Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Carbohydrates
Intracellular Carbohydrate Metabolism

Intracellular Carbohydrate METABOLISM includes the Synthesis and Breakdown of Glycogen in skeletal Muscles and the Liver, the Breakdown and Oxidation of glucose with energy release, and the de novo Synthesis of glucose from non-carbohydrate precursors.

Glycogen Biosynthesis — Carbohydrate Deposition

Excess glucose entering via the bloodstream is stored as a reserve, primarily in The Liver and skeletal muscles. The synthesis and accumulation of glycogen is known as carbohydrate deposition. Glycogen serves as the body's primary carbohydrate energy reserve. The duration of sustained Muscle work depends directly on its reserves in skeletal muscles and the liver; therefore, specialized Methods for Enhancing tissue glycogen storage are widely employed in sports practice.

The synthesis of glycogen from glucose molecules is mediated by glycogen synthase, utilizing uridine triphosphate (UTP) as an energy source. This process is preceded by several glucose conversion reactions, as illustrated in Fig. 62. Catalyzed by hexokinase and ATP, glucose is converted into glucose-6-phosphate, which is subsequently isomerized to glucose-1-phosphate by phosphoglucomutase. From glucose-1-phosphate, in the presence of UTP, the active form UDP-glucose is synthesized, which is then attached to the terminal glycogen residue by glycogen synthase:

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Fig. 62. Scheme of glycogen biosynthesis

Thus, the glycogen molecule is extended by a single glucose residue. Glycogen synthesis via glycogen synthase requires a pre-existing trace amount of glycogen, known as a primer. As the glycogen chain elongates, the enzyme's activity increases. Branching of the molecule is catalyzed by the transglycosylase enzyme.

Glycogen synthesis requires the expenditure of ATP, UTP, and pyrophosphate energy. The addition of just one glucose residue to the growing glycogen molecule consumes 41 kJ of energy. Consequently, this process proceeds intensively in Tissues under aerobic conditions, fueled by ATP generated through Oxidative Phosphorylation. In the liver, glycogen accumulates during periods of abundant Nutrition, whereas in muscles, it accumulates after the depletion of existing reserves, such as following prolonged physical exertion. The highest rate of glycogen synthesis is observed during the recovery period, specifically 30–40 minutes following carbohydrate ingestion. This factor must be taken into account when planning nutritional strategies during competition to ensure that enhanced anabolic synthesis does not impair glycogen mobilization during muscular activity.

Glycogen synthesis is upregulated by the pancreatic hormone Insulin, which regulates cellular glucose uptake, and is inhibited by adrenaline due to its suppressive effect on glycogen synthase activity. The concentration of glycogen in the liver and other tissues can only increase up to a certain threshold, as high intracellular concentrations inhibit glycogen synthase activity.

Glycogen Breakdown — Carbohydrate Mobilization

The process of degrading glycogen into glucose molecules is termed carbohydrate mobilization. Glycogenolysis occurs primarily via phosphorolysis, mediated by Glycogen phosphorylase in the presence of phosphoric acid (H₃PO₄). A glucose molecule is cleaved from the glycogen polymer in the form of glucose-1-phosphate:

The resulting glucose-1-phosphate is rapidly converted into glucose-6-phosphate. In the liver, it is hydrolyzed by Phosphatases into free glucose and phosphoric acid. Free glucose molecules readily enter the bloodstream and serve as an energy substrate for numerous tissues throughout the body. Skeletal muscles lack such phosphatases; hence, glycogen stored in Muscle tissue is utilized exclusively for local energy demands.

The rate of glycogen breakdown in muscles depends on their functional activity, whereas in the liver, it is regulated by Blood glucose levels. During muscular activity, the rate of hepatic glycogen mobilization depends on workload intensity: moderate work increases the rate 2- to 3-fold, whereas intense work accelerates it 7- to 10-fold compared to resting levels.

Glycogen degradation in the liver continues into the rest period. The resulting glucose contributes to the restoration of glycogen reserves in cardiac and skeletal muscles, effectively redistributing CARBOHYDRATES among various tissues.

