BELARUSIAN STATE UNIVERSITY OF INFORMATICS AND RADIO ELECTRONICS
Department of ETT
« Aerobic oxidation of carbohydrates. Biological oxidation and reduction"
MINSK, 2008
Aerobic oxidation of carbohydrates- the main way of energy production for the body. Indirect - dichotomous and direct - apotomic.
The direct pathway of glucose breakdown is pentose cycle– leads to the formation of pentoses and the accumulation of NADPH 2. The pentose cycle is characterized by the sequential elimination of each of its 6 carbon atoms from glucose molecules with the formation of 1 molecule of carbon dioxide and water during one cycle. The breakdown of the entire glucose molecule occurs over 6 repeating cycles.
The importance of the pentose phosphate cycle of carbohydrate oxidation in metabolism is great:
1. It supplies reduced NADP, necessary for the biosynthesis of fatty acids, cholesterol, etc. Due to the pentose cycle, 50% of the body's need for NADPH 2 is covered.
2. Supply of pentose phosphates for synthesis nucleic acids and many coenzymes.
The reactions of the pentose cycle occur in the cytoplasm of the cell.
In a number of pathological conditions, the proportion of the pentose pathway of glucose oxidation increases.
Indirect path– breakdown of glucose to carbon dioxide and water with the formation of 36 molecules of ATP.
1. Breakdown of glucose or glycogen to pyruvic acid
2. Conversion of pyruvic acid to acetyl-CoA
Oxidation of acetyl-CoA in the Krebs cycle to carbon dioxide and water
C 6 H 12 O 6 + 6 O 2 ® 6 CO 2 + 6 H 2 O + 686 kcal
In the case of aerobic conversion, pyruvic acid undergoes oxidative decarboxylation to form acetyl-CoA, which is then oxidized to carbon dioxide and water.
The oxidation of pyruvate to acetyl-CoA is catalyzed by the pyruvate dehydrogenase system and occurs in several stages. Total reaction:
Pyruvate + NADH + NS-CoA ® acetyl-CoA + NADH 2 + CO 2 reaction is almost irreversible
Complete oxidation of acetyl-CoA occurs in the tricarboxylic acid cycle or Krebs cycle. This process takes place in mitochondria.
The cycle consists of 8 consecutive reactions:
In this cycle, a molecule containing 2 carbon atoms (acetic acid in the form of acetyl-CoA) reacts with a molecule of oxalic acid acetic acid, resulting in the formation of a compound with 6 carbon atoms - citric acid. During the process of dehydrogenation, decarboxylation and preparatory reaction, citric acid is converted back into oxaloacetic acid, which easily combines with another acetyl-CoA molecule.
1) acetyl-CoA + oxaloacetate (SCHUK) ®citric acid
citrate synthase
2) citric acid® isocitric acid
aconitate hydratase
3) isocitric acid + NAD®a-ketoglutaric acid + NADH 2 + CO 2
isocitrate dehydrogenase
4) a-ketoglutaric acid + NS-CoA + NAD®succinylSCoA + NADH 2 + CO 2
5) succinyl-CoA+GDP+Fn®succinic acid+GTP+HS-CoA
succinyl CoA synthetase
6) succinic acid+FAD®fumaric acid+FADN 2
succinate dehydrogenase
7) fumaric acid + H 2 O® L malic acid
fumarate hydratase
8) malate + NAD®oxaloacetate + NADH 2
malate dehydrogenase
In total, when a glucose molecule is broken down in tissues, 36 ATP molecules are synthesized. Undoubtedly, this is an energetically more efficient process than glycolysis.
The Krebs cycle is the common final pathway by which the metabolism of carbohydrates, fatty acids and amino acids is completed. All these substances are included in the Krebs cycle at one stage or another. Next, biological oxidation or tissue respiration occurs, main feature which is that it occurs gradually, through numerous enzymatic stages. This process occurs in mitochondria, cellular organelles in which a large number of enzymes are concentrated. The process involves pyridine-dependent dehydrogenases, flavin-dependent dehydrogenases, cytochromes, coenzyme Q - ubiquinone, proteins containing non-heme iron.
The rate of respiration is controlled by the ATP/ADP ratio. The lower this ratio, the more intense respiration occurs, ensuring the production of ATP.
