Tricarboxylic acid cycle

Tricarboxylic acid cycle (Krebs cycle, citrate cycle) - the central part of the general path of catabolism, a cyclic biochemical aerobic process during which the conversion of two- and three-carbon compounds formed as intermediate products in living organisms during the breakdown of carbohydrates, fats and proteins occurs to CO 2. In this case, the released hydrogen is sent to the tissue respiration chain, where it is further oxidized to water, directly participating in the synthesis of a universal energy source - ATP.

The Krebs cycle is a key stage in the respiration of all cells that use oxygen, the intersection of many metabolic pathways in the body. In addition to the significant energy role, the cycle also has a significant plastic function, that is, it is an important source of precursor molecules, from which, during other biochemical transformations, compounds important for the life of the cell are synthesized, such as amino acids, carbohydrates, fatty acids, etc.

Functions

1. Integrative function- the cycle is the link between the reactions of anabolism and catabolism.
2. Catabolic function- transformation of various substances into cycle substrates:
   • Fatty acids, pyruvate, Leu, Phen - Acetyl-CoA.
   • Arg, Gis, Glu - α-ketoglutarate.
   • Hairdryer, shooting range - fumarate.
3. Anabolic function- use of cycle substrates for the synthesis of organic substances:
   • Oxalacetate - glucose, Asp, Asn.
   • Succinyl-CoA - heme synthesis.
   • CO 2 - carboxylation reactions.
4. Hydrogen donor function- the Krebs cycle supplies protons to the respiratory chain of mitochondria in the form of three NADH.H + and one FADH 2.
5. Energy function- 3 NADH.H + gives 7.5 mol of ATP, 1 FADH 2 gives 1.5 mol of ATP on the respiratory chain. In addition, in the cycle, 1 GTP is synthesized by substrate phosphorylation, and then ATP is synthesized from it by transphosphorylation: GTP + ADP = ATP + GDP.

Mnemonic rules

To make it easier to memorize the acids involved in the Krebs cycle, there is a mnemonic rule:

A Whole Pineapple and a Piece of Soufflé Is Actually My Lunch Today, which corresponds to the series - citrate, (cis-)aconitate, isocitrate, (alpha-)ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

There is also the following mnemonic poem (its author is assistant at the Department of Biochemistry of KSMU E. V. Parshkova):

Shchuk y acetyl lemon il, but nar cis With and con I was afraid, He was above him isolimon But Alpha-ketoglutar came. Succinyl Xia coenzyme oh, Amber was fumar ovo, Yabloch ek stored for the winter, Turned around pike oh again.

(oxaloacetic acid, citric acid, cis-aconitic acid, isocitric acid, α-ketoglutaric acid, succinyl-CoA, succinic acid, fumaric acid, malic acid, oxaloacetic acid).

Another version of the poem

PIKE ate acetate, it turns out citrate through cis-aconitate, it will be isocitrate hydrogens giving up NAD, it loses CO 2 to this, it is immensely happy alpha-ketoglutarate oxidation is coming - NAD stole hydrogen TDP, coenzyme A takes CO 2 and energy barely appeared in succinyl immediately GTP was born and remained succinate now he got to FAD - he needed hydrogens fumarate water drank, and it turned into malate here NAD came to malate, acquired hydrogens PIKE showed up again and quietly hid Watch for acetate...

Notes

Links

• Tricarboxylic acid cycle

The tricarboxylic acid cycle was first discovered by the English biochemist Krebs. He was the first to postulate the importance of this cycle for the complete combustion of pyruvate, the main source of which is the glycolytic conversion of carbohydrates. It was subsequently shown that the tricarboxylic acid cycle is a “focus” at which almost all metabolic pathways converge.

So, acetyl-CoA formed as a result of oxidative decarboxylation of pyruvate enters the Krebs cycle. This cycle consists of eight consecutive reactions (Fig. 91). The cycle begins with the condensation of acetyl-CoA with oxaloacetate and the formation of citric acid. ( As will be seen below, in the cycle it is not acetyl-CoA itself that undergoes oxidation, but a more complex compound - citric acid (tricarboxylic acid).)

Then citric acid (a six-carbon compound), through a series of dehydrogenations (removal of hydrogen) and decarboxylation (elimination of CO 2), loses two carbon atoms and again oxaloacetate (a four-carbon compound) appears in the Krebs cycle, i.e., as a result of a complete revolution of the cycle, the acetyl-CoA molecule burns to CO 2 and H 2 O, and the oxaloacetate molecule is regenerated. Below are all eight sequential reactions (stages) of the Krebs cycle.

In the first reaction, catalyzed by the enzyme citrate synthase, acetyl-CoA is condensed with oxaloacetate. As a result, citric acid is formed:

Apparently, in this reaction, citril-CoA bound to the enzyme is formed as an intermediate product. The latter is then spontaneously and irreversibly hydrolyzed to form citrate and HS-CoA.

