BIOCHEMISTRY: A TEXTBOOK FOR UNIVERSITIES - E. S. Severin - 2004

CHAPTER 7. CARBOHYDRATE METABOLISM

VIII. Glucose Catabolism

Glucose Catabolism is the primary source of energy for the body's vital metabolic processes.

A. Major PATHWAYS OF GLUCOSE Catabolism

Oxidation of glucose to CO₂ and H₂O (aerobic degradation). Aerobic glucose degradation can be expressed by the overall equation:

С₆Н₁₂О₆ + 6 О₂ —> 6 СО₂ + Н₂О + 2820 кДж/моль.

This process comprises several stages (Fig. 7-33).

Class="center">Fig. 7-33. Aerobic degradation of glucose. 1-10 — reactions of aerobic Glycolysis; 11 — malate-aspartate shuttle mechanism for hydrogen transport into Cell/35.html">Mitochondria; 2 (circled) — stoichiometric coefficient.

✵ Aerobic glycolysis — The process of glucose oxidation resulting in The formation of two Pyruvate molecules;

✵ The common pathway of catabolism, including The conversion of pyruvate to acetyl-CoA and its subsequent oxidation in The Tricarboxylic Acid Cycle;

✵ The Electron Transport Chain coupled to oxygen, linked with dehydrogenation reactions occurring during glucose degradation.

Anaerobic Degradation

Under certain conditions, oxygen supply to Tissues may fail to meet their metabolic demands. For instance, during the Initial Stages of intense muscular exertion or under stress, Heart rate may not increase sufficiently, while the oxygen demand of Muscles for aerobic glucose catabolism remains high. In such cases, a pathway is activated that proceeds in the absence of oxygen and culminates in the formation of lactate from pyruvic acid. This process is referred to as anaerobic degradation, or anaerobic glycolysis. Although anaerobic glucose degradation is energetically inefficient, it can become the sole source of energy for Muscle Cells under these circumstances. Later, once the oxygen supply to the muscles becomes adequate due to accelerated heart rate, anaerobic degradation shifts to aerobic METABOLISM. The pathways of glucose catabolism and their energy yields are illustrated in Fig. 7-34.

Fig. 7-34. Pathways of glucose catabolism. 1 — aerobic glycolysis; 2, 3 — common pathway of catabolism; 4 — aerobic degradation of glucose; 5 — anaerobic degradation of glucose (boxed); 2 (circled) — stoichiometric coefficient.

B. Aerobic Glycolysis

Aerobic glycolysis is defined as The oxidation of glucose to pyruvic acid in the presence of oxygen. All Enzymes catalyzing the reactions of this pathway are localized in The Cell Cytosol.

1. Stages of Aerobic Glycolysis

Aerobic glycolysis can be divided into two main stages.

1. The preparatory stage, during which glucose is phosphorylated and cleaved into two phosphotriose molecules. This series of reactions consumes 2 molecules of ATP.

2. The stage coupled with ATP synthesis. As a result of this series of reactions, phosphotrioses are converted into pyruvate. The energy released at this stage is utilized to synthesize 10 moles of ATP.

2. Reactions of Aerobic Glycolysis

Conversion of glucose-6-phosphate into 2 molecules of glyceraldehyde-3-phosphate

Glucose-6-phosphate, formed as a result of glucose phosphorylation involving ATP, is converted into fructose-6-phosphate in the subsequent reaction. This reversible isomerization reaction is catalyzed by the enzyme glucose phosphate isomerase.

This is followed by another phosphorylation reaction utilizing a phosphate residue and ATP energy. During this reaction, catalyzed by Phosphofructokinase, fructose-6-phosphate is converted into fructose-1,6-bisphosphate. Similar to the hexokinase reaction, this step is practically irreversible and represents the slowest reaction in glycolysis. The reaction catalyzed by phosphofructokinase determines the overall rate of glycolysis; therefore, regulating phosphofructokinase activity allows for the modulation of glucose catabolism rates.

Fructose-1,6-bisphosphate is subsequently cleaved into two triose phosphates: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. The reaction is catalyzed by the enzyme fructose bisphosphate aldolase, or simply aldolase. This enzyme catalyzes both aldol Cleavage and aldol Condensation reactions, making it reversible. The products of the aldol cleavage reaction are isomers. Since only glyceraldehyde-3-phosphate is utilized in subsequent glycolytic reactions, dihydroxyacetone phosphate is converted into glyceraldehyde-3-phosphate with the participation of the enzyme triose phosphate isomerase (Fig. 7-35).

