Biology Reference
In-Depth Information
Fig. 11.33 (continued)
to take place: (a) the generation of a charge on myosin which increases the
actin-binding affinity and (b) the paying back of the thermal energy borrowed
from the environment to extend the S-2 subfragment in going from State a to
State b. Actin and myosin are now tightly coupled electrostatically and mechanical
energy is stored in myosin (which corresponds to State 3 in Fig.
11.33d
). As S-
2 relaxes, the thin filament is pushed toward left as indicated by the arrow in State b.
When S-2 contracts to a critical distance, through allosteric interactions, the
phosphoryl group in the myosin head (i.e., S-1) is thought to be transferred from
X to Y (which could well be bound H
2
O) (see State c) and the actin-binding affinity
is drastically reduced so that myosin becomes detached from the thin filament (see
c
d), thus completing one machine cycle.
It is known that one ATP split is capable of moving the thin filament by a
maximum of about 100
˚
or 10 nm (Huxley and Hanson 1960). This finding was the
basis for the assumption that one turnover of ATP hydrolysis causes myosin to be
displaced by about 100
˚
(or 10 nm) in two steps, from States b to c (accompanied
by the release of ADP from myosin) and from States c to d (associated with the
release of Pi from the same). The sequential releases of ADP and Pi were postulated
on the basis of the analogy drawn between the
electron
and the highly unstable
phosphoryl group,
PO3
, which was conveniently termed the
phosphoron
(Ji 1974b). Just as the electron flow from carrier A to carrier B in Fig.
8.1
led to
the generation of conformons (see the cocked spring stabilized by two opposite
charges at State c), it was thought plausible to generate conformons in myosin by
transferring the phosphoron from ATP to a hypothetical
phosphoron
carrier X and
then to another
phosphoron
carrier, Y, thereby generating two conformons, each
!
