Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors: - The Major Proteins of Muscle Are Myosin and Actin

The contractile force of muscle is generated by the in teraction of two proteins, myosin and actin. These proteins are arranged in filaments that undergo transient interactions and slide past each other to bring about contraction. Together, actin and myosin make up more than 80% of the protein mass of muscle. Myosin (Mr 540,000) has six subunits: two heavy chains (each of Mr 220,000) and four light chains (each of Mr 20,000). The heavy chains account for much of the overall structure. At their carboxyl termini, they are arranged as extended α helices, wrapped around each other in a fibrous, left-handed coiled coil similar to that of -keratin (Fig. 5–29a). At its amino terminus, each heavy chain has a large globular domain containing a site where ATP is hydrolyzed. The light chains are as sociated with the globular domains. When myosin is treated briefly with the protease trypsin, much of the fibrous tail is cleaved off, dividing the protein into com ponents called light and heavy meromyosin (Fig. 5–29b). The globular domain, called myosin sub fragment 1, or S1, or simply the myosin head group, is liberated from heavy meromyosin by cleavage with papain. The S1 fragment produced by this procedure is the motor domain that makes muscle contraction possible. S1 fragments can be crystallized and their structure has been determined. The overall structure of the S1 frag ment as determined by Ivan Rayment and Hazel Holden is shown in Figure 5–29c.

tractile unit. Within a thick filament, several hundred myosin molecules are arranged with their fibrous “tails” associated to form a long bipolar structure. The globular domains project from either end of this structure, in regular stacked arrays. The second major muscle protein, actin, is abundant in almost all eukaryotic cells. In muscle, molecules of monomeric actin, called G-actin (globular actin; Mr 42,000), associate to form a long polymer called F-actin (filamentous actin). The thin filament (Fig. 5–30b) consists of F-actin, along with the proteins troponin and tropomyosin. The filamentous parts of thin filaments as semble as successive monomeric actin molecules add to one end. On addition, each monomer binds ATP, then hydrolyzes it to ADP, so every actin molecule in the fil ament is complexed to ADP. This ATP hydrolysis by actin functions only in the assembly of the filaments; it does not contribute directly to the energy expended in muscle contraction. Each actin monomer in the thin fil ament can bind tightly and specifically to one myosin head group (Fig. 5–30c).

FIGURE 5–29 (at left) Myosin. (a) Myosin has two heavy chains (in two shades of pink), the carboxyl termini forming an extended coiled coil (tail) and the amino termini having globular domains (heads). Two light chains (blue) are associated with each myosin head. (b) Cleavage with trypsin and papain separates the myosin heads (S1 fragments) from the tails. (c) Ribbon representation of the myosin S1 fragment. The heavy chain is in gray, the two light chains in two shades of blue. (From coordinates supplied by Ivan Rayment.)

FIGURE 5–30 The major components of muscle. (a) Myosin aggre gates to form a bipolar structure called a thick filament. (b) F-actin is a filamentous assemblage of G-actin monomers that polymerize two by two, giving the appearance of two filaments spiraling about one another in a right-handed fashion. An electron micrograph and a model of the myosin thick filament and F-actin are shown. (c) Space filling model of an actin filament (shades of red) with one myosin head (gray and two shades of blue) bound to an actin monomer within the filament. (From coordinates supplied by Ivan Rayment.)

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مواضيع ذات صلة

Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors: -Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -The Antibody-Antigen Interaction Is the Basis for a Variety of Important Analytical Procedures

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Antibodies Bind Tightly and Specifically to Antigen

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -Antibodies Have Two Identical Antigen-Binding Sites

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -Self Is Distinguished from Nonself by the Display of Peptides on Cell Surfaces

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -The Immune Response Features a Specialized Array of Cells and Proteins

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Sickle-Cell Anemia Is a Molecular Disease of Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate

Protein Denaturation and Folding: - Loss of Protein Structure Results in Loss of Function

Protein Tertiary and Quaternary Structures: -There Are Limits to the Size of Proteins

Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Protein-Ligand Interactions Can Be Described Quantitatively