Principles of Protein Structure - G. Schultz 1982

Structural basis of the mechanism, action, and function of proteins
Skeletal muscle: a system where protein action can be correlated with the overall activity of the organ
Structural and functional organization of contractile proteins

The main STRUCTURAL AND FUNCTIONAL unit of a Muscle Cell is the sarcomere (Fig. 11.5)—a cylindrical Structure with a diameter of 1.5 µm and a length of 2 µm, containing about 2 x 2000 thin and 1000 thick protein filaments. The left part of Fig. 11.5 shows The formation of a thick filament from approximately 200 Myosin molecules. A thin filament is formed by the association of 2 x 175 globular Actin monomers, 2 x 25 Tropomyosin molecules, and 2 x 25 units of the three-component protein troponin. The actin filament is a double helix (Sect. 5.1) with a repeat period of 360 to 370 Å [755, 758]. The sarcomere is one of many macromolecular assemblies formed in vivo that possess a spatial Symmetry group.

The cross-bridge cycle is the elementary process of Muscle contraction. Contraction occurs As a result of thin filaments sliding along thick filaments toward the center of the sarcomere [759, 760]. The force between the filaments arises from cross-bridges extending from the thick filaments. These bridges, also referred to as myosin heads, represent biochemically active Regions of the myosin molecule that interact with Mg-ATP and actin in so-called cross-bridge cycles (Fig. 11.6). A single bridge is capable of performing 10 to 100 cycles per second. Because the cycles of the myosin heads (there are a full million of them per sarcomere!) are not synchronized, the result is a continuously acting force and smooth movement.

Class="center">Table 11.1 The mammalian muscle cell as a system in which signals (nerve impulses or hormone binding) lead to a controlled response^a

^a The primary function of The Cell consists in movement based on contraction and relaxation, and in performing mechanical work. Auxiliary Functions relate to the storage of chemical energy intended for conversion into mechanical energy. In this regard, an important stage is the mobilization of Glycogen, which serves an energy-storage function.

Fig. 11.5. Structural Organization of the Skeletal Muscle contraction apparatus [754].

As schematically shown at the bottom left, the myosin molecule (b and d) consists of a long coiled-coil α-Helix, a "tail," and two globular heads. The "tail" is formed by the heavy chains of the molecule, which, together with four light chains, also participate in forming both heads. Bipolar aggregates of myosin molecules, the so-called thick filaments (e), are located in the sarcomeres (h). The thin filament has a length of 1200 nm and a diameter of 10 nm; its molecular mass is 10⁵. Polymerization of actin (a) leads to long helical filaments (F-actin). Two other Proteins, troponin and tropomyosin, bind to F-actin; as a result (an example of self-assembly [754]), a thin filament is formed (g; length 1000 nm, diameter 5 nm, molecular mass 2.5x10⁷). Thin filaments attach to the Z-discs that separate the sarcomeres from one another. Thin filaments extending from opposite sides of the Z-line have opposite polarity. Within the sarcomere, thin and thick filaments form a hexagonal lattice (i). As can be seen from electron micrographs (provided by Hoffmann), the ordering of the thick and thin filaments is close to crystalline (k). Myofibrils (l), located at the next level of the structural hierarchy, are built from sarcomeres. The ensemble of myofibrils forms the contractile apparatus of the muscle cell.

Fig. 11.6. Protein Interactions in the contraction apparatus [615].

Shown on the left is a cross-section of a thin filament in the relaxed state. C, I, and T are the protein Components of the troponin complex. Tropomyosin blocks the binding sites of the myosin heads. Upon the binding of Ca²⁺ to troponin C, the thin filament switches to the active state (right). Tropomyosin moves toward the center of the actin helix groove, and the myosin heads (cross-bridges) can interact with actin. The cycle begins with the attachment of the myosin HEAD to the actin filament [755]. Then the myosin tilts, pulling the active filament toward the center of the sarcomere (out of the plane of the paper in the figure) by approximately 80 Å. Afterward, ATP dissociates the thin filament from the myosin head, which can then initiate another cross-bridge cycle further along the actin filament.

Contraction is based on The conversion of chemical energy into mechanical work. When a muscle is activated, its chemical energy consumption in the form of ATP increases by one to two orders of magnitude. One proposed mechanism is as follows. In the relaxed state of the muscle, Actin and myosin do not interact, and the myosin head exhibits weak ATPase activity, which is limited by the product release step, mainly Mg-ADP. In an active muscle, actin enhances the ATPase activity of myosin by approximately 100-fold through the displacement of Mg-ADP. It remains unclear whether the action of actin on the ADP-binding site of myosin is direct or allosteric.

The "response" of myosin ATPase demonstrates that both the action of the protein and the control over this action are equally important. Myofibrillar myosin exhibits ATPase activity only During muscle contraction; high ATPase activity in the absence of actin-myosin interaction would be a wasteful expenditure of chemical energy.

The long lifetimes of Mg²⁺-nucleotide complexes can explain their widespread occurrence as substrates. To answer the question of why the physiological substrate for myosin ATPase and many Other Enzymes is Mg-ATP rather than Ca-ATP, kinetic data can be invoked. The stability constants of Mg-nucleotide and Ca-nucleotide complexes are nearly identical, but the dissociation of Ca²⁺ complexes proceeds 1000 times faster than that of the corresponding Mg²⁺ complexes [762]. Not calcium, but magnesium—whose ATP and ADP complexes have a half-life on the order of milliseconds—was selected to suppress the ATPase activity of myosin in the muscle relaxation state and to drive relatively slow conformational changes (t_1/2 > 1 ms) that occur during the catalytic steps of actin-activated myosin ATPase, as well as certain other enzymes [758].

Such an example demonstrates that to evaluate The Effect of complexes on protein function, the equilibrium and kinetic properties of these complexes, as well as their structural isomers, must be studied. For this purpose, techniques for investigating fast (10^-7 s) and ultra-fast (10^-9 s) reactions can be employed [763–766], the rates of which approach those of diffusion-controlled processes (10^-10 s).