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Fig. 11.34 A pictorial representation of the conformon model of muscle contraction discussed in
Fig.
11.31
(Reproduced from Ji 1974b). The S-1 subfragment of myosin is depicted as an ellipsoid
and the S-2 subfragment as a mechanical spring, but both are thought to be involved in storing
mechanical energy, i.e.,
conformons
. The
upper bar
represents the thin filament composed of a
linear aggregate of actin monomers and the
lower bar
is the thick filament, an intertwined chain of
myosin subfragments S-2 with subfragments S-1 protruding from the body of the thick filament.
(a) ATP bound to the myosin head which is detached from the actin filament. (b) ATP is split into
ADP and Pi and the Pi-bund myosin head attaches to the actin filament. (c) The S-2 subfragment
contracts and pushes the actin filament to the left. (d) The myosin head releases Pi and detaches
from the actin filament to restart the cycle. In some publications, the order of the release of ADP
and Pi from the myosin head is the reverse of what is given in this figure, which will not
substantially alter the basic mechanisms underlying the conformon-mediated coupling between
chemical reactions and mechanical processes
hydrolyzed and the resulting ADP dissociates from myosin as indicated by the rapid
decrease in the fluorescence signal at t
¼
10 s in Fig.
11.33d
which is associated with
the
a
b
transition postulated in Fig.
11.34
, and the ADP-free myosin then exerts
a mechanical force on actin as shown by the relatively slow displacement of actin
around t
!
d transition in Fig.
11.34
.
The fact that the fluorescence drop in State (3) is faster than the velocity of the
associated displacement of actin is consistent with the proposed mechanism where
ATP hydrolysis leading to conformon generation in myosin precedes the displace-
ment of actin by myosin.
Therefore, it may be concluded that
the experimental data
presented in Fig.
11.33
d provide the first direct experimental evidence to validate
the following mechanism of free energy transduction in molecular biology that was
proposed more than three decades ago (Ji
1974b
):
¼
10 s in Fig.
11.33d
, supporting the c
!
