HUMAN BIOCHEMISTRY - L. V. Kapilevich - 2016

PART 1. SPORTS BIOCHEMISTRY

BIOCHEMISTRY OF MUSCLES, MUSCULAR CONTRACTION AND RELAXATION

General characteristics of Muscles

Animals and humans have two primary Types of Muscles:

✵ striated (attached to bones, i.e., the Skeleton, and thus also referred to as skeletal muscles; cardiac Muscle is also distinguished as having its own unique features);

✵ smooth (the musculature of the walls of hollow Organs and the Skin).

Structure of Muscle Cells

A striated Muscle consists of numerous elongated muscle cells. Motor nerves enter the muscle fiber at various points, transmitting an electrical impulse that triggers contraction. A muscle fiber is typically viewed as a Giant multinucleated Cell enclosed in an elastic membrane, the sarcolemma. The diameter of a functionally mature striated muscle fiber generally ranges from 10 to 100 µm, and its length often corresponds to the length of the muscle itself.

The sarcoplasm of muscle fibers contains various structures: Mitochondria, microsomes, Ribosomes, tubules and cisternae of the sarcoplasmic reticulum, diverse vacuoles, Glycogen granules, and lipid inclusions that serve as energy reserves, among others.

Within each muscle fiber, embedded in the semi-fluid sarcoplasm along its length, lie numerous thread-like formations known as myofibrils (typically less than 1 µm in thickness), often arranged in bundles and exhibiting the same transverse striation as the fiber as a whole. This transverse striation—caused by the optical heterogeneity of protein substances localized at the same level across all myofibrils—is easily observed when examining Skeletal Muscle fibers under a polarizing or phase-contrast microscope.

The repeating unit of a striated myofibril is the sarcomere, a segment of the myofibril bounded by narrow Z-lines. Each myofibril consists of several hundred sarcomeres, with an average sarcomere length of 2.5–3.0 µm. In the middle of the sarcomere lies a zone 1.5–1.6 µm long that appears dark under a phase-contrast microscope. In polarized light, it exhibits strong birefringence. This zone is designated as the A-band (anisotropic band). At the center of the A-band is the M-line, which can only be observed using an Electron microscope. The central portion of the A-band is occupied by the H-zone, which displays weaker birefringence. Finally, there are isotropic bands, or I-bands, with very weak birefringence; under a phase-contrast microscope, they appear lighter than the A-bands. The length of the I-bands is approximately 1 µm. Each I-band is divided into two equal halves by the Z-membrane, or Z-line. According to current models, the A-bands contain thick filaments composed primarily of the protein Myosin, as well as thin filaments generally consisting of the second component of the Actomyosin system, the protein Actin. Thin (actin) filaments originate near the Z-line within each sarcomere, extend through the I-band, penetrate into the A-band, and terminate at the edge of the H-zone.

Examination of thin muscle sections under an electron microscope revealed that these protein filaments are arranged in a strictly ordered manner. Thick filaments with a diameter of 12–16 nm and a length of roughly 1.5 µm are packed in a hexagonal pattern with a diameter of 40–50 nm, spanning the entire A-band. Interspersed between these thick filaments are thin filaments with a diameter of 8 nm, extending from the Z-line for a distance of about 1 µm (Fig. 3). Studies of contracting muscle have shown that the I-bands nearly vanish, while the region of overlap between the thick and thin filaments increases (in a contracting skeletal muscle, the sarcomere shortens to 1.7–1.8 µm).

Class="center">Fig. 3. Diagram of sarcomere structure

According to the model proposed by A. Huxley and R. Niedergerke, as well as H. Huxley and J. Hanson, during myofibrillar contraction one system of filaments penetrates the other—meaning the filaments begin to slide past one another, which drives Muscle contraction.