Also, the citric acid cycle is the main source of carbon dioxide in the cell for carboxylation reactions, which begin the synthesis of fatty acids and gluconeogenesis. The same carbon dioxide supplies carbon for urea and some units of the purine and pyrimidine rings.
The relationship between the processes of carbohydrate and nitrogen metabolism is also achieved through intermediate products of the citric acid cycle.
There are several pathways through which citric acid cycle intermediates are incorporated into the process of lipogenesis. The breakdown of citrate leads to the formation of acetyl-CoA, which plays the role of a precursor in the biosynthesis of fatty acids.
Isocitrate and malate provide the formation of NADP, which is consumed in the subsequent reductive stages of fat synthesis.
The role of the key factor determining the conversion of NADH is played by the state of adenine nucleotides. High ADP and low ATP indicate low energy reserves. In this case, NADH is involved in the reactions of the respiratory chain, enhancing the processes of oxidative phosphorylation associated with energy storage. The opposite phenomenon is observed at low ADP content and high ATP content. By limiting the electron transport system, they promote the use of NADH in other recovery reactions, such as glutamate synthesis and gluconeogenesis.
Biological oxidation and reduction.
Cellular respiration is the totality of enzymatic processes occurring in each cell, as a result of which molecules of carbohydrates, fatty acids and amino acids are ultimately broken down into carbon dioxide and water, and the released biologically useful energy is stored by the cell and then used. Many enzymes that catalyze these reactions are located in the walls and cristae of mitochondria.
It is known that for all manifestations of life - growth, movement, irritability, self-reproduction - a cell must expend energy. All living cells obtain biologically useful energy through enzymatic reactions in which electrons are transferred from one energy level to another. For most organisms, the final electron acceptor is oxygen, which reacts with electrons and hydrogen ions to form a water molecule. The transfer of electrons to oxygen occurs with the participation of the enzyme system contained in the mitochondria - the electron transfer system. ATP serves as the “energy currency” of the cell and is used in all metabolic reactions that require energy. Energy-rich molecules do not move freely from one cell to another, but are formed in that place. where they should be used. For example, high-energy ATP bonds, which serve as a source of energy for reactions associated with muscle contraction, are formed in the muscle cells themselves.
The process in which atoms or molecules lose electrons (e -) is called oxidation, and the reverse process - the addition (attachment) of electrons to an atom or molecule - is called reduction.
A simple example of oxidation and reduction is the reversible reaction - Fe 2+ ®Fe 3+ + e -
Reaction going to the right - oxidation, removal of an electron
To the left - reduction (addition of an electron)
All oxidative reactions(in which an electron is removed) must be accompanied by reduction - a reaction in which electrons are captured by some other molecule, because they do not exist in a free state.
The transfer of electrons through the electron transport system occurs through a series of sequential oxidation-reduction reactions, which together are called biological oxidation. If the energy of the electron flow accumulates in the form of high-energy phosphate bonds (~P), then the process is called oxidative phosphorylation. Specific compounds that form an electron transport system and that are alternately oxidized and reduced are called cytochromes. Each of the cytochromes is a protein molecule to which is attached a chemical group called heme; at the center of the heme is an iron atom, which is alternately oxidized and reduced, giving or accepting one electron.
All biological oxidation reactions occur with the participation of enzymes, and each enzyme is strictly specific and catalyzes either the oxidation or the reduction of very specific chemical compounds.
Another component of the electron transfer system, ubiquinone or coenzyme Q, is capable of acquiring or donating electrons.
Mitochondria are contained in the cytoplasm of the cell and are microscopic rod-shaped or other shaped formations, the number of which in one cell amounts to hundreds or thousands.
What are mitochondria, what is their structure? The internal space of mitochondria is surrounded by two continuous membranes, with the outer membrane being smooth and the inner one forming numerous folds or cristae. The intramitochondrial space, bounded by the inner membrane, is filled with the so-called matrix, which consists of approximately 50% protein and has a very fine structure. Mitochondria contain a large number of enzymes. The outer membrane of mitochondria does not contain any of the components of the respiratory catalyst chain. Based on the enzyme set outer membrane, it is still difficult to answer the question of what its purpose is. Perhaps it plays the role of a partition separating the internal, working part of the mitochondria from the rest of the cell. Enzymes of the respiratory chain are associated with the inner membrane. The matrix contains a number of Krebs cycle enzymes.