In the second reaction of the cycle, the resulting citric acid undergoes dehydration to form cis-aconitic acid, which, by adding a water molecule, becomes isocitric acid. These reversible hydration-dehydration reactions are catalyzed by the enzyme aconitate hydratase:

In the third reaction, which appears to be the rate-limiting reaction of the Krebs cycle, isocitric acid is dehydrogenated in the presence of NAD-dependent isocitrate dehydrogenase:

(There are two types of isocitrate dehydrogenases in tissues: NAD- and NADP-dependent. It has been established that NAD-dependent isocitrate dehydrogenase plays the role of the main catalyst for the oxidation of isocitric acid in the Krebs cycle.)

During the isocitrate dehydrogenase reaction, isocitric acid is decarboxylated. NAD-dependent isocitrate dehydrogenase is an allosteric enzyme that requires ADP as a specific activator. In addition, the enzyme requires Mg 2+ or Mn 2+ ions to exhibit its activity.

In the fourth reaction, α-ketoglutaric acid is oxidatively decarboxylated to succinyl-CoA. The mechanism of this reaction is similar to the reaction of oxidative decarboxylation of pyruvate to acetyl-CoA. The α-ketoglutarate dehydrogenase complex is similar in structure to the pyruvate dehydrogenase complex. In both cases, five coenzymes take part in the reaction: TDP, lipoic acid amide, HS-CoA, FAD and NAD. In total, this reaction can be written as follows:

The fifth reaction is catalyzed by the enzyme succinyl-CoA synthetase. During this reaction, succinyl-CoA, with the participation of GDP and inorganic phosphate, is converted into succinic acid (succinate). At the same time, the formation of a high-energy phosphate bond of GTP1 occurs due to the high-energy thioester bond of succinyl-CoA:

(The resulting GTP then donates its terminal phosphate group to ADP, resulting in the formation of ATP. The formation of a high-energy nucleoside triphosphate during the succinyl-CoA synthetase reaction is an example of phosphorylation at the substrate level.)

In the sixth reaction, succinate is dehydrogenated to fumaric acid. The oxidation of succinate is catalyzed by succinate dehydrogenase, in the molecule of which the coenzyme FAD is covalently bound to the protein:

In the seventh reaction, the resulting fumaric acid is hydrated under the influence of the enzyme fumarate hydratase. The product of this reaction is malic acid (malate). It should be noted that fumarate hydratase is stereospecific - during this reaction L-malic acid is formed:

Finally, in the eighth reaction of the tricarboxylic acid cycle, under the influence of mitochondrial NAD-dependent malate dehydrogenase, L-malate is oxidized to oxaloacetate:

As you can see, in one turn of the cycle, consisting of eight enzymatic reactions, complete oxidation (“combustion”) of one molecule of acetyl-CoA occurs. For continuous operation of the cycle, a constant supply of acetyl-CoA into the system is necessary, and coenzymes (NAD and FAD), which have passed into a reduced state, must be oxidized again and again. This oxidation occurs in the electron transport system (or chain of respiratory enzymes) located in the mitochondria.

The energy released as a result of the oxidation of acetyl-CoA is largely concentrated in the high-energy phosphate bonds of ATP. Of the four pairs of hydrogen atoms, three pairs are transferred through NAD to the electron transport system; in this case, for each pair in the biological oxidation system, three ATP molecules are formed (in the process of conjugate oxidative phosphorylation), and therefore a total of nine ATP molecules. One pair of atoms enters the electron transport system through FAD, resulting in the formation of 2 ATP molecules. During the reactions of the Krebs cycle, 1 molecule of GTP is also synthesized, which is equivalent to 1 molecule of ATP. So, the oxidation of acetyl-CoA in the Krebs cycle produces 12 ATP molecules.

As already noted, 1 molecule of NADH 2 (3 molecules of ATP) is formed during the oxidative decarboxylation of pyruvate into acetyl-CoA. Since the breakdown of one molecule of glucose produces two molecules of pyruvate, when they are oxidized to 2 molecules of acetyl-CoA and the subsequent two turns of the tricarboxylic acid cycle, 30 molecules of ATP are synthesized (hence, the oxidation of one molecule of pyruvate to CO 2 and H 2 O produces 15 molecules ATP).