Fig. 7-35. Conversion of glucose-6-phosphate into triose phosphates.

In the described series of reactions, phosphorylation involving ATP occurs twice. However, the consumption of two ATP molecules (per glucose molecule) is subsequently offset by the synthesis of a larger amount of ATP.

Conversion of glyceraldehyde-3-phosphate to pyruvate

This phase of aerobic glycolysis encompasses reactions associated with ATP synthesis. The most complex step in this series is the conversion of glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. This transformation represents the first oxidation reaction in glycolysis. The reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase, which is an NAD-dependent enzyme. The Significance of this reaction lies not only in the formation of a reduced coenzyme—whose oxidation in the Respiratory Chain is coupled with ATP synthesis—but also in the concentration of the Free energy of oxidation within a high-energy bond of the reaction product. Glyceraldehyde-3-phosphate dehydrogenase contains a Cysteine residue in its active center, whose sulfhydryl group directly participates in catalysis. Oxidation of glyceraldehyde-3-phosphate leads to NAD reduction and, with the participation of H₃PO₄, the formation of a high-energy anhydride bond at position 1 of 1,3-bisphosphoglycerate. In the next reaction, the high-energy phosphate is transferred to ADP to yield ATP. The enzyme catalyzing this transformation is named phosphoglycerate kinase based on the reverse reaction (Kinases are named after the substrate positioned on the same side of the equation as ATP). This series of reactions is illustrated in Fig. 7-36.

Fig. 7-36. Conversion of glyceraldehyde-3-phosphate to 3-phosphoglycerate.

ATP formation via this mechanism is independent of the Respiratory Chain and is referred to as substrate-level phosphorylation of ADP. The resulting 3-phosphoglycerate no longer contains a high-energy bond. Subsequent reactions involve intramolecular rearrangements designed to convert a low-energy phosphate ester into a compound containing a high-energy phosphate. These intramolecular transformations involve The transfer of the phosphate group from position 3 of phosphoglycerate to position 2. Next, a Water molecule is eliminated from the resulting 2-phosphoglycerate with the participation of the enzyme enolase. The dehydrating enzyme is named after the reverse reaction. This reaction yields a substituted enol—phosphoenolpyruvate. The resulting phosphoenolpyruvate is a high-energy compound whose phosphate group is transferred in the next reaction to ADP with the participation of pyruvate kinase (this enzyme is also named after the reverse reaction, in which pyruvate phosphorylation occurs, although such a reaction does not proceed in that direction physiologically).

The conversion of phosphoenolpyruvate to pyruvate is an irreversible reaction. This is the second substrate-level phosphorylation reaction in glycolysis. The resulting enol form of pyruvate then spontaneously (non-enzymatically) tautomerizes into the more thermodynamically stable keto form. The described series of reactions is presented in Fig. 7-37.

Fig. 7-37. Conversion of 3-phosphoglycerate to pyruvate.

The pathway of the 10 reactions occurring during aerobic glycolysis and the subsequent Oxidation of Pyruvate are shown in Fig. 7-33.

3. Oxidation of cytoplasmic NADH in the mitochondrial respiratory chain. Shuttle systems

NADH generated during the oxidation of glyceraldehyde-3-phosphate in aerobic glycolysis undergoes oxidation via the transfer of hydrogen atoms into the mitochondrial respiratory chain. However, cytosolic NADH cannot directly transfer hydrogen to the respiratory chain because the mitochondrial membrane is impermeable to it. Hydrogen is transported across the membrane via specialized systems known as shuttle systems. In these systems, hydrogen is ferried across the membrane with the participation of substrate pairs linked to corresponding dehydrogenases; that is, a specific dehydrogenase is located on both sides of the mitochondrial membrane. Two Shuttle systems are known. In the first system, hydrogen from cytosolic NADH is transferred to dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenase (an NAD-dependent enzyme, named after the reverse reaction). The glycerol-3-phosphate formed in this reaction is subsequently oxidized by an inner mitochondrial membrane enzyme—glycerol-3-phosphate dehydrogenase (an FAD-dependent enzyme). Protons and electrons from FADH₂ are then transferred to ubiquinone and subsequently along The electron transport chain (ETC) (Fig. 7-38).

Fig. 7-38. Glycerophosphate shuttle system. 1 — glyceraldehyde-3-phosphate dehydrogenase; 2 — glycerol-3-phosphate dehydrogenase (cytosolic enzyme, named after the reverse reaction); 3 — glycerol-3-phosphate dehydrogenase (mitochondrial flavin enzyme).