At the first stage, glucose is split into 2 trioses:
Thus, at the first stage of glycolysis, 2 ATP molecules and 2 molecules of 3-phosphoglyceraldehyde are formed.
In the second stage, 2 molecules of 3-phosphoglyceraldehyde are oxidized to two molecules of lactic acid.
The significance of the lactate dehydrogenase reaction (LDH) is to oxidize NADH 2 to NAD under oxygen-free conditions and allow the glycerophosphate dehydrogenase reaction to occur.
The overall equation of glycolysis: glucose + 2ADP + 2H 3 PO 4 → 2 lactate + 2ATP + 2H 2 O
Glycolysis occurs in the cytosol. Its regulation is carried out by key enzymes - hexokinase, phosphofructokinase And pyruvate kinase. These enzymes are activated by ADP and NAD and inhibited by ATP and NADH 2 .
The energy efficiency of anaerobic glycolysis comes down to the difference between the number of ATP molecules consumed and the number of ATP molecules produced. 2 ATP molecules are consumed per glucose molecule in the hexokinase reaction and the phosphofructokinase reaction. 2 molecules of ATP are formed per molecule of triose (1/2 glucose) in the glycerokinase reaction and pyruvate kinase reaction. For a molecule of glucose (2 trioses), 4 molecules of ATP are formed, respectively. Total balance: 4 ATP – 2 ATP = 2 ATP. 2 ATP molecules accumulate ≈ 20 kcal, which is about 3% of the energy of complete oxidation of glucose (686 kcal).
Despite the relatively low energy efficiency of anaerobic glycolysis, it has an important biological significance in that it the only one a method of generating energy in oxygen-free conditions. In conditions of oxygen deficiency, it ensures the performance of intense muscle work and the beginning of muscle work.
In children anaerobic glycolysis is very active in fetal tissues under conditions of oxygen deficiency. It remains active during the neonatal period, gradually giving way to aerobic oxidation.
Further conversion of lactic acid.
- With an intensive supply of oxygen under aerobic conditions, lactic acid is converted into PVA and, through acetyl CoA, is included in the Krebs cycle, providing energy.
- Lactic acid is transported from muscles to the liver, where it is used for glucose synthesis - the Cori cycle.
Measles cycle
- At high concentrations of lactic acid in tissues, it can be excreted through the kidneys to prevent acidosis.
Stages:
1. H 3 C – CO – COOH + TDF – E 1 = H 3 C – CHOH - TDF – E 1 + CO 2
2. H 3 C – CHOH - TDP – E 1 + Lipoic kt.a – E2 = H 3 C – CO~ dihydrolipoic kt.a – E2 + TDF – E 1
3. H 3 C – CO~ dihydrolipoic kt.a – E2 + HS-KoA = CH3 – CO ~ S – KoA+ dihydrolipoic kt. – E2
4. dihydrolipoic kta – E2 + E3 – FAD = Lipoic kta – E2 + E3-FADH2
5.E3-FADH2+NAD+=E3-FAD + NADH + H+
E 1 - pyruvate dehydrogenase; E 2 - di-hydrolipoylacetyltransferase; E 3 -dihydrolipoyl dehydrogenase
Total reaction:
H 3 C – CO – COOH+ HS-KoA+NAD+ = CH3 – CO ~ S – KoA+ CO 2 + NADH + H+
Description:
The oxidation of pyruvate to acetyl-CoA occurs with the participation of a number of enzymes and coenzymes, united structurally into a multienzyme system called the “pyruvate dehydrogenase complex.”
On I stage of this process, pyruvate loses its carboxyl group as a result of interaction with thiamine pyrophosphate (TPP) in the active site of the enzyme pyruvate dehydrogenase (E 1). On II stage, the oxyethyl group of the E 1 –TPP–CHOH–CH 3 complex is oxidized to form an acetyl group, which is simultaneously transferred to lipoic acid amide (coenzyme) associated with the enzyme dihydrolipoylacetyltransferase (E 2). This enzyme catalyzes III stage - transfer of the acetyl group to coenzyme CoA (HS-KoA) with the formation of the final product acetyl-CoA, which is a high-energy (macroergic) compound.