To this we must add 2 ATP molecules formed during aerobic glycolysis, and 4 ATP molecules synthesized through the oxidation of 2 molecules of extramitochondrial NADH 2, which are formed during the oxidation of 2 molecules of glyceraldehyde-3-phosphate in the dehydrogenase reaction. In total, we find that when 1 glucose molecule is broken down in tissues according to the equation: C 6 H 12 0 6 + 60 2 -> 6CO 2 + 6H 2 O, 36 ATP molecules are synthesized, which contributes to the accumulation of adenosine triphosphate in high-energy phosphate bonds 36 X 34.5 ~ 1240 kJ (or, according to other sources, 36 X 38 ~ 1430 kJ) free energy. In other words, of all the free energy released during aerobic oxidation of glucose (about 2840 kJ), up to 50% of it is accumulated in mitochondria in a form that can be used to perform various physiological functions. There is no doubt that, energetically, the complete breakdown of glucose is a more efficient process than glycolysis. It should be noted that the NADH 2 molecules formed during the conversion of glyceraldehyde-3-phosphate 2 subsequently, upon oxidation, produce not 6 ATP molecules, but only 4. The fact is that the molecules of extramitochondrial NADH 2 themselves are not able to penetrate through the membrane into the mitochondria. However, the electrons they donate can be included in the mitochondrial chain of biological oxidation using the so-called glycerophosphate shuttle mechanism (Fig. 92). As can be seen in the figure, cytoplasmic NADH 2 first reacts with cytoplasmic dihydroxyacetone phosphate to form glycerol 3-phosphate. The reaction is catalyzed by NAD-dependent cytoplasmic glycerol-3-phosphate dehydrogenase.

Brief historical information

Our favorite cycle is the TCA cycle, or the tricarboxylic acid cycle - life on Earth and under the Earth and in the Earth... Stop, in general this is the most amazing mechanism - it is universal, it is a way of oxidizing the breakdown products of carbohydrates, fats, proteins in the cells of living organisms, as a result We get energy for the activities of our body.

This process was discovered by Hans Krebs himself, for which he received the Nobel Prize!

He was born in August 25 - 1900 in the German city of Hildesheim. He received a medical education from the University of Hamburg and continued biochemical research under the leadership of Otto Warburg in Berlin.

In 1930, together with his student, he discovered the process of neutralizing ammonia in the body, which was present in many representatives of the living world, including humans. This cycle is the urea cycle, which is also known as the Krebs cycle #1.

When Hitler came to power, Hans emigrated to Great Britain, where he continues to study science at the Universities of Cambridge and Sheffield. Developing the research of the Hungarian biochemist Albert Szent-Györgyi, he received an insight and made the most famous Krebs cycle No. 2, or in other words, the “Szent-Györgyö – Krebs cycle” - 1937.

The research results are sent to the journal Nature, which refuses to publish the article. Then the text flies to the magazine "Enzymologia" in Holland. Krebs received the Nobel Prize in 1953 in physiology or medicine.

The discovery was surprising: in 1935 Szent-Györgyi found that succinic, oxaloacetic, fumaric and malic acids (all 4 acids are natural chemical components of animal cells) enhance the oxidation process in the pectoral muscle of the pigeon. Which was shredded.

It is in it that metabolic processes occur at the highest speed.

F. Knoop and K. Martius in 1937 found that citric acid is converted into isocitric acid through an intermediate product, cis - aconitic acid. In addition, isocitric acid could be converted into a-ketoglutaric acid, and that into succinic acid.

Krebs noticed the effect of acids on the absorption of O2 by the pectoral muscle of a pigeon and identified an activating effect on the oxidation of PVC and the formation of Acetyl-Coenzyme A. In addition, the processes in the muscle were inhibited by malonic acid, which is similar to succinic acid and could competitively inhibit enzymes whose substrate is succinic acid .

When Krebs added malonic acid to the reaction medium, the accumulation of a-ketoglutaric, citric and succinic acids began. Thus, it is clear that the combined action of a-ketoglutaric and citric acids leads to the formation of succinic acid.

Hans examined more than 20 other substances, but they did not affect oxidation. Comparing the data obtained, Krebs received a cycle. At the very beginning, the researcher could not say for sure whether the process began with citric or isocitric acid, so he called it the “tricarboxylic acid cycle.”

Now we know that the first is citric acid, so the correct name is the citrate cycle or the citric acid cycle.

In eukaryotes, TCA cycle reactions occur in mitochondria, while all enzymes for catalysis, except 1, are contained in a free state in the mitochondrial matrix; the exception is succinate dehydrogenase, which is localized on the inner membrane of the mitochondrion and is embedded in the lipid bilayer. In prokaryotes, the reactions of the cycle occur in the cytoplasm.

Let's meet the participants of the cycle:

1) Acetyl Coenzyme A:
- acetyl group
- coenzyme A - Coenzyme A:

2) PIKE – Oxaloacetate - Oxaloacetic acid:
seems to consist of two parts: oxalic and acetic acid.

