Essential Cell Biology 5th edition

Muscle Contraction
603
minus
end
myosin head
actin filament
lever arm
plus
end
ATTACHED At the start of the cycle shown in this figure,
a myosin head lacking a bound ATP or ADP is attached
tightly to an actin filament in a rigor configuration (so
named because it is responsible for rigor mortis, the
rigidity of death). In an actively contracting muscle, this
state is very short-lived, being rapidly terminated by the
binding of a molecule of ATP to the myosin head.
ATP
ATP
myosin filament
RELEASED A molecule of ATP binds to the large cleft on
the “back” of the myosin head (that is, on the side
furthest from the actin filament) and immediately causes
a slight change in the conformation of the domains that
make up the actin-binding site. This reduces the affinity
of the head for actin and allows it to let go of the
filament. (The space drawn here between the head and
actin emphasizes this change, although in reality the
head probably remains very close to the actin.)
ATP HYDROLYSIS
ADP
P
COCKED The cleft closes like a clam shell around the
ATP molecule, triggering a large shape change that
causes the head to be displaced along the actin filament
by a distance of about 5 nm. Hydrolysis of ATP occurs, but
the ADP and inorganic phosphate (P) produced remain
tightly bound to the myosin head. Dotted lines show the
position of myosin head prior to ATP hydrolysis.
lever arm
ADP
P
FORCE-GENERATING Weak binding of the myosin head
to a new site on the actin filament causes release of the
inorganic phosphate produced by ATP hydrolysis. This
release triggers the power stroke—the force-generating
change in shape during which the head regains its
original conformation. In the course of the power stroke,
the head loses its bound ADP, thereby returning to the
start of a new cycle.
POWER STROKE
ADP
minus
end
plus
end
ATTACHED At the end of the cycle, the myosin head is
again bound tightly to the actin filament in a rigor
configuration. Note that the head has moved to a new
position on the actin filament, which has slid to the left
along the myosin filament.
the myosin filament. The concerted action of many myosin heads pulling
the actin and myosin filaments past each other causes the sarcomere to
contract. After a contraction is completed, the myosin heads all lose contact
with the actin filaments, and the muscle
ECB5 e17.43/17.43
relaxes.
A myosin filament has about 300 myosin heads. Each myosin head can
attach and detach from actin about five times per second, allowing the
myosin and actin filaments to slide past one another at speeds of up to
15 μm per second. This speed is sufficient to take a sarcomere from a fully
extended state (3 μm) to a fully contracted state (2 μm) in less than onetenth
of a second. All of the sarcomeres of a muscle are coupled together
and are triggered simultaneously by the signaling system we describe
next, so the entire muscle contracts almost instantaneously.
Figure 17–43 The head of a myosin-II
molecule walks along an actin filament
through an ATP-dependent cycle of
conformational changes. Two actin
monomers are highlighted to make the
movement of the actin filament easier to
see. Movie 17.12 shows actin and myosin in
action. (Based on I. Rayment et al., Science
261:50–58, 1993.)