Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Proteins of blood and muscle tissues
Proteins of the contractile system
Organization of skeletal muscles

Data regarding the size, shape, and geometric arrangement of the protein components of Muscle contractile elements have been obtained primarily through optical and Electron Cell/15.html">Microscopy. It has been established that Skeletal Muscle fibers are highly elongated, multinucleated Cells. The Plasma Membrane of these cells is termed the sarcolemma. The bulk of the muscle (see Fig. 3.1) consists of contractile elements known as myofibrils, which are bundled in parallel. Myofibrils are suspended in an intracellular fluid called sarcoplasm.

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Fig. 3.1 Structure of a skeletal muscle (adapted from: Berezov, T.T., Korovkin, B.F. Biological Chemistry. Moscow: Meditsina, 1990)

I - A-bands; II - I-bands; III - H-zone; 1 - Z-line; 2 - T-system; 3 - sarcoplasmic reticulum; 4 - opening of the T-system; 5 - Glycogen; 6 - Mitochondria; 7 - sarcolemma.

The sarcoplasm contains glycogen, glycolytic Enzymes, ATP, creatine phosphate, inorganic electrolytes, as well as significant amounts of Amino Acids and Peptides. Highly active aerobic Muscles are exceptionally rich in mitochondria, which are arranged in an orderly fashion along the myofibrils in a largely fixed position. In less active muscles, mitochondria are fewer in number and exhibit a less regular distribution.

Muscle fiber myofibrils appear as long, thin threads featuring periodically repeating longitudinal structural elements (approximately every 2.5 µm). The diameter of a myofibril is about 1-2 µm, while that of a muscle fiber ranges from 50 to 100 µm. In skeletal, or striated, muscles, the repeating units of parallel-aligned myofibrils produce a characteristic cross-striated appearance (see Fig. 3.2)

Fig. 3.2 Electron micrograph of a Cytology/practical/54.html">Longitudinal section of skeletal muscle. Clearly visible are Z-discs, A- and I-bands, thick and thin filaments, as well as longitudinal (left) and transverse (right) sections of T-tubules.

The figure is adapted from the monograph: Metzler, D. Biochemistry. In 3 volumes. Moscow: Mir, 1980, Vol. 1.

The striation results from the alternation of zones with varying optical density. The light bands are designated as I-bands and are isotropic, whereas the dark bands are termed A-bands and are anisotropic.

Fig. 3.3 Structure of a sarcomere—the fundamental structural unit of a myofibril.

The illustration is taken from: Enok, R.M. Fundamentals of Kinesiology. Kyiv: Olympic Literature, 1998.

The distinction between isotropic and anisotropic structures lies in the fact that the Physical Properties of anisotropic structures depend on the direction of measurement, whereas those of isotropic structures do not. Muscle A-bands are optically anisotropic; they exhibit different refractive indices in various directions, a phenomenon known as birefringence. In general, birefringence is characteristic of solid bodies whose constituent molecules are asymmetrical and preferentially oriented in a single direction.

In resting muscle, A-bands are approximately 1.6 µm long, and I-bands are about 1 µm long. The I-band region is divided into two halves by a dense transverse Z-disc (or Z-line) approximately 800Å wide. Situated in the central part of the A-band is the less dense H-zone, which is roughly 0.5 µm wide and bisected by a transverse dark line known as the M-band. The entire repeating unit located between two adjacent Z-discs is called a sarcomere (see Fig. 3.3).

Adjacent myofibrils are aligned such that their Z-lines lie in the same plane, i.e., they overlap (see figure):

Using electron microscopy, Huxley and Hanson established that each myofibril consists of numerous microscopic threads called filaments, which are of two types: thick and thin. I-bands contain exclusively thin filaments with a diameter of about 60Å. The dense A-regions contain both thin and thick filaments (the latter with a diameter of 150-170 Å), which accounts for the birefringence of the A-bands. In transverse sections, thick filaments are spaced 450 Å apart and arranged in a hexagonal lattice. Each thick filament is surrounded by six thin filaments, which likewise form a two-dimensional hexagonal array in cross-section. Thus, within the bundle, every filament of one type is surrounded by six filaments of the other type.

In the resting state of the muscle, thick filaments are continuous throughout the entire A-band. Thin filaments are discontinuous within the A-band region; they originate at the Z-discs, run continuously across the I-bands, and terminate at the edge of the H-zone within the A-band. Because thin filaments are arranged longitudinally, their ends clearly delineate the BOUNDARIES OF THE H-zone. Consequently, in an unstretched muscle, thin filaments are absent from the H-zone, while thick filaments are absent from the I-band.

Thick filaments are composed of several Proteins, Myosin being the primary one, whereas Actin is the principal protein of thin filaments. High-resolution electron microscopy reveals regularly spaced cross-bridges between the thick and thin filaments in the dense Zones of the A-bands. These represent projections extending from the thick filaments. In skeletal muscle, there are 6 such bridges for every 430Å. They are arranged in pairs, with each pair rotated by 120⁰ relative to the preceding one. Cross-bridges serve as the sole structural link between thick and thin filaments.

During Muscle contraction, thick and thin filaments move (slide) past one another. Upon full contraction, the opposing thin filaments partially overlap, while the thick filaments reach the Z-discs with their ends (see model and Fig. 3.4).

Model of muscle contraction (sliding filament theory)

Fig. 3.4 Stages of muscle contraction: relaxed state (a); during contraction (b); final, fully contracted state (c).

It has been established that A-bands (thick filaments) do not change their length during muscle contraction. Nor does the distance between the Z-line and the edge of the H-zone change, meaning that thin filaments also retain their length. The width of the H-zone and I-bands decreases during contraction. However, this is not the result of A change in the length of thick and thin filaments, but rather a consequence of filament overlap. During contraction, sarcomeres shorten by 20-50%. Upon stretching, they can be elongated to 120% of normal.