Myosin is a protein with a molecular weight of 520 kDa. It consists of 6 polypep-tide chains, which consist of two identical heavy chains of 220 kDa each, two pairs of light chains of 20 kDa each and a globular cross-bridge head and rod-like tail. The head has a site for binding ATP and also contains an ATPase enzyme.

e Limited proteolysis with trypsin results in dissociation into two fragments, light

0 meromyosin and heavy meromyosin. Heavy meromyosin can be further split p y into two identical globular subfragments and one rod-shaped subfragment by

1 papain. It has ATPase activity.

g The biological activities of myosin can be categorised as:

Myosin molecules spontaneously assemble into filaments in solutions of physiological ionic strength and pH. It acts as an ATPase, hydrolysing ATP to ADP and Pi, being activated when the myosin head binds to actin. This provides the free energy for contraction. It binds to the polymerised form of actin (F-actin), the major constituent of thin filaments.

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