On IV stage, the oxidized form of lipoamide is regenerated from the reduced dihydrolipoamide–E 2 complex. With the participation of the enzyme dihydrolipoyl dehydrogenase (E 3), hydrogen atoms are transferred from the reduced sulfhydryl groups of dihydrolipoamide to FAD, which acts as a prosthetic group of this enzyme and is tightly bound to it. At stage V, the reduced FADH 2 dihydro-lipoyl dehydrogenase transfers hydrogen to the coenzyme NAD to form NADH + H +.
The process of oxidative decarboxylation of pyruvate occurs in the mitochondrial matrix. It involves (as part of a complex multienzyme complex) 3 enzymes (pyruvate dehydrogenase, dihydrolipoyl acetyltransferase, dihydrolipoyl dehydrogenase) and 5 coenzymes (TPF, lipoic acid amide, coenzyme A, FAD and NAD), of which three are relatively firmly associated with enzymes (TPF-E 1, lipoamide-E 2 and FAD-E 3), and two are easily dissociated (HS-KoA and NAD).
All these enzymes, which have a subunit structure, and coenzymes are organized into a single complex. Therefore, intermediate products are able to quickly interact with each other. It has been shown that the polypeptide chains of the subunits of dihydrolipoyl acetyltransferase that make up the complex constitute the core of the complex, around which pyruvate dehydrogenase and dihydrolipoyl dehydrogenase are located. It is generally accepted that the native enzyme complex is formed by self-assembly.
The overall reaction catalyzed by the pyruvate dehydrogenase complex can be represented as follows:
Pyruvate + NAD + + HS-CoA –> Acetyl-CoA + NADH + H + + CO 2 .
The reaction is accompanied by a significant decrease in standard free energy and is practically irreversible.
Acetyl-CoA formed during oxidative decarboxylation undergoes further oxidation with the formation of CO 2 and H 2 O. Complete oxidation of acetyl-CoA occurs in the tricarboxylic acid cycle (Krebs cycle). This process, as well as the oxidative decarboxylation of pyruvate, occurs in cell mitochondria.
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Respiration in microbes is considered as an energy process or as a set of various chemical reactions, oxidation and breakdown of substances of organic and inorganic origin. As a result of these chemical reactions, energy is released, which is used by microbes for the absorption of nutrients, the synthesis of proteins in their body, movement, growth, reproduction and other functions of the living organism.
An example of energy release is the oxidation of glucose, which can be expressed by the following compound:
C6 H2O + 6 O 2 6 H2O+ 6CO2 + 674 kcal.
As can be seen from the equations, as a result of the complete oxidation of one glucose molecule to the final products (water and carbon dioxide), 674 large calories of heat are released.
Energy process Respiration in microbes is much more complicated and depends on the nature of the nutrient material used.
Based on the type of respiration, microbes are divided into aerobes and anaerobes; there are microbes with a transitional type of respiration.
Aerobes can live and develop with free access to oxygen. They obtain the necessary energy for life processes by absorbing oxygen and oxidizing nutritional materials.
Anaerobes are able to develop without access to oxygen. Free oxygen in the air has a harmful, destructive effect on these microbes. Strict (obligate) anaerobes (tetanus bacillus, the causative agent of butyric acid fermentation) do not tolerate oxygen at all. They obtain the necessary energy by breaking down the organic substances of carbohydrates, proteins, fats, organic acids, and alcohols.
Facultative anaerobes use the same substances, but in relation to the conditions of their existence they can change the anaerobic type of their respiration to aerobic. Thus, top-fermenting yeast “with limited air flow decomposes sugar into alcohol and carbon dioxide; with abundant aeration, they develop aerobic respiration with complete oxidation sugar to carbon dioxide and water. Lactic acid bacteria under anaerobic conditions convert glucose into lactic acid, while releasing slightly less energy than aerobes. When there is a lack of oxygen, denitrifying bacteria use nitrate oxygen to oxidize organic compounds.
The examples given show the variety of energy sources and methods of obtaining it. different types microbes; For these reasons, bacteria do not and may have a single mechanism of respiration.
Most aerobic microorganisms oxidize organic nutrients during respiration to CO2 and water. Since in a CO 2 molecule it is achieved highest degree oxidation of carbon, in this case they speak of complete oxidation and distinguish this type of respiration from incomplete oxidations, in which partially oxidized organic compounds are released as metabolic products.
By “complete oxidation” we mean only that no release of any organic substances occurs; but this does not mean at all that the entire absorbed substrate is oxidized. In each case, a significant part of the substrate (40-70%) is assimilated, i.e. turns into cell substances.
