Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
ATP Cycle and Cellular Bioenergetics
ATP is used to provide energy for muscle contraction

ATP not only energizes precursor molecules and thereby primes material for The Biosynthesis of various cellular components; it also supplies chemical energy for the two primary forms of work performed by The Cell: mechanical work, associated with Muscle contraction, and osmotic work, which drives the Transport of substances against a concentration gradient.

The contractile system of Skeletal Muscle comprises Two Types of filaments (Fig. 14-10). Thick filaments consist of bundles formed by parallel, rod-shaped Myosin molecules, whereas thin filaments are composed of two strands of filamentous Actin (F-actin) twisted around each other. F-actin, in turn, is made up of globular actin (G-actin) molecules linked together like a string of beads. Within myofibrils, thick and thin filaments are arranged in an orderly fashion; in sarcomeres—the repeating units of myofibrils—they lie parallel and overlap to a greater or lesser extent. During muscle contraction, the thick filaments of each sarcomere slide into the gaps between the thin filaments, causing the entire muscle fiber to shorten. The chemical energy that drives this sliding motion is provided by the Hydrolysis of ATP to ADP and phosphate. As seen in Fig. 14-10, each myosin molecule in a thick filament features a HEAD. These myosin heads, distributed uniformly along the thick filaments, are essentially Enzymes. By repeatedly making contact with the thin filaments, they catalyze ATP hydrolysis, generating a sliding force that causes the thick filaments to move along the thin ones toward the ends of the sarcomere. ATP hydrolysis is thought to induce A change in the shape, or conformation, of the myosin head, which in turn generates mechanical force. Through this mechanism, myosin, actin, and other specialized Proteins of the contractile system convert the chemical energy of ATP into the mechanical energy of muscle contraction.

The contraction and relaxation of skeletal muscle are regulated by the cytosolic concentration of Ca²⁺. At rest, the concentration of Ca²⁺ in muscle is typically very low. Upon stimulation of a muscle fiber by motor nerve impulses, Ca²⁺ is released from the transverse membrane tubules of the muscle cell. This released Ca²⁺ binds to troponin, a complex regulatory protein whose molecules are attached at regular intervals along the thin filaments. Troponin molecules act as a trigger, or switching mechanism. They undergo a conformational change that affects the myosin heads in the thick filaments, stimulating their ATPase activity and thereby initiating contraction. Troponin remains active as long as Ca²⁺ is present in the Cytosol of the muscle fiber. Muscle relaxation occurs once nerve impulses cease and Ca²⁺ is pumped out of the sarcoplasm and back into the cisternae of the sarcoplasmic reticulum by a membrane-bound ATPase acting as a calcium pump. Thus, ATP is required not only for muscle contraction but also for relaxation. As we shall see later, The energy released by ATP hydrolysis is likewise utilized to transport other ions across membranes.

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Fig. 14-10. The contractile system of skeletal muscle. A. A skeletal Muscle consists of bundles of parallel muscle fibers, which are very long, multinucleated Cells. B. Each muscle fiber contains numerous myofibrils—bundles of parallel filaments. The myofibrils are divided into sarcomeres by distinct dark regions called Z lines. C. Each sarcomere is composed of regularly arranged thick and thin filaments, with thick filaments capable of sliding along thin ones. D. Thick filaments are formed from bundles of elongated, rod-shaped molecules of the protein myosin. Each myosin molecule consists of two α-helical polypeptide chains twisted around each other. One end of the polypeptide chain (the head) has a globular Structure AND Functions as an enzyme that catalyzes the hydrolysis of ATP to ADP and P_i.

D. Each thin filament consists of two F-actin chains twisted around each other in a helical structure. Each such chain is composed of globular G-actin molecules linked together like a string of beads. E. Globular myosin heads protrude from the thick filaments. It is hypothesized that in the presence of ATP, these myosin heads act as levers. By attaching to the thin filaments and pulling them toward the center of the sarcomere, they shorten the sarcomeres, resulting in myofibril contraction. At the same time, the myosin heads catalyze the hydrolysis of ATP to ADP and P_i. This muscle contraction process is regulated by a specialized Ca²⁺-binding protein, troponin, molecules of which are attached at regular intervals along the actin filaments.

Fig. 14-11. Thanks to the well-coordinated action of powerful skeletal Muscles, a cheetah is capable of reaching speeds up to 125 km/h over distances of several hundred meters.

Muscles vary greatly in their specialization. The smooth Muscles of the intestines, for instance, contract extremely slowly, whereas the flight muscles of insects such as flies and mosquitoes are characterized by an extraordinarily high frequency of contraction and relaxation. Some muscles are capable of "locking" in a contracted state. Examples include the catch muscles, or adductor muscles, of bivalve Mollusks such as oysters.

Cardiac muscle is characterized by rhythmic contractions. Skeletal muscles, in turn, are divided into several specialized groups. White skeletal muscle belongs to "fast" muscles that can function anaerobically, whereas red skeletal muscle, which contracts slowly, requires oxygen. In some animals, muscles are exceptionally powerful and operate with high efficiency (Fig. 14-11). However, regardless of muscle specialization, the molecular components of their contractile elements are always actin, myosin, and troponin, while the energy source for muscle contraction is invariably ATP. Other types of movement (Chap. 2) are powered by Cilia and flagella found in certain Eukaryotic cells (Fig. 14-12). Chromosome segregation during mitosis, driven by the contraction of spindle fibers, also relies on ATP energy, which facilitates the twisting and sliding of microtubules relative to one another.

The actin-myosin-troponin system represents a unique type of chemical engine, as it directly converts chemical energy into mechanical work at constant Temperature and pressure. To our knowledge, no human-made machine is capable of such energy conversion. Consequently, living organisms utilize a mode of energy Transduction that engineers have yet to replicate.