosins. It was proposed that such different
stroke sizes for molecules with same lever arm length result from different extend of converter domain rotation. Only for class-I
myosins, however, an about 30 degrees larger rotation of the converter was found so far by
protein crystallography. This, however, is far too small to account for the almost 3-fold larger
stroke size reported from single molecule studies. In this contribution we discuss some factors that might account for the apparent discrepancies between single molecule studies on the one hand and
protein crystallography as well as some fiber studies on the other hand. In addition, we present some modeling to illustrate that the power
stroke very likely is underestimated to a large extent in current single molecule approaches. We further show that differences in the
stroke size for various classes of
myosins reported from single molecule studies might be related to small differences in the probability to execute the power
stroke kinetics. We demonstrate that such small changes in power
stroke kinetics can seriously affect the extent to which the 'true' power
stroke is underestimated by present single molecule approaches.