Cardiomyocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 209

actin

myosin

titin

actin

myosin heads

troponin

Z disc M line

Fig. 5.5 Interacting myosin and actin in the sarcomere. The thick filaments are composed of
myosin and its partners (myosin-binding proteins-C, -H, and -X). The thin filaments contain cardiac
actin, -tropomyosin, and TnT, TnI, and TnC troponins. Myocardium contraction is generated

by cyclic interactions between actin and myosin. Conformational changes in the actin-myosin
binding interface are related to ADP release and ATP binding. Nanoscale conformational changes
are converted into macroscale myocardium displacements. The dissociation rate of actomyosin
bonds was postulated to depend on strain in the Huxley model. The lifetime of the actomyosin
bond indeed depends on the instantaneous load and loading history.

α

TnC that relieves the inhibition once TnC binds Ca 2 + . Troponin-T is a tropomyosin-
binding protein. The troponin-C-Ca 2 + complex deforms TnC-TnI element and
weakens TnI-TnT, exposing the myosin-binding site on actin.
Troponin-I and myosin-binding protein-C are substrates of protein kinase-A. As
it phosphorylates troponin-I, PKA reduces the Ca 2 + sensitivity, but not the cross-
bridge cycling rate [ 364 ]. Protein kinase-C targets troponin-I, troponin-T, myosin
light chain-2, and myosin-binding protein-C. Protein kinase-C may reduce the cross-
bridge cycling rate.
Two actin polymers placed side by side form a double helix that behaves like a
moving rack (Fig. 5.5 ). The screw-shaped myosin has a twisted tail and a head that
emerges to form grooves in which the actin filaments slide like a wrench jaw.

5.3.3.1 Actin-Myosin Interactions

Contraction is induced by the 4-time nanomotor composed of interdigitated myosin
and actin filaments (Fig. 5.6 )[ 365 ]: (1) the myosin head detaches from actin and
fixes ATP; (2) ATP is hydrolized and the myosin head binds to actin; (3) the
myosin head releases the phosphate and undergoes a conformational change. Actin
displacement is generated by a bascule motion of the myosin head that pulls on