Mechanism of muscle Contraction

The transmission of excitation from a motor neuron to a muscle fiber is mediated by the neurotransmitter acetylcholine (ACh). The interaction of ACh with the cholinergic receptors of the motor endplate leads to the activation of ACh-gated channels and the generation of an endplate potential, which can reach up to 60 mV. As a result, the endplate region becomes a source of stimulating current for the muscle fiber membrane, triggering an Action Potential (AP) in the areas of The Cell membrane adjacent to the endplate that propagates in both directions at a velocity of approximately 3–5 m/s at 36 °C.

The Second Stage involves the Propagation of the AP deep into the muscle fiber via the transverse tubular (T) system, which acts as a connecting link between the surface membrane and the contractile apparatus. The T-system is in close contact with the terminal cisternae of the sarcoplasmic reticulum belonging to two adjacent sarcomeres. Electrical stimulation of this contact zone activates Enzymes localized there, leading to The production of Inositol trisphosphate. Inositol trisphosphate activates calcium channels in the membranes of the terminal cisternae, causing the release of Ca²⁺ ions from the cisternae and an increase in intracellular Ca²⁺ concentration from 10^-7 to 10^-5 M. The sequence of processes resulting in this elevated intracellular Ca²⁺ concentration constitutes the Third Stage of muscle contraction. Thus, the initial stages involve The conversion of the electrical AP signal into a chemical signal—the rise in intracellular Ca²⁺ concentration—representing electrochemical coupling.

Upon the increase in intracellular Ca²⁺ concentration, Tropomyosin shifts into the groove between the actin filaments, thereby exposing binding sites on the actin filaments where the cross-bridges of myosin can attach. This displacement of tropomyosin is driven by a conformational change in the troponin protein molecule upon binding Ca²⁺. Consequently, the involvement of Ca²⁺ ions in the actin-myosin interaction mechanism is mediated through troponin and tropomyosin.

The next step in electromechanical coupling is the attachment of the cross-bridge HEAD to the actin filament at the first of several sequentially arranged stable binding sites. The myosin head then pivots around its axis, as it possesses multiple active sites that interact sequentially with corresponding sites on the actin filament. This Rotation of the head increases the elastic tension in the neck of the cross-bridge and elevates muscle tension. At any given moment during The Development of a contraction, a fraction of the cross-bridge heads are bound to the actin filament while others are unbound, creating a coordinated sequence of interactions with the actin filament that ensures a smooth contraction process. The fourth and fifth stages comprise chemomechanical conversion.

The successive attachment and detachment cycles of the cross-bridge heads with the actin filament drive the sliding of thin and thick filaments relative to each other, reducing the size of the sarcomere and the overall length of the muscle, which constitutes the sixth stage. The totality of these described processes forms The basis of the sliding filament theory.

Mechanism of Muscle Relaxation

The mechanisms of muscle fiber shortening outlined above suggest that muscle relaxation requires, first and foremost, a decrease in the concentration of Ca²⁺ ions. It has been experimentally proven that the sarcoplasmic reticulum possesses a specialized mechanism—the calcium pump—which actively transports calcium back into the cisternae. The activation of the calcium pump is triggered by inorganic phosphate generated during ATP Hydrolysis, and the energy required for its operation is likewise derived from The energy released by ATP hydrolysis. Thus, ATP serves as the second critical factor absolutely essential for the relaxation process.

Furthermore, following muscle contraction, the Elastic properties of the thin myofilaments prompt them to return to their original resting position.

For some time after death, muscles remain soft due to the cessation of the tonic influence of motor Neurons. Subsequently, the ATP concentration drops below a critical level, and the myosin head can no longer detach from the actin filament. This leads to rigor mortis, characterized by pronounced skeletal muscle stiffness.

Structural Features of Smooth Muscles

The smooth muscles of Internal Organs differ significantly from skeletal muscles in their innervation, excitation, and contraction patterns. Waves of excitation and contraction occur at a very slow pace in smooth muscles. The development of an "fatigue-resistant" smooth muscle tone is associated, as in tonic skeletal fibers, with the sluggishness of the contraction waves, which merge even during rare rhythmic stimulation. Smooth muscles are also characterized by automatism, meaning The ability to function without nerve impulses arriving from the Central Nervous system. It has been established that not only the Nerve Cells present within smooth muscles, but also the smooth muscle cells themselves, possess the ability for rhythmic spontaneous excitation and contraction.

