Protein Chemistry. Structure, Properties, and Research Methods - Shendryk A.N. 2022

Proteins of blood and muscle tissues
Proteins of the contractile system
Functions of muscle tissue proteins - Myosin

In Muscle, Myosin serves as the primary structural element of thick filaments and performs three essential Functions.

1. At physiological pH and Ionic strength, myosin spontaneously forms filaments.

2. Myosin exhibits ATPase activity, driving Muscle contraction by harnessing energy from the high-energy bonds of ATP.

3. Myosin binds to the polymerized form of Actin, an interaction that plays a pivotal role in the sliding of thin and thick filaments During muscle contraction.

Myosin molecules are long (approximately 1600Å) asymmetric strands with a Molecular Weight of 470,000 - 480,000. Each molecule consists of two identical peptide chains, which can be dissociated by Treatment with concentrated urea or guanidine solutions. These chains contain 1,800 amino acid residues (see Fig. 3.5).

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Fig. 3.5 Structure of a myosin molecule (a) and a myosin (thick) filament (b)

Each of the two peptide chains of myosin is, in turn, composed of one heavy chain (MW = 200,000) and two light chains (MW approximately 20,000). Over most of their length, the heavy chains adopt an a-helical conformation, winding around each other to form a coiled-coil superhelix. At the exit point of the superhelix, both peptide chains terminate in a globular HEAD consisting of the two light chains.

Information regarding The structure of myosin and its ATPase activity has been obtained through Limited proteolysis experiments using Trypsin or Chymotrypsin. Short-term treatment with these proteases cleaves the myosin double superhelix near its midpoint, yielding heavy and light fragments known respectively as heavy meromyosin and light meromyosin. These fragments have been isolated and purified.

Papain Digestion of myosin cleaves the globular regions away from the double superhelix, producing SF₁ fragments. These fragments can also be obtained by treating heavy meromyosin with papain (F₁ + F₂). All proteolytic fragments of myosin have been isolated in pure form and thoroughly characterized (see Table 3.3).

Light meromyosin (LMM), much like intact myosin, forms filaments but lacks ATPase activity and does not bind actin. Heavy meromyosin (HMM), conversely, catalyzes ATP Hydrolysis and binds actin. It is suggested that both the ATPase activity and the actin-binding capacity of the myosin molecule and the HMM fragment reside in the same sulfhydryl groups.

Table 3.3 Characteristics of myosin molecule fragments