Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter VI. BIOCHEMISTRY OF PHYSIOLOGICAL FUNCTIONS AND SPECIALIZED TISSUES

CHAPTER 32. BIOCHEMISTRY OF MUSCLES AND MUSCLE CONTRACTION

32.3. BIOENERGETICS OF MUSCLE TISSUE

Skeletal Muscle operating at maximum activity requires a significant expenditure of energy in the form of ATP, with the transition from a state of physiological rest to functional activity occurring within fractions of a second.

The efficient conversion of ATP energy into the mechanical energy of Muscle contraction requires a continuous supply of chemical Energy Sources to the Actin-Myosin system.

Class="center">Sources of ATP in Muscles

1. Glycogenolysis, which supplies glucose-6-phosphate oxidized via The Glycolytic Pathway under either aerobic or anaerobic mechanisms.

The functioning of this ATP source is particularly prominent in white muscles (see below) and utilizes Glycogen reserves contained in granules adjacent to the I-discs.

2. Oxidation of glucose delivered to the muscles via the bloodstream.

3. Oxidative Phosphorylation in sarcosomes — a process of primary importance in aerobic red muscles, which are rich in Cell/35.html">Mitochondria.

4. The adenylate kinase reaction:

This reaction, catalyzed by adenylate kinase, has a dual significance:

- it serves as a reserve mechanism for the rapid production of ATP;

- it generates AMP, which acts as an allosteric activator of Phosphofructokinase, thereby accelerating glycolytic reactions.

5. The creatine phosphokinase (creatine kinase) reaction:

The generation of ATP from creatine phosphate is the most rapid mechanism for producing the ATP required for the immediate onset of muscle contraction. The operation of the creatine phosphokinase (CPK) mechanism can sustain intensive muscle work for 2–5 seconds — the period necessary for other bioenergetic mechanisms to engage.

During muscle relaxation, the resynthesis of creatine phosphate occurs via the reverse flux of the creatine kinase reaction.

Upon damage to skeletal or cardiac muscle (such as myocardial infarction), the CPK enzyme leaks through the Plasma Membranes of myocytes into the Blood, which is utilized for the Diagnosis of necrotic processes in Muscle tissue.

Red and white muscles

Based on specific ultrastructural and metabolic characteristics, skeletal muscles are divided into two types:

- red (slow) skeletal muscles;

- white (fast) skeletal muscles.

Red muscles are a type of muscle well-supplied with blood and rich in Myoglobin, an O₂-binding protein that forms oxygen reserves within muscle Cells. Red muscles contain numerous mitochondria and possess a high capacity for oxidative processes, utilizing glucose, Fatty acids, and Ketone Bodies as substrates. This muscle type is best adapted for prolonged physical activity, although the mobilization of its energy reserves occurs slowly.

White muscles are a type of muscle containing a small number of mitochondria; due to the high activity of Glycogen phosphorylase and glycolytic Enzymes, they are primarily adapted to generating energy via the anaerobic breakdown of glycogen. Driven by anaerobic glycogenolysis and Glycolysis, white muscles transition to maximum activity with a high contraction frequency much faster than red muscles, but they fatigue more quickly.

Many animal species possess distinctly separate types of these muscles. Human skeletal muscles contain both red and white muscle fibers.

Features of Bioenergetic Processes in the Myocardium

In terms of its Structure and Biochemical properties, the myocardium is similar to striated muscle, particularly to red skeletal muscle.

A distinctive feature of METABOLISM/26.html">Energy Metabolism in the myocardium is its almost exclusively aerobic nature, which makes it highly sensitive to impaired oxygen supply, such as that caused by coronary artery stenoses. As a substrate for Biological Oxidation, the myocardium predominantly utilizes fatty acids, The oxidation of which accounts for about 70 % of the O₂ consumed by The Heart muscle (A.Ya. Nikolayev, 1998).

The share of fatty acids in supplying the myocardium with ATP decreases slightly after a meal and during physical exertion; under these conditions, the oxidation of glucose and lactic acid increases correspondingly.

The Regulation of Muscle contraction in the myocardium occurs via mechanisms similar to those of skeletal muscles. However, unlike skeletal muscles, the main source of Ca²⁺ for contraction is extracellular calcium, which enters through the plasma membranes upon myocardial excitation.