BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 34. MUSCLE CONTRACTION AND CELL MOTILITY

How is the energy of chemical bonds transformed into coordinated movement? This is one of the most pressing questions in modern molecular biology. Directed movement occurs during chromosome segregation in Cell Division, bacteriophage DNA injection into a bacterial host, the beating of Cilia and flagella, the active Transport of Molecules, RNA translocation during Protein Synthesis, and—most notably—in Muscle contraction. In this chapter, we will primarily examine the structural basis of contraction in vertebrate striated muscle, as this process has been studied in the greatest detail. The contractile system of striated muscle consists of overlapping protein filaments that slide past one another. Contraction is driven by The energy released during ATP Hydrolysis. In striated muscle, it depends on the Ca²+ concentration, which in turn is regulated by the sarcoplasmic reticulum—a specialized membrane system that sequesters Ca²+ in the resting state and releases it upon stimulation of the muscle fiber by a Nerve Impulse.

34.1. Muscle Consists of Interacting Thick and Thin Protein Filaments

Under a Light Microscope, vertebrate voluntary Muscles appear striated (Fig. 34.1). They consist of Cells surrounded by an electrically excitable membrane, the sarcolemma. A muscle cell contains A large number of parallel myofibrils, each about 1 µm in diameter. The myofibrils are embedded in an intracellular fluid called the sarcoplasm, which contains Glycogen, ATP, phosphocreatine, and glycolytic Enzymes. Actively functioning muscles also contain numerous Mitochondria arranged in regular arrays along the myofibrils.

Class="center">Fig. 34.1. Skeletal Muscle fiber viewed under a phase-contrast light microscope. Fiber diameter is approximately 50 µm. A-bands are dark, I-bands are light.

Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF longitudinal sections of myofibrils reveal a wealth of structural detail (Figs. 34.2 and 34.3). The functional unit is the sarcomere, which repeats every 2.3 µm (23,000 А) along the long axis of the fibril. Dark A-bands and light I-bands alternate regularly. The middle of the A-band, known as the H-zone, has a lower electron density. A dark M-line runs down the center of the H-zone, whereas the I-band is bisected by a narrow, highly electron-dense Z-disc.

Fig. 34.2. Electron micrograph of a Longitudinal section of a skeletal muscle fiber.

Fig. 34.3. Electron micrograph of a longitudinal section of a skeletal muscle myofibril. A schematic diagram of the corresponding cross-sections is shown below the micrograph.

The molecular architecture of the sarcomere in cross-section can be inferred from electron micrographs of myofibrils. These images reveal Two Types of interacting protein filaments (myofilaments). Thick filaments are approximately 150 А in diameter, whereas thin filaments are about 70 А. Thick filaments are composed primarily of Myosin, while thin filaments contain Actin, Tropomyosin, and troponin. In addition, the Z-disc contains α-actinin, and the M-line contains M-protein.

The I-band consists exclusively of thin filaments, whereas the H-zone of the A-band contains only thick filaments. Other Regions of the A-band contain both types of filaments. Cross-sections clearly demonstrate the hexagonal lattice of the myofibril, in which each thin filament is surrounded by three thick filaments, and each thick filament is surrounded by six thin filaments (Fig. 34.3). The interaction between thick and thin filaments is mediated by cross-bridges, which represent domains of the myosin molecules. Projecting at regular intervals from the thick filaments, these cross-bridges span the 130 А gap between the surfaces of the thick and thin filaments (Figs. 34.4 and 34.5). Essentially, it is the interaction between myosin cross-bridges and actin units within the thin filaments that generates the contractile force.

Fig. 34.4. Electron microscopic view of cross-bridges between thick and thin filaments.

Fig. 34.5. Schematic diagram of The Structure of striated muscle, illustrating the overlap between thick and thin filaments, alongside a corresponding ultra-thin section electron micrograph.