The glycerophosphate shuttle system operates in white Skeletal Muscle and hepatocyte cells. However, mitochondrial glycerol-3-phosphate dehydrogenase is absent in cardiac muscle cells. The second shuttle system, involving malate, cytosolic and mitochondrial malate dehydrogenases, is more universal. In the Cytoplasm, NADH reduces oxaloacetate to malate (Fig. 7-39, reaction 1), which crosses into the mitochondria via a transporter and is oxidized back to oxaloacetate by an NAD-dependent malate dehydrogenase (reaction 2). The NAD reduced during this reaction delivers hydrogen to the mitochondrial ETC. However, the oxaloacetate produced from malate cannot independently exit the mitochondria into the cytosol because the mitochondrial membrane is impermeable to it. Consequently, oxaloacetate is converted into aspartate, which is transported into the cytosol and reconverted into oxaloacetate. The conversions between oxaloacetate and aspartate involve the addition and removal of an amino group (Transamination reactions, see Section 9). This shuttle system is termed the malate-aspartate shuttle (Fig. 7-39). Its primary function is the regeneration of cytoplasmic NAD⁺ from NADH.

Fig. 7-39. Malate-aspartate shuttle system. 1, 2 — oxidation-reduction reactions ensuring hydrogen Transport from the cytosol to the mitochondrial ETC; 3, 4 — translocases mediating The transport of α-ketoglutarate, aspartate, and glutamate across the mitochondrial membrane.

Both shuttle systems differ significantly in The amount of ATP synthesized. In the first system, the P/O ratio is 2, as hydrogen enters the ETC at the level of CoQ. The second system is energetically more efficient because it transfers hydrogen into the ETC via mitochondrial NAD⁺, yielding a P/O ratio close to 3.

4. ATP balance in aerobic glycolysis and glucose degradation to CO₂ and H₂O

ATP yield in aerobic glycolysis

The Formation of fructose-1,6-bisphosphate from a single glucose molecule requires 2 molecules of ATP (reactions 1 and 3 in Fig. 7-33). The reactions associated with ATP synthesis occur after glucose is split into 2 phosphotriose molecules, i.e., during the Second Stage of glycolysis. At this stage, 2 Substrate-Level Phosphorylation Reactions take place, synthesizing 2 molecules of ATP (reactions 7 and 10). In addition, one molecule of glyceraldehyde-3-phosphate is dehydrogenated (reaction 6), and NADH transfers hydrogen to the Mitochondrial Electron Transport chain (ETC), where 3 molecules of ATP are synthesized via Oxidative Phosphorylation. In this case, the amount of ATP (3 or 2) depends on the type of shuttle system. Consequently, the oxidation of one molecule of glyceraldehyde-3-phosphate to pyruvate is coupled with the synthesis of 5 ATP molecules. Given that glucose yields 2 phosphotriose molecules, this value must be multiplied by 2 and then reduced by the 2 ATP molecules consumed in The First stage. Thus, the net ATP yield in aerobic glycolysis is (5 x 2) - 2 = 8 ATP.

ATP yield during the aerobic breakdown of glucose to end products

Glycolysis yields pyruvate, which is subsequently oxidized to CO₂ and H₂O in the tricarboxylic acid cycle and ETC, as described in Section 6. We can now evaluate the energetic efficiency of glycolysis and oxidative metabolism, which together comprise the aerobic degradation of glucose to its end products (Table 7-4).

Table 7-4. Stages of aerobic glucose degradation

Thus, the ATP yield from the oxidation of 1 mol of glucose to CO₂ and H₂O is 38 mol of ATP.

The aerobic degradation of glucose involves 6 dehydrogenation reactions: one occurs in glycolysis and 5 in the oxidative pathway (see Section 6). The substrates for specific NAD-dependent dehydrogenases are glyceraldehyde-3-phosphate, pyruvate, isocitrate, α-ketoglutarate, and malate. One dehydrogenation reaction in The Citric Acid Cycle is catalyzed by succinate dehydrogenase and utilizes the FAD coenzyme. The total amount of ATP synthesized via Oxidative phosphorylation is 17 mol per 1 mol of glyceraldehyde phosphate. To this, we must add 3 mol of ATP synthesized via substrate-level phosphorylation (two reactions in glycolysis and one in the citric acid cycle).

Given that glucose splits into 2 phosphotrioses and that the stoichiometric coefficient for subsequent transformations is 2, the resulting value must be multiplied by 2, and the 2 mol of ATP consumed in the first stage of glycolysis subtracted from the result.

