Chapter 7 - Protein Function: Myoglobin and Hemoglobin, Muscle Contraction, and Antibodies - Exercises - Page 219: 11

A single myosin molecule consists of two heavy chains and four light chains. It is effectively a dimer of two heterotrimer. The latter has a globular head and a long alpha-helical tail. In fact the two tails of the complete molecule form a parallel coiled-coil. A myosin molecule has three functions: 1. binding to other myosin molecules to form filaments; this occurs spontaneously in physiological conditions. At high ionic strengths, myosin exists as individual molecules. 2. binding to actin filaments 3. it is an ATPase, i.e. it hydrolyzes ATP to give ADP and a phosphate (Pi). The two different structural domains are responsible for different functions, as is revealed by treatment of myosin with proteases to give different subunits. Cleavage of myosin with trypsin gives two products: 1. light meromyosin (LMM), an 850Å coiled-coil, i.e. a large section of the myosin "tail". LMM aggregates to form filaments, but does not bind to actin filaments, and does not hydrolyze ATP. 2. heavy meromyosin (HMM), which consists of the globular heads and a shorter section of tail. It does not aggregate to form filaments, but it hydrolyzes ATP and binds to thin filaments. Treatment of HMM with the protease papain cleaves the two globular heads from the tail section (S2). The two heads (termed S1) are not surprisingly found to be the site of ATPase activity; they also bind to actin filaments. Myosin is therefore unusual in that it is both a fibrous protein, and a globular enzyme.

A single myosin molecule consists of two heavy chains and four light chains. It is effectively a dimer of two heterotrimer. The latter has a globular head and a long alpha-helical tail. In fact the two tails of the complete molecule form a parallel coiled-coil. A myosin molecule has three functions: 1. binding to other myosin molecules to form filaments; this occurs spontaneously in physiological conditions. At high ionic strengths, myosin exists as individual molecules. 2. binding to actin filaments 3. it is an ATPase, i.e. it hydrolyzes ATP to give ADP and a phosphate (Pi). The two different structural domains are responsible for different functions, as is revealed by treatment of myosin with proteases to give different subunits. Cleavage of myosin with trypsin gives two products: 1. light meromyosin (LMM), an 850Å coiled-coil, i.e. a large section of the myosin "tail". LMM aggregates to form filaments, but does not bind to actin filaments, and does not hydrolyze ATP. 2. heavy meromyosin (HMM), which consists of the globular heads and a shorter section of tail. It does not aggregate to form filaments, but it hydrolyzes ATP and binds to thin filaments. Treatment of HMM with the protease papain cleaves the two globular heads from the tail section (S2). The two heads (termed S1) are not surprisingly found to be the site of ATPase activity; they also bind to actin filaments. Myosin is therefore unusual in that it is both a fibrous protein, and a globular enzyme.