Biology Reference
In-Depth Information
Fig. 11.33 Simultaneous measurements of the mechanical movement of the myosin head and
the associated kinetics of ATP hydrolysis (Reproduced from Ishii and Yanagida 2007) with
modification: I added the star symbol,
*
, to myosin in State (3) to indicate the presence of
conformational strains
or
the conformons postulated to be stored
in myosin. (a) In the presence
of ATP, a molecule of myosin head immobilized on a block exerts a force on an actin filament
attached to two beads on its ends. One of these beads is fixed with a laser tweezers and the
movement of the other is measured with another optical tweezers. (b) An alternative way of
measuring the myosin's mechanical activity. The myosin molecule attached to the tip of a micro-
needle is brought to an actin filament fixed on a glass surface so that it can touch actin molecules.
Given ATP in the medium, the myosin head exerts a force on the actin filament thereby
producing an equal and opposite force on the tip of the micro-needle (following the Third Law
of Newtonian mechanics), which causes the micro-needle to undergo displacements as recorded
in (c). (c) Expanding the rising phase of the step movement in (c) revealed substeps of 5.5 nm,
the diameter of the actin monomer. (d) The results of a simultaneous measurement of the
mechanical movement of the myosin head and the hydrolysis of one molecule of a fluorescent
ATP analogue. The upper trace records the movement of myosin along the actin filament, and the
lower trace shows the binding activity of ATP to myosin. The high level of fluorescence indicates
the binding of a molecule of ATP to the myosin head and the low level of fluorescence signals the
desorption of ATP and ADP from myosin. It should be noted here that the superscript
*
added
to the myosin head in State (3) (to denote the conformationally strained and hence mechanically
energized state of the myosin head) is a modification proposed in this topic for the first time
based on the conformon theory of molecular machines (Green and Ji 1972a, b; Ji 1974a, b, 1979,
1991, 2000)