The end products of “incomplete oxidations” can be acetic, gluconic, fumaric, citric, lactic acids and a number of other compounds. Since these products are similar to those formed during fermentations (propionic, butyric, succinic, lactic acids, etc.), and also due to the fact that industrial fermentation processes require special technical devices (fermenters), incomplete oxidations are also called “ oxidative fermentation" or "aerobic fermentation". The words “fermentation” and “fermentation” in this case reflect more of a technological aspect.
We will also classify as “incomplete oxidations” the simple abstraction of hydrogen from the substrate and the use of microorganisms to catalyze certain reactions that have no significance for them under normal conditions. Some examples of such oxidations will be discussed below.
Respiration of bacteria
Breathing is the most advanced form of the oxidative process and the most efficient way to obtain energy. The main advantage of respiration is that the energy of the oxidized substance - the substrate on which the microorganism grows - is used most fully. Therefore, during the process of respiration, much less substrate is processed to obtain a certain amount of energy than, for example, during fermentation.
Fig. 10. The role of pyruvic acid in the processes of respiration and fermentation.
The process of respiration consists in the fact that carbohydrates (or proteins, fats and other reserve substances of the cell) decompose, oxidized by atmospheric oxygen, to carbon dioxide and water. The energy released in this case is spent on maintaining the vital functions of organisms, growth and reproduction. Bacteria, due to the negligible size of their bodies, cannot accumulate significant amounts of reserve substances. Therefore, they mainly use the nutrient compounds of the environment.
IN general view breathing can be represented by the following equation:
С6Н12О6 + 602 = 6С02 + 6Н20 + 2.87-106 J.
glucose oxygen carbon dioxide water energy
Behind this simple formula lies a complex chain of chemical reactions, each of which is catalyzed by a specific enzyme.
Rice. 11. Scheme of the glycolytic pathway for the breakdown of carbohydrates.
The enzymatic reactions that occur during respiration are now well studied. The reaction scheme turned out to be universal, i.e., in principle, the same in animals, plants and many microorganisms, including bacteria. The respiration process during glucose oxidation consists of the following main stages (Fig. 10).
First, phosphorus esters of glucose are formed - monoactivated glucose in the form of diphosphate is further split into two triose phosphates (three-carbon compounds): phosphoglyceraldehyde and dihydroxyacetone phosphate, which can be reversibly converted into each other.
Rice. 12. Tricarboxylic acid cycle. The arrows indicate the direction, and the numbers indicate the order of reactions.
Next, phosphoglyceraldehyde enters into the exchange and is oxidized to diphosphoglyceric acid. The purpose of this process is the abstraction of hydrogen atoms from the oxidized substrate and the transfer of hydrogen with the help of specific oxidative enzymes to atmospheric oxygen (see Fig. 10, 11).
Hydrogen from phosphoglyceraldehyde attaches to the enzyme nicotine amide dinucleotide (NAD); in this case, the aldehyde is oxidized to an acid and energy is released. Some of this energy is spent on the formation of ATP; in this case, phosphoric acid is added to adenosine diphosphate y-ADP. During hydrolysis ATP energy is released and can be spent on various processes of protein synthesis and other cell needs.
Phosphoglyceric acid is oxidized to pyruvic acid. At the same time, ATP is also formed, i.e. energy is stored.
This completes the first - anaerobic - stage of the respiration process, which is called the glycolytic pathway or the Embden-Meyerhof-Parnas pathway. Oxygen is not required for these reactions to occur. The resulting pyruvic acid (CH3COCOOH) is an interesting and very important compound. The pathways for the breakdown of glucose during respiration and many fermentations, up to the formation of pyruvic acid, proceed in exactly the same way, which was first established by the Russian biochemist S.P. Kostychev. Pyruvic acid is the central point from which the paths of respiration and fermentation diverge, from where a chain of enzymatic transformations specific to a given process begins - a specific chain of chemical reactions (Fig. 11).
During respiration, pyruvic acid enters the tricarboxylic acid cycle (Fig. 12). This is a complex vicious circle of transformations, as a result of which organic acids with 4, 5 and 6 carbon atoms (malic, lactic, fumaric, a-ketoglutaric and citric) and carbon dioxide is split off.