Glycolysis

The extraction of metabolic energy from carbohydrates occurs in nearly all Cells of The Human Body and comprises two primary phases: anaerobic oxidation (glycolysis), which takes place in the Cytosol, predominantly in skeletal muscles; and aerobic oxidation, which occurs within the Cell/35.html">Mitochondria via the Enzymes of The Citric Acid Cycle and the Electron Transport Chain.

Glycolysis is the stepwise breakdown of a glucose or glycogen molecule (glycogenolysis) into two molecules of pyruvic acid, which under anaerobic conditions is further reduced to lactic acid. It comprises ten Chemical Reactions, depicted in Fig. 63, and is divided into two main stages: the preparatory (investment) phase and the oxidative (payoff) phase. In the preparatory stage, a glucose molecule is progressively cleaved into two molecules of 3-phosphoglyceraldehyde, consuming two ATP molecules in the process. In the oxidative stage, these intermediates undergo further oxidation to yield Pyruvate and four ATP molecules. Glycolysis begins with the ATP-dependent activation of glucose to form glucose-6-phosphate, or via glycogen phosphorolysis yielding glucose-1-phosphate. The phosphorylation of glucose is catalyzed by hexokinase and requires Mg²⁺ ions. Hexokinase is a regulatory allosteric enzyme whose activity is modulated by intracellular ATP levels. At low ATP concentrations, the enzyme is active, whereas at high concentrations, it is inhibited, halting glycolysis when energy demands are low. Subsequently, glucose-6-phosphate is isomerized to fructose-6-phosphate by glucose phosphate isomerase. Fructose-6-phosphate is then phosphorylated using ATP energy, yielding fructose-1,6-bisphosphate in a reaction catalyzed by Phosphofructokinase (PFK).

Fig. 63. Scheme of glycolysis (a) and its chemical reactions (b)

Phosphofructokinase is a key allosteric enzyme governing the rate of glycolysis. Its activity is regulated by the concentrations of ATP and other metabolites (such as lactic acid and citrate). In resting muscles, ATP concentrations are relatively high, keeping glycolysis suppressed. During Muscle contraction, ATP is rapidly consumed, which relieves inhibition, increases PFK activity, and accelerates glycolysis. However, the accumulation of lactic acid—the end product of anaerobic glycolysis—inhibits this enzyme, thereby limiting the overall rate of glycolysis.

The first phase of glycolysis concludes with the Cleavage of fructose-1,6-bisphosphate into two triose phosphates: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, a reaction catalyzed by aldolase. These resulting trioses are isomers capable of interconversion. Two molecules of glyceraldehyde 3-phosphate enter the subsequent reactions of glycolysis.

The oxidative phase begins with The oxidation of glyceraldehyde 3-phosphate in the presence of phosphoric acid and an NAD-dependent dehydrogenase. In this reaction, the coenzyme NAD accepts hydrogen and is reduced to NADH₂. Under aerobic conditions, NADH₂ can transfer its hydrogen to oxygen, yielding 3 ATP molecules. The resulting 1,3-bisphosphoglycerate contains a high-energy bond and is capable of participating in a transphosphorylation reaction with ADP, producing ATP and 3-phosphoglyceric acid. This Mechanism of ATP synthesis is termed substrate-level phosphorylation and is catalyzed by phosphoglycerate kinase.

Catalyzed by phosphoglyceromutase, 3-phosphoglyceric acid is converted into 2-phosphoglyceric acid. The latter loses a Water molecule in a reaction facilitated by enolase, yielding phosphoenolpyruvate. Through an intramolecular redox process at the second carbon atom of this compound, a high-energy phosphate bond is formed. Upon its cleavage—mediated by pyruvate kinase—the phosphate group is transferred from phosphoenolpyruvate to ADP (the second substrate-level phosphorylation), culminating in The formation of two molecules of pyruvic acid and two molecules of ATP.

