Biology Reference
In-Depth Information
Table 11.14 A comparison between the two explanations for the single-molecule measurement
data on the myosin-actin interactions coupled to ATP hydrolysis (Fig.
11.33
)
Mechanism based on the conformon
theory (Green and Ji 1972a, b; Ji 1974b,
States in
Fig.
11.33d
Mechanism proposed by Ishii and
Yanagida (2007)
1
Myosin strongly interacts with actin and
dissociates upon binding ATP
The conformon stored in myosin exerts
force on actin
2
Myosin weakly interacts with actin
Myosin dissociates from actin upon
binding ATP
3
Myosin generates mechanical motion
when Pi and ADP dissociates from
myosin
1. ATP is hydrolyzed and a part of the
chemical free energy supplied
thereby is stored in myosin as
mechanical energy or
conformons
(as denoted by the superscript
*
on
the myosin molecule)
2. The mechanical energy stored in
myosin is used to exert forces on
actin in two steps, each step
involving a 5.5-nm displacement
(see Fig.
11.33c
)
3. Each 5.5-nm displacement of actin is
allowed if and only if one of the two
products of ATP hydrolysis
dissociates from myosin. For
example, ADP can dissociate from
myosin and the other product, Pi,
remains bound to myosin to prevent
its slippage from actin filament,
thereby preventing the futile cycling
(i.e., hydrolyzing ATP without
producing any processive movement
of actin), in agreement with the
mechanism of muscle contraction
proposed in Ji (1974b) (see
Fig.
11.34
)
4
Myosin interacts with actin strongly
After both ADP and Pi dissociate from
myosin, myosin binds and exerts
force on actin as in State (1)
carrying about 8 kcal/mol of free energy. Thus it was assumed that in States b and c
in Fig.
11.34
, myosin was phosphorylated at the hypothetical carriers X and Y,
respectively (Ji 1974b, p. 221).
Thus, the conformon-based mechanism of muscle
contraction proposed in Ji (
1974b
) predicted the two, 5 nm-step motion of the
myosin along the actin filament which was confirmed by the single-molecule
measurements of Ishijima et al. (
1998
) shown in Fig.
11.33
c.
From the point of view of the conformon hypothesis, the most important steps in
Fig.
11.33d
are States (2) and (3), since these steps represent the first
direct experi-
mental evidence
for the conversion of
chemical energy
to
mechanical energy
stored
in myosin, i.e., conformons. In State (2), ATP is bound to myosin and actin is relaxed
as indicated by the two related traces labeled (2) in Fig.
11.33d
. In State (3), ATP is