First of all, CO2 is split off from pyruvic acid containing three carbon atoms - acetic acid is formed, which with coenzyme A forms an active compound - acetyl coenzyme A. It transfers the remainder of acetic acid (acetyl) to oxaloacetic acid (4 carbon atoms), and citric acid is formed (6 carbon atoms). Citric acid undergoes several transformations, as a result CO2 is released and a five-carbon compound is formed - a-ketoglutaric acid. CO2 (the third molecule of carbon dioxide) is also split off from it, and succinic acid (4 carbon atoms) is formed, which then turns into fumaric, malic and, finally, oxaloacetic acid.
This completes the cycle. Oxaloacetic acid can enter the cycle again.
Thus, three-carbon pyruvic acid enters the cycle, and in the course of transformations 3 CO2 molecules are released.
The hydrogen of pyruvic acid, released during dehydrogenation under aerobic conditions, does not remain free - it enters the respiratory chain (just like the hydrogen of glyceraldehyde, taken away when it is converted into glyceric acid). This is a chain of oxidative enzymes.
Enzymes that are the first to take on hydrogen from the substrate being oxidized are called primary dehydrogenases.
They contain di- or tripyridine nucleotides: NAD or NADP and a specific protein. The mechanism of hydrogen addition is the same:
Oxidizable substance - H2 + NAD -> oxidized substance + NAD*H2
The hydrogen produced by the dehydrogenase is then added to the next enzyme system, flavin enzymes (FMN or FAD).
From flavin enzymes, electrons go to cytochromes - iron-containing proteins (complex proteins). It is not the hydrogen atom that is transferred along the cytochrome chain, but only electrons. In this case, the valency of iron changes: Fe++ - e->Fe++
The final reaction of respiration is the addition of a proton and an electron to oxygen in the air and the formation of water. But first, the oxygen molecule is activated under the action of the enzyme cytochrome oxidase. Activation comes down to the fact that oxygen acquires a negative charge due to the addition of an electron from the oxidized substance. Hydrogen (proton) attaches to activated oxygen, forming water.
In addition to the aforementioned chain of electron and hydrogen carriers, others are known. This process is much more complex than the diagram outlined.
The biological meaning of these transformations is the oxidation of substances and the formation of energy. As a result of the oxidation of a sugar molecule (glucose), 12.6-1053 J of energy is stored in ATP, the sugar molecule itself contains 28.6-106 J, therefore, 44% of the energy is usefully used. This is a very high efficiency factor when compared with the efficiency of modern machines.
The process of breathing produces a huge amount of energy. If all of it were released at once, the cell would cease to exist. But this does not happen, because the energy is not released all at once, but in stages, in small portions. The release of energy in small doses is due to the fact that respiration is a multi-stage process, at individual stages of which various intermediate products are formed (with different lengths of the carbon chain) and energy is released. The released energy is not consumed in the form of heat, but is stored in the universal macroergic compound - ATP. When ATP is broken down, energy can be used in any processes necessary to maintain the vital functions of the body: for the synthesis of various organic substances, mechanical work, maintaining the osmotic pressure of protoplasm, etc.
Breathing is a process that provides energy, but its biological significance is not limited to this. As a result of chemical reactions that accompany respiration, a large number of intermediate compounds are formed. From these compounds, which have different numbers of carbon atoms, a wide variety of cell substances can be synthesized: amino acids, fatty acids, fats, proteins, vitamins.
Therefore, carbohydrate metabolism determines other metabolisms (proteins, fats). This is its great significance.
With the process of breathing, it chemical reactions One of the amazing properties of microbes is connected - the ability to emit visible light - to luminesce.
It is known that a number of living organisms, including bacteria, can emit visible light. Luminescence caused by microorganisms has been known for centuries. The accumulation of luminescent bacteria in symbiosis with small marine animals sometimes leads to a glow in the sea; luminescence was also encountered during the growth of certain bacteria on meat, etc.
The main components, the interaction between which leads to the emission of light, include reduced forms of FMN or NAD, molecular oxygen, the enzyme luciferase and the oxidizable compound - luciferin. It is assumed that reduced NAD or FMN reacts with luciferase, oxygen and luciferin, as a result of which electrons in some molecules go into an excited state and the return of these electrons to the ground level is accompanied by the emission of light. Luminescence in microbes is considered a “wasteful process”, since it reduces the energy efficiency of respiration.