Under anaerobic conditions, glycolysis concludes with the reduction of pyruvic acid to lactic acid, catalyzed by the enzyme Lactate dehydrogenase. Hydrogen is supplied by NADH₂ molecules generated during the oxidation of 3-phosphoglyceraldehyde. Thus, the final product of anaerobic glycolysis is lactic acid. Under aerobic conditions, pyruvic acid does not convert into lactic acid but is further oxidized in The Citric Acid cycle into the final metabolic products. The overall equation for glycolysis can be represented as follows:

During glycolysis, 196 kJ of energy is gradually released. Most of it is dissipated as heat (135 kJ), while a smaller portion is stored in the high-energy bonds of two ATP molecules. The energy storage efficiency in the form of ATP during glycolysis is 40%. The majority of the energy stored within the glucose molecule (2880 kJ) remains in the glycolysis product—two molecules of lactic acid—and can only be released through their aerobic oxidation. Glycolysis also produces numerous substances required for plastic processes within cells. A particularly large amount of lactic acid accumulates in the process; it rapidly diffuses from skeletal muscles into the blood, affecting the body's acid-base balance. The blood lactate level only partially reflects the intensity of glycolysis in muscles because some of the acid is metabolized locally. Normally, blood lactic acid concentration ranges from 1 to 1.5 mmol ∙ L^-1.

In an aqueous environment, lactic acid dissociates into a hydrogen ion (H⁺) and an acid residue anion:

In aqueous solution, the acid residue anion of lactic acid can interact with metal cations (Na⁺, K⁺) to form salts known as lactates. Consequently, lactic acid is frequently referred to as lactate.

Glycolysis and glycogenolysis play a crucial role in anaerobic muscular activity. They provide the energy required for intense exercise lasting anywhere from 30 seconds to 2–5 minutes.

Lactic acid is produced in muscles at its highest rate during 40–45 seconds of intense physical exertion due to the maximal engagement of anaerobic glycolysis. Blood lactate levels increase 4-fold to 5-fold under these conditions and, within 1–5 minutes following strenuous work, can reach 10 mmol ∙ L^-1 or more. This leads to acidification of the body's internal environment (acidosis). If buffer capacities are exhausted, blood pH may drop from 7.34 down to 7.0 or even 6.8 during exhaustive exercise. Such acidification impacts the Functions of The Nervous system and skeletal muscles, diminishing performance and inducing fatigue. Because blood lactic acid concentration depends on glycolytic intensity in muscles—which, in turn, is driven by exercise intensity and the individual's training status—blood lactate levels are widely used in the Biochemical Monitoring of an athlete's functional state (see Chapter 24).

Following the cessation of activity, about 55–70% of lactic acid is utilized by tissues, including muscles, as an energy source, about 5–7% is excreted in the urine, and the remainder is used by the liver for Gluconeogenesis and glycogen replenishment in muscles. The clearance of lactic acid from muscles and its subsequent oxidation after exercise are accelerated by active recovery.

Aerobic Oxidation of Carbohydrates

Aerobic oxidation of glucose is a multi-step process breaking down its molecule into the final metabolic products CO₂ and H₂O, yielding 38 ATP molecules and releasing thermal energy (Fig. 64). It proceeds in the presence of oxygen, which is delivered to tissues by the protein Hemoglobin. This carbohydrate oxidation pathway is one of the primary mechanisms of ATP generation in body tissues. It comprises the following main stages:

✵ glycolytic breakdown of the glucose molecule into two molecules of pyruvic acid (PA);

✵ conversion of PA into acetyl-CoA;

✵ oxidation of acetyl-CoA in the citric acid cycle and The electron transport chain.

The breakdown of glucose into pyruvic acid proceeds identically under both anaerobic and aerobic conditions, as described above (see "Glycolysis"). The pyruvic acid produced during the glycolytic stage then undergoes oxidative decarboxylation, yielding the high-energy compound acetyl-CoA, reduced NADH₂, and one molecule of CO₂. If lactic acid was formed during glycolysis, it is converted back into pyruvic acid under aerobic conditions.

Acetyl-CoA then enters the citric acid cycle, where it is broken down into CO₂ and H₂O. Water is formed within the respiratory enzyme system when hydrogen—generated during Biological Oxidation REACTIONS—reacts with the atomic oxygen of inhaled air. The overall equation for the aerobic oxidation of a glucose molecule can be represented as follows:

The chemical reactions involved in the oxidation of acetyl-CoA in the citric acid cycle are discussed in Chapter 3 (see Fig. 18).