B. Anaerobic breakdown of glucose (anaerobic glycolysis)

Anaerobic glycolysis refers to The breakdown of glucose resulting in the formation of lactate as the end product. This process proceeds without oxygen and is therefore independent of the mitochondrial respiratory chain. ATP is generated through substrate-level phosphorylation reactions. The overall equation for the process is:

С₆Н₁₂О₆ + 2 Н₃РО₄ + 2 АДФ = 2 С₃Н₆О₃ + 2 АТФ + 2 Н₂О.

1. Reactions of anaerobic glycolysis

During anaerobic glycolysis (Fig. 7-40), all 10 reactions identical to aerobic glycolysis take place in the cytosol. Only the 11th reaction, involving the reduction of pyruvate by cytosolic NADH, is specific to anaerobic glycolysis (Fig. 7-41). The reduction of pyruvate to lactate is catalyzed by Lactate dehydrogenase (this reaction is reversible, and the enzyme is named after the reverse reaction). This reaction ensures the regeneration of NAD⁺ from NADH independently of the mitochondrial respiratory chain during conditions of inadequate cellular oxygen supply. Pyruvate serves as the hydrogen acceptor from NADH (much like oxygen in the respiratory chain). Thus, the physiological significance of pyruvate reduction lies not in lactate production itself, but in the fact that this cytosolic reaction ensures the regeneration of NAD⁺. Furthermore, lactate is not merely a metabolic end product to be eliminated from the body; it is released into the Blood and utilized—either converted back to glucose in the Liver or, when oxygen is available, converted into pyruvate to enter the general catabolic pathway, being oxidized to CO₂ and H₂O. The Structure of lactate dehydrogenase, its MECHANISM OF ACTION, and the Diagnostic significance of measuring its activity were discussed earlier in Section 2.

Fig. 7-40. Anaerobic glycolysis.

Fig. 7-41. Reduction of pyruvate to lactate.

ATP balance in anaerobic glycolysis

Anaerobic glycolysis is less efficient than aerobic glycolysis. In this process, the catabolism of 1 mol of glucose without the involvement of the mitochondrial respiratory chain is accompanied by the synthesis of 2 mol of ATP and 2 mol of lactate. ATP is generated via 2 substrate-level phosphorylation reactions. Since glucose breaks down into 2 phosphotrioses, accounting for a stoichiometric coefficient of 2, the number of moles of synthesized ATP is 4. Accounting for the 2 mol of ATP consumed in the initial stage of glycolysis, we obtain a net energetic yield of 2 mol of ATP. Thus, the 10 cytosolic enzymes catalyzing the conversion of glucose to pyruvate, together with lactate dehydrogenase, provide for the synthesis of 2 mol of ATP (per 1 mol of glucose) in anaerobic glycolysis without the involvement of oxygen.

C. Significance of glucose catabolism

The primary physiological purpose of glucose catabolism is to harness the energy released during this process for the synthesis of ATP.

The energy released during the complete degradation of glucose to CO₂ and H₂O is 2880 kJ/mol. Comparing this value with the energy of high-energy bond Hydrolysis—38 mol of ATP (50 kJ per mol of ATP)—we get: 50 x 38 = 1900 kJ, which represents 65% of the total energy released during the complete breakdown of glucose. This is the efficiency of utilizing glucose breakdown energy for ATP synthesis. It should be noted that the actual efficiency of the process may be lower. The ATP yield can be precisely estimated only for substrate-level phosphorylation, whereas the ratio between hydrogen delivery to the respiratory chain and ATP synthesis is approximate.

The aerobic breakdown of glucose occurs in many Organs and tissues and serves as the primary, though not exclusive, source of energy for vital activity. Certain tissues are highly dependent on glucose catabolism as an energy source. For instance, Brain cells consume up to 100 g of glucose per day, oxidizing it aerobically. Consequently, an inadequate supply of glucose to the brain or Hypoxia manifests as symptoms indicating impaired brain function (dizziness, seizures, loss of consciousness).

The anaerobic breakdown of glucose occurs in muscles During the first minutes of physical exertion, in erythrocytes (which lack mitochondria), and in various organs under conditions of limited oxygen supply, including tumor cells. Tumor cell metabolism is characterized by an acceleration of both aerobic and anaerobic glycolysis. However, preferential anaerobic glycolysis and increased lactate synthesis serve as indicators of an elevated Cell Division rate coupled with insufficient blood vessel supply.

