Cardiomyocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 210

myosin

release

P

ADP

Ca ATP

troponin complex

actin

MBS

tropomyosin

Ca

A

ATP

A−M−ATP

A−M

M−ATP

M−ADP−P

A−M−ADP

A−M−ADP−P

Fig. 5.6 Sliding-filament model of actin-myosin interactions: actin and myosin fixation, sliding,
and detachment cycle (nanomotor cycle). In the absence of Ca 2 + , tropomyosin locks the myosin-
binding sites of actin. When Ca 2 + binds to the troponin complex, troponin and tropomyosin free
the mysosin binding site (MBS) of actin. The myosin head then binds to actin, forming a cross-
bridge. Myosin detachment from actin is caused by ATP binding. ATP hydrolysis induces a rotation
and a weak then strong rebinding of myosin to adjoining myosin-binding site of actin, associated
with release of phosphate (P) and then ADP (phosphate release induces a rotation in myosin head).
ADP unbinding leads to a new cycle if Ca 2 + and ATP are available.

the actin filament. The myosin head rotates with an angle of about 45 degrees. This
rotation leads to an actin displacement of about 10 nm. (4) The myosin releases ADP
and remains anchored to actin. The interaction force is equal to about 1 pN [ 366 ].
Interdigitated actin and myosin filaments slide over each other to shorten the
sarcomere during contraction (sliding-filament model).
As a sarcomere contracts, Z lines are closer, and the widths of the I bands and
H zone decrease, whereas the width of the A band remains constant. Conversely, as
a myofibril is stretched, the width of the I bands and H zones increases (constant
A-band width). Sarcomere shortening generates myofibril and muscle shortening.
Each molecule of myosin contains a myosin head that is a binding site for
actin and ATP (Fig. 5.7 ). Troponin and tropomyosin allow actin to interact with
myosin heads in the presence of Ca 2 + ions. Activation of the muscle fiber causes
the myosin heads to bind to actin. Actin is drawn a short distance ( 10 nm) past

myosin. When linkages break, bonds are reformed farther along actin to repeat the
process. 34 The sarcomere length is maximum near the endocardium [ 367 ].

∼

34 The filaments are pulled past each other in a ratchet-like action.