Energy Efficiency of AEROBIC GLUCOSE OXIDATION

During the Complete oxidation of a single glucose molecule to CO₂ and H₂O, Energy is stored in the form of 10 NADH₂ (2 of which are produced during glycolysis, 2 during The conversion of PA to acetyl-CoA, and 6 in the citric acid cycle), as well as 2 FADH₂, 2 GTP molecules (equivalent to ATP), and 2 ATP generated directly in glycolysis (see Fig. 64). The transfer of hydrogen along the Respiratory Chain from NADH₂ to oxygen is coupled with the synthesis of three ATP molecules, whereas transfer from FADH₂ yields two ATP molecules. Consequently, 10 NADH₂ generate 30 ATP, and 2 FADH₂ generate 4 ATP. The total net yield of ATP per glucose molecule is 38 ATP (Table 15).

Fig. 64 Diagram of aerobic glucose oxidation and the energy efficiency of the process

TABLE 15 Energy accumulation at individual stages of glucose molecule oxidation

However, in muscle and nerve tissues, the 2 NADH₂ molecules produced in the Cytoplasm during glycolysis do not enter the mitochondria directly; instead, they transfer their hydrogen to the FAD carrier. Consequently, hydrogen enters the respiratory chain from 2 FADH₂ rather than NADH₂, resulting in the synthesis of only 4 ATP instead of 6. Therefore, complete oxidation of a glucose molecule in Skeletal Muscle yields 36 ATP.

Aerobic glucose metabolism is 19 (18) times more efficient in terms of ATP yield than Anaerobic Metabolism. It has a high efficiency rate (around 45%), since out of the 2,880 kJ of Free energy released during glucose oxidation, 1,311 kJ is stored in ATP. Aerobic carbohydrate oxidation is the primary mechanism for meeting energy demands during prolonged aerobic muscle activity lasting several hours.

The Pentose Phosphate Pathway of Carbohydrate Oxidation

In certain body tissues (such as the liver, erythrocytes, and adipose tissue), an alternative aerobic pathway for the direct oxidation of carbohydrates, known as The pentose phosphate cycle, can also occur. This cycle generates energy in the form of NADPH₂, which is utilized in biosynthetic processes. It also produces pentoses (such as ribose) essential for nucleotide synthesis (ATP, NAD, FAD, and Nucleic Acids), as well as glyceraldehyde, which can be converted into pyruvic acid or enter glycolysis. The overall equation for the pentose phosphate cycle is as follows:

6 glucose-6-phosphate + 12NADP + 7H₂O →

→ 5 glucose-6-phosphate + 12NADPH₂ + 6CO₂ + H₃PO₄

During this cycle, a glucose-6-phosphate molecule is completely oxidized to CO₂. The four MAIN MECHANISMS OF the pentose phosphate pathway of carbohydrate oxidation are illustrated in Fig. 65.

Fig. 65. Four mechanisms of the pentose phosphate pathway of carbohydrate oxidation

Gluconeogenesis

The de novo synthesis of glucose in body tissues from non-carbohydrate precursors is called gluconeogenesis. Glucose can be synthesized from pyruvic and lactic acids, as well as from acetyl-CoA, glycerol, and Amino Acids (Fig. 66). All of these precursors, except glycerol, pass through a pyruvic acid formation stage. Many gluconeogenesis reactions are the reverse of the corresponding reactions in glycolysis. However, there are additional bypass reactions, such as the formation of phosphoenolpyruvate and pyruvic acid.

The process of glucose synthesis actively occurs in the liver, Kidneys, and, during physical exertion, in skeletal muscles. This process prevents a drastic drop in blood glucose and liver glycogen levels (for example, during prolonged muscular work).

The conversion of lactic acid into glucose—known as the Cori cycle—plays a vital role in restoring glucose and glycogen levels in the liver and skeletal muscles. Likewise, the gradual conversion of many amino acids into glucose molecules via Alanine is facilitated by another cycle: the glucose-alanine cycle (Fig. 67).

A key feature of these cycles is that lactic acid and alanine, produced in skeletal muscles, are transported via the bloodstream to the liver, where they are converted into glucose. This is an energy-dependent process. From the liver, glucose enters the bloodstream and is reused by the muscles to replenish glycogen reserves. This mechanism is of critical importance during physical activity, as it prevents a sharp depletion of muscle glycogen stores and blood glucose levels.