The unique contractile function of vertebrate smooth muscles is determined not only by their innervation and histological structure, but also by their specific chemical composition: a lower content of contractile Proteins (actomyosin) and high-energy compounds (particularly ATP), low myosin ATPase activity, and the presence of a Water-soluble modification of actomyosin known as tonoactomyosin, among other factors.

The ability of smooth muscles to change length without increasing tension (such as during the filling of hollow organs like the Urinary Bladder or Stomach) is of vital importance to the Organism.

Energetics of Muscle Activity

As noted previously, both phases of muscle activity—contraction and relaxation—obligatorily consume the energy released during ATP hydrolysis.

However, ATP reserves in muscle cells are negligible (at rest, the ATP concentration in muscles is about 5 mmol/L), which is only enough to sustain muscle work for 1–2 seconds. Therefore, to support prolonged muscular activity, ATP stores must be continuously replenished. The formation of ATP in muscle cells directly during physical work is called ATP resynthesis and proceeds with energy consumption.

Thus, during muscle functioning, two processes occur simultaneously: ATP hydrolysis, which provides the necessary energy for contraction and relaxation, and ATP resynthesis, which replenishes the depleted substance. While the chemical energy of ATP is used exclusively to drive muscle contraction and relaxation, the ATP resynthesis can utilize the chemical energy of A wide variety of compounds, including CARBOHYDRATES, fats, Amino Acids, and creatine phosphate.

Structure and Biological Role of ATP

Adenosine triphosphate (ATP) is a nucleotide. An ATP molecule consists of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphoric acid residues linked together by high-energy bonds. The hydrolysis of these bonds releases a large amount of energy. ATP is the cell's primary high-energy compound and acts as an energy accumulator in the form of high-energy chemical bonds.

Under physiological conditions—that is, the conditions found within a living cell—the Cleavage of one mole of ATP (506 g) is accompanied by the release of 12 kcal, or 50 kJ, of energy.

Pathways of ATP Generation

Aerobic Oxidation (Tissue Respiration)

Synonyms: Oxidative Phosphorylation, respiratory phosphorylation, aerobic phosphorylation.

This pathway takes place in the mitochondria.

The Tricarboxylic Acid Cycle was first discovered by the English biochemist H. Krebs (Fig. 4).

Fig. 4. Tricarboxylic acid cycle (Krebs cycle)

The first reaction is catalyzed by the enzyme citrate synthase, in which the acetyl group of acetyl-CoA condenses with oxaloacetate to form citric acid. Apparently, this reaction proceeds via an enzyme-bound citryl-CoA intermediate. Subsequently, the latter undergoes spontaneous and irreversible hydrolysis to yield citrate and HS-CoA.

In the second reaction, the resulting citric acid undergoes dehydration to form cis-aconitic acid, which then adds a water molecule to become isocitric acid (isocitrate). These reversible Hydration-dehydration reactions are catalyzed by the enzyme aconitate hydratase (aconitase). As a result, H and OH groups are interchanged within the citrate molecule.

The third reaction appears to rate-limit the Krebs cycle. Isocitric acid is dehydrogenated in the presence of NAD-dependent isocitrate dehydrogenase. During this isocitrate dehydrogenase reaction, isocitric acid is simultaneously decarboxylated. NAD-dependent isocitrate dehydrogenase is an allosteric enzyme that requires ADP as a specific activator. Additionally, the enzyme requires Mg²⁺ or Mn²⁺ ions for its activity.

The fourth reaction involves The oxidative decarboxylation of α-ketoglutaric acid to yield the high-energy compound succinyl-CoA. Mechanistically, this reaction is similar to the Oxidative Decarboxylation of Pyruvate to acetyl-CoA; the α-ketoglutarate dehydrogenase complex structurally resembles the pyruvate dehydrogenase complex. In both cases, five Coenzymes participate in the reaction: TPP, lipoamide, HS-CoA, FAD, and NAD⁺.