In addition to its energetic function, glucose catabolism can also fulfill anabolic Functions. Glycolysis metabolites are utilized for the synthesis of new compounds. For example, fructose-6-phosphate and glyceraldehyde-3-phosphate participate in the formation of ribose-5-phosphate, a structural component of NUCLEOTIDES; 3-phosphoglycerate can be channeled into the synthesis of Amino Acids such as Serine, Glycine, and cysteine (see Section 9). In The Liver and adipose tissue, acetyl-CoA derived from pyruvate serves as a substrate for the Biosynthesis of Fatty acids and Cholesterol, while dihydroxyacetone phosphate acts as a substrate for the synthesis of glycerol-3-phosphate (see Section 8).

D. REGULATION OF GLUCOSE Catabolism

Since the primary purpose of glycolysis is ATP synthesis, its rate must correlate with the body's energy demands.

Most glycolytic reactions are reversible, except for three catalyzed by hexokinase (or glucokinase), phosphofructokinase, and pyruvate kinase. Regulatory factors that alter The rate of glycolysis—and consequently ATP production—target these irreversible reactions. The accumulation of ADP and AMP serves as an indicator of ATP consumption. The latter is formed in the reaction catalyzed by adenylate kinase: 2 ADP <-> AMP + ATP

Even a minor consumption of ATP leads to a noticeable increase in AMP. The ratio of ATP to ADP and AMP levels characterizes the energy status of the cell (see Section 6), and its components act as Allosteric regulators of the rate of both the general catabolic pathway and glycolysis. Figure 7-42 illustrates the Allosteric Regulation of glucose catabolism rate in skeletal muscle.

Fig. 7-42. Regulation of glucose catabolism in skeletal muscle.

Changes in phosphofructokinase activity are crucial for the Regulation of glycolysis because this enzyme, as mentioned earlier, catalyzes the slowest reaction in the pathway.

Phosphofructokinase is activated by AMP and inhibited by ATP. By binding to the allosteric site of phosphofructokinase, AMP increases the enzyme's affinity for fructose-6-phosphate and accelerates its phosphorylation. The Effect of ATP on this enzyme is an example of homotropic allosterism (see Section 2), since ATP can interact with both the allosteric site and the Active Site, in the latter case acting as a substrate.

At physiological ATP levels, the Active Site of phosphofructokinase is always saturated with substrates (including ATP). An increase in the ATP-to-ADP ratio slows down the reaction, because under these conditions ATP acts as an inhibitor: it binds to the allosteric site of the enzyme, induces conformational changes, and decreases its affinity for its substrates.

Changes in phosphofructokinase activity help regulate the rate of glucose phosphorylation by hexokinase. A decrease in phosphofructokinase activity at high ATP levels leads to the accumulation of both fructose-6-phosphate and glucose-6-phosphate, with the latter inhibiting hexokinase. It should be recalled that in many tissues (except for the liver and pancreatic β-cells), hexokinase is inhibited by glucose-6-phosphate.

When ATP levels are high, the rates of the citric acid cycle and the respiratory chain decrease. Under these conditions, glycolysis also slows down. It is worth noting that the allosteric Regulation of the Enzymes of the common catabolic pathway and the respiratory chain is likewise linked to Changes in the concentrations of such key products as NADH, ATP, and certain metabolites. For instance, NADH, when it accumulates because it cannot be oxidized quickly enough in the respiratory chain, inhibits several allosteric enzymes of the tricarboxylic acid cycle (see Section 6).

The Physiological Role of glycolysis in the liver and adipose tissue differs somewhat from that in other tissues. In the liver and adipose tissue, during Digestion, glycolysis functions primarily as a source of substrates for lipid synthesis. The regulation of glycolysis in the liver has specific features and will be discussed later.

E. The 2,3-Bisphosphoglycerate Cycle

An auxiliary reaction can occur in The Glycolytic Pathway, catalyzed by bisphosphoglycerate mutase, which converts 1,3-bisphosphoglycerate into 2,3-bisphosphoglycerate (2,3-BPG). This intermediate can then be converted into 3-phosphoglycerate—a glycolytic metabolite—with the participation of 2,3-bisphosphoglycerate phosphatase (Fig. 7-43).

Fig. 7-43. Formation and conversion of 2,3-bisphosphoglycerate.

In most tissues, 2,3-BPG is produced in small amounts. In erythrocytes, however, this metabolite is synthesized in significant quantities and serves as an allosteric regulator of Hemoglobin function. By binding to hemoglobin, 2,3-BPG lowers its oxygen affinity, facilitating oxygen dissociation and its release into tissues (see Section 1).

The formation of 2,3-BPG entails the loss of the high-energy bond in 1,3-bisphosphoglycerate; this energy is not transferred to ATP but is instead dissipated as heat, which means a reduction in the overall energy yield of glycolysis.