Aerobic oxidation of glucose includes 3 stages:
Stage 1 occurs in the cytosol and involves the formation of pyruvic acid:
Glucose → 2 PVK + 2 ATP + 2 NADH 2;
Stage 2 occurs in mitochondria:
2 PVC → 2 acetyl - CoA + 2 NADH 2;
Stage 3 occurs inside mitochondria:
2 acetyl-CoA → 2 TCA cycle.
Due to the fact that 2 molecules of NADH 2 are formed in the cytosol at the first stage, and they can only be oxidized in the mitochondrial respiratory chain, hydrogen transfer from NADH 2 of the cytosol to the intramitochondrial electron transport chain is necessary. Mitochondria are impermeable to NADH 2 , so special shuttle mechanisms exist for the transfer of hydrogen from the cytosol to the mitochondria. Their essence is reflected in the diagram, where X is the oxidized form of the hydrogen carrier, and XH 2 is its reduced form:
Depending on which substances are involved in the transfer of hydrogen across the mitochondrial membrane, several shuttle mechanisms are distinguished.
Glycerophosphate shuttle mechanism in which the loss of two ATP molecules occurs, because instead of two molecules of NADH 2 (potentially 6 molecules of ATP), 2 molecules of FADH 2 are formed (actually 4 molecules of ATP).
Malate shuttle mechanism works to remove hydrogen from the mitochondrial matrix:
Energy efficiency of aerobic oxidation.
- glucose → 2 PVK + 2 ATP + 2 NADH 2 (→8 ATP).
- 2 PVK → 2 acetyl CoA + 2 NADH 2 (→ 6 ATP).
- 2 acetyl CoA → 2 TCA cycle (12*2 = 24 ATP).
In total, 38 ATP molecules can be formed, from which it is necessary to subtract 2 ATP molecules lost in the glycerophosphate shuttle mechanism. Thus, it is formed 36 ATP.
36 ATP (about 360 kcal) is from 686 kcal. 50-60% is the energy efficiency of aerobic glucose oxidation, which is twenty times higher than the efficiency of anaerobic glucose oxidation. Therefore, when oxygen enters the tissues, the anaerobic pathway is blocked, and this phenomenon is called Pasteur effect. In newborns the aerobic pathway begins to activate in the first 2-3 months of life.
6.5. 2. Biosynthesis of glucose (gluconeogenesis)
Gluconeogenesis is a pathway for the synthesis of glucose in the body from non-carbohydrate substances, which is capable of maintaining glucose levels for a long time in the absence of carbohydrates in the diet. The starting materials for it are lactic acid, PVC, amino acids, glycerin. Gluconeogenesis occurs most actively in the liver and kidneys. This process is intracellularly localized partly in the cytosol, partly in the mitochondria. In general, gluconeogenesis is the reverse process of glycolysis.
Glycolysis has three irreversible stages catalyzed by enzymes:
· pyruvate kinase;
· phosphofructokinase;
· hexokinase.
Therefore, in gluconeogenesis Instead of these enzymes, there are specific enzymes that bypass these irreversible stages:
- pyruvate carboxylase and carboxykinase (“bypass” pyruvate kinase);
- fructose-6-phosphatase (“bypasses” phosphofructokinase);
- glucose-6-phosphatase (“bypasses” hexokinase).
The key enzymes for gluconeogenesis are pyruvate carboxylase And fructose 1,6-biphosphatase. The activator for them is ATP (the synthesis of one glucose molecule requires 6 ATP molecules).
Thus, a high concentration of ATP in cells activates gluconeogenesis, which requires energy, and at the same time inhibits glycolysis (at the stage of phosphofructokinase), leading to the formation of ATP. This situation is illustrated by the graph below.
Vitamin H
Vitamin H (biotin, antiseborrheic vitamin) is involved in gluconeogenesis, which chemical nature is a sulfur-containing heterocycle with valeric acid residues. It is widely distributed in animal and plant products (liver, yolk). The daily requirement for it is 0.2 mg. Vitamin deficiency manifests itself as dermatitis, nail damage, an increase or decrease in the formation of sebum (seborrhea). Biological role of vitamin H:
- participates in carboxylation reactions;
- participates in transcarboxylation reactions;
- participates in the exchange of purine bases and some amino acids.
Gluconeogenesis is active in recent months intrauterine development. After the birth of a child, the activity of the process increases, starting from the third month of life.