260
Chapter 9
3
3
2
4
1
Resting
muscle
Cross-bridge
binds to actin
Cross-bridge
moves
ADP + P
i
ATP binds to myosin,
causing cross-bridge
to detach
No ATP
(after death)
Rigor mortis
Energized
cross-bridge
Thin filament (actin, A)
Thick filament (myosin, M)
M line
Z line
[A
M
ADP
P
i
]
[A
M]
[A
M]
Hydrolysis
of ATP
energizes
cross-bridge
[A + M
ATP]
ATP
[Ca
2+
] rises
ADP
P
i
ADP
P
i
ATP
[A + M
ADP
P
i
]
Figure 9–8
Chemical (shown in brackets) and mechanical
representations of the four stages of a cross-bridge
cycle. In a resting muscle fi ber, contraction begins
when calcium activates the thin fi lament.
Figure 9–8
physiological
inquiry
Under certain experimental conditions it is
possible to extract the protein troponin from a
skeletal muscle fi ber. Predict how cross-bridge
cycling in a skeletal muscle fi ber would be
affected in the absence of troponin.
Answer can be found at end of chapter.
The binding of energized myosin to actin triggers the release
of the strained conformation of the energized bridge, which pro-
duces the movement of the bound cross-bridge (sometimes called
the
power stroke
) and the release of P
i
and ADP (step 2):
Step 2
A · M · ADP · P
i
⎯→
A · M + ADP + P
i
cross-bridge
movement
This sequence of energy storage and release by myosin is analo-
gous to the operation of a mousetrap: Energy is stored in the
trap by cocking the spring (ATP hydrolysis) and released after
springing the trap (binding to actin).
During the cross-bridge movement, myosin is bound
very fi
rmly to actin, and this linkage must be broken to allow
the cross-bridge to be re-energized and repeat the cycle. The
binding of a new molecule of ATP to myosin breaks the link
between actin and myosin (step 3):
Step 3
A · M + ATP
⎯→
A + M · ATP
cross-bridge
dissociation from actin
The dissociation of actin and myosin by ATP is an example of
allosteric regulation of protein activity. The binding of ATP at
one site on myosin decreases myosin’s affi nity for actin bound
at another site. Note that ATP is not split in this step; that is,
it is not acting as an energy source, but only as an allosteric
modulator of the myosin head that weakens the binding of
myosin to actin.
Following the dissociation of actin and myosin, the ATP
bound to myosin is split (step 4), thereby re-forming the ener-
gized state of myosin and returning the cross-bridge to its pre-
power-stroke position.
Step 4
A + M · ATP
⎯→
A + M · ADP · P
i
ATP hydrolysis
Note that the hydrolysis of ATP (step 4) and the movement of
the cross-bridge (step 2) are not simultaneous events. If cal-
cium is still present at this time, the cross-bridge can reattach
to a new actin molecule in the thin fi lament and the cross-
bridge cycle repeats. (In the event that the muscle is gener-
ating force without actually shortening, the cross-bridge will
reattach to the same actin molecule as in the previous cycle.)
Thus, ATP performs two distinct roles in the cross-
bridge cycle: (1) The energy released from ATP
hydrolysis
ulti-
mately provides the energy for cross-bridge movement, and
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