The fifth reaction is catalyzed by the enzyme succinyl-CoA synthetase. In this reaction, succinyl-CoA is converted into succinic acid (succinate) with the participation of GTP and inorganic phosphate. Concurrently, a high-energy GTP phosphate bond is formed at the expense of the high-energy thioester bond of succinyl-CoA.

In the sixth reaction, succinate is dehydrogenated to fumaric acid. The oxidation of succinate is catalyzed by succinate dehydrogenase, whose FAD coenzyme is tightly (covalently) bound to the protein molecule. In turn, succinate dehydrogenase is firmly anchored to The inner mitochondrial membrane.

The seventh reaction is catalyzed by the enzyme fumarate hydratase (fumarase). The resulting fumaric acid is hydrated, yielding malic acid (malate) as the reaction product.

Finally, during the eighth reaction of the tricarboxylic acid cycle, mitochondrial NAD-dependent malate dehydrogenase drives the oxidation of L-malate to oxaloacetate.

For every single turn of the cycle, the oxidation of one molecule of acetyl-CoA in the Krebs cycle and the oxidative phosphorylation system can generate 12 ATP molecules.

Anaerobic Oxidation

Synonyms: substrate-level phosphorylation, anaerobic ATP synthesis. This process takes place in the Cytoplasm, where the split-off hydrogen is transferred to another acceptor molecule. Depending on the substrate, There are two pathways of anaerobic ATP resynthesis: the creatine phosphate (creatine kinase, alactic) pathway and the glycolytic (Glycolysis, lactic) pathway. In the first case, the substrate is creatine phosphate; in the second, it is glucose.

These pathways operate without the involvement of oxygen.

Quantitative Criteria for ATP Resynthesis Pathways

To quantitatively characterize the various ATP resynthesis pathways, the following criteria are typically used:

a) maximum power, or maximum rate, which represents the maximum amount of ATP that can be produced per unit of time via a given resynthesis pathway;

b) activation time, defined as the minimum time required for ATP resynthesis to reach its maximum rate;

c) maintenance time, or endurance at maximum power, which is the maximum duration for which a given ATP resynthesis pathway can operate at peak power;

d) metabolic capacity, representing the total amount of ATP that can be generated during muscular work through a given ATP resynthesis pathway.

Aerobic Pathway of ATP Resynthesis

Maximum power is 350–450 cal/min·kg.

Activation time is 3–4 minutes (and can be around 1 minute in well-trained athletes).

The duration of work at maximum power spans tens of minutes.

Advantages: high efficiency, versatility in substrate utilization, and prolonged operational capacity.

Disadvantages: obligate oxygen consumption, requirement for an intact membrane, slow activation time, and relatively low maximum power.

Consequently, the muscular activity characteristic of most sports cannot be sustained solely by this ATP resynthesis pathway; muscles are thus compelled to additionally engage anaerobic mechanisms of ATP production, which feature faster activation times and greater maximum power.

In sports practice, the following indicators are frequently used to assess aerobic phosphorylation:

VO2 max (maximum oxygen uptake) is the maximum possible rate of oxygen consumption by the body during physical exertion.

AT (aerobic threshold) is the highest relative workload intensity, measured by oxygen consumption as a percentage of VO2 max.

AnT (anaerobic threshold) is the minimum relative workload intensity, measured by oxygen consumption as a percentage of VO2 max.

Oxygen intake is The amount of oxygen consumed during a given workload to support aerobic ATP resynthesis.

Creatine Phosphate Pathway of ATP Resynthesis

The energy source is creatine phosphate. It either turns into creatinine and is excreted from the body, or binds with ADP to form creatine and ATP.

The synthesis of Creatine phosphate in muscle cells occurs during rest through the interaction of creatine with excess ATP.

Creatine is produced in the Liver using Three amino acids: Glycine, Methionine, and Arginine.

Maximum power output reaches 900–1000 cal/min•kg.

Activation time is only 1–2 s.

The duration of work at maximum speed is merely 8–10 s, which is due to the small initial reserves of creatine phosphate in the muscles.

Advantages: extremely short activation time and high power output.

Disadvantage: very short operational lifespan.

The biochemical Assessment of the creatine phosphate pathway for ATP resynthesis is typically based on two indicators: the creatinine coefficient and the alactacid oxygen debt.

The creatinine coefficient is the daily urinary excretion of creatinine calculated per 1 kg of body weight.

The alactacid oxygen debt is the elevated oxygen consumption (above resting levels) During the first 4–5 minutes following a short-term maximal-effort exercise.

Glycolytic pathway of ATP resynthesis

Energy Sources: muscle Glycogen and Blood glucose.

Maximum power output: 750–850 cal/min•kg.

Activation time: 20–30 s.

Duration of work at maximum power: 2–3 min.

Advantages: does not require the involvement of mitochondria or oxygen.

Disadvantages: low metabolic efficiency, along with the production and accumulation of lactate.

Indicators of this pathway's activity include measuring blood and urine lactate concentrations after physical exertion, blood pH, blood alkaline reserve, and lactacid oxygen debt.

The alkaline reserve of blood refers to the alkaline components of all Blood Buffer Systems. When lactic acid enters the bloodstream during muscular work, it is initially neutralized through interaction with blood buffer systems (specifically their alkaline components), which leads to a decrease in the blood's alkaline reserve.

The lactacid oxygen debt is the elevated oxygen consumption observed during the first 1–1.5 hours following the cessation of muscular work. This excess oxygen is required to clear and process the lactic acid.

Adenylate kinase (myokinase) reaction

There are two Perspectives regarding the timing of this reaction. One view holds that the reaction occurs in muscles during fatigue. The second view is that it operates continuously in parallel with other ATP resynthesis pathways. The reaction is catalyzed by the enzyme adenylate kinase (myokinase). During this reaction, one ADP molecule transfers its phosphate group to another ADP molecule, resulting in the formation of ATP and AMP.

Interrelationship between various ATP resynthesis pathways during muscular work

During any type of muscular work, all three pathways of ATP resynthesis function simultaneously, though they are engaged sequentially. In the first seconds of work, ATP resynthesis is driven by the creatine phosphate reaction, followed by glycolysis, and finally, as work continues, tissue respiration takes over from glycolysis.

The specific contribution of each ATP production mechanism to the Energy supply of muscular movement depends on the intensity and duration of physical exertion.

Review Questions

1. What are the MAIN TYPES OF muscles distinguished in The Human Body?

2. What Organelles does a muscle cell contain?

3. What compounds are present in a myocyte?

4. WHAT IS A myofibril?

5. What does a sarcomere look like under a microscope?

6. What serves as the trigger for muscle contraction?

7. Why is ATP energy required for muscle contraction?

8. Is energy needed for muscle fiber relaxation?

9. Which organs incorporate smooth muscle in their structure?

10. What type of chemical compound is ATP?

11. What compounds are referred to as high-energy compounds?

12. How much energy is released during the hydrolysis of one mole of ATP under physiological conditions?

13. What are the primary consumers of ATP in the human body?

14. What pathways of ATP production are distinguished in the human body?

15. Define the term “tissue respiration”.

16. In which part of the cell do aerobic and anaerobic ATP synthesis take place?

17. What compound is the end product of anaerobic ATP synthesis?

18. Through which mechanisms are ATP reserves replenished?

19. List the Advantages and disadvantages of the aerobic pathway of ATP resynthesis.

20. What is the maximum duration, in minutes, of work at peak power supported by the aerobic pathway of ATP resynthesis?

21. List the advantages and disadvantages of the lactate pathway of ATP resynthesis.

22. What is the maximum power output of The Glycolytic Pathway of ATP resynthesis?

23. List the advantages and disadvantages of the alactate pathway of ATP resynthesis.

24. What is the deployment time of the creatine phosphate pathway for ATP resynthesis?