3-4) Citric and Isocitric acids:

5) a-Ketoglutaric acid:

6) Succinyl-Coenzyme A:

7) Succinic acid:

8) Fumaric acid:

9) Malic acid:

How do reactions occur? In general, we are all accustomed to the appearance of the ring, which is shown below in the picture. Below everything is described step by step:

1. Condensation of Acetyl Coenzyme A and Oxaloacetic acid ➙ citric acid.

The transformation of Acetyl Coenzyme A begins with condensation with Oxaloacetic acid, resulting in the formation of citric acid.

The reaction does not require the consumption of ATP, since the energy for this process is provided as a result of hydrolysis of the thioether bond with Acetyl Coenzyme A, which is high-energy:

2. Citric acid passes through cis-aconitic acid into isocitric acid.

Isomerization of citric acid into isocitric acid occurs. The conversion enzyme - aconitase - first dehydrates citric acid to form cis-aconitic acid, then connects water to the double bond of the metabolite, forming isocitric acid:

3. Isocitric acid is dehydrogenated to form α-ketoglutaric acid and CO2.

Isocitric acid is oxidized by a specific dehydrogenase, the coenzyme of which is NAD.

Simultaneously with oxidation, decarboxylation of isocitric acid occurs. As a result of transformations, α-ketoglutaric acid is formed.

4. Alpha-ketoglutaric acid is dehydrogenated by ➙ succinyl-coenzyme A and CO2.

The next stage is the oxidative decarboxylation of α-ketoglutaric acid.

Catalyzed by the α-ketoglutarate dehydrogenase complex, which is similar in mechanism, structure and action to the pyruvate dehydrogenase complex. As a result, succinyl-CoA is formed.

5. Succinyl coenzyme A ➙ succinic acid.

Succinyl-CoA is hydrolyzed to free succinic acid, the energy released is stored by the formation of guanosine triphosphate. This stage is the only one in the cycle at which energy is directly released.

6. Succinic acid is dehydrogenated ➙ fumaric acid.

The dehydrogenation of succinic acid is accelerated by succinate dehydrogenase, its coenzyme is FAD.

7. Fumaric acid is hydrated ➙ malic acid.

Fumaric acid, which is formed by dehydrogenation of succinic acid, is hydrated and malic acid is formed.

8. Malic acid is dehydrogenated ➙ Oxalic-acetic acid - the cycle closes.

The final process is dehydrogenation of malic acid, catalyzed by malate dehydrogenase;

The result of the stage is the metabolite with which the tricarboxylic acid cycle begins - Oxalic-Acetic acid.

In reaction 1 of the next cycle, another quantity of Acetyl Coenzyme A will enter.

How to remember this cycle? Just!

1) A very figurative expression:
A Whole Pineapple and a Piece of Soufflé Is Actually My Lunch Today, which corresponds to - citrate, cis-aconitate, isocitrate, (alpha-)ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

2) Another long poem:

PIKE ate acetate, it turns out citrate,
Through cisaconitate it will become isocitrate.
Having given up hydrogen to NAD, it loses CO2,
Alpha-ketoglutarate is extremely happy about this.
Oxidation is coming - NAD has stolen hydrogen,
TDP, coenzyme A takes CO2.
And the energy barely appeared in succinyl,
Immediately ATP was born and what remained was succinate.
Now he got to the FAD - he needs hydrogen,
The fumarate drank from the water and turned into malate.
Then NAD came to malate, acquired hydrogen,
The PIKE showed up again and quietly hid.

3) The original poem - in short:

PIKE ACETYL LIMONIL,
But the horse was afraid of narcissus,
He is above him ISOLIMON
ALPHA - KETOGLUTARASED.
SUCCINALIZED WITH COENZYME,
AMBER FUMAROVO,
Stored up some APPLES for the winter,
Turned into a PIKE again.

This metabolic pathway is named after the author who discovered it, G. Krebs, who received (together with F. Lipman) the Nobel Prize for this discovery in 1953. The citric acid cycle captures most of the free energy generated by the breakdown of proteins, fats and carbohydrates in food. The Krebs cycle is the central metabolic pathway.

Acetyl-CoA, formed as a result of oxidative decarboxylation of pyruvate in the mitochondrial matrix, is included in a chain of successive oxidation reactions. There are eight such reactions.

1st reaction - formation of citric acid. Citrate is formed by condensation of the acetyl residue of acetyl-CoA with oxalacetate (OA) using the enzyme citrate synthase (with the participation of water):

This reaction is practically irreversible, since it disintegrates the energy-rich thioether bond acetyl~S-CoA.

2nd reaction - formation of isocitric acid. This reaction is catalyzed by an iron-containing (Fe - non-heme) enzyme - aconitase. The reaction proceeds through the formation stage cis-aconitic acid (citric acid undergoes dehydration to form cis-aconitic acid, which, by adding a water molecule, turns into isocitric acid).

3rd reaction - dehydrogenation and direct decarboxylation of isocitric acid. The reaction is catalyzed by the NAD+-dependent enzyme isocitrate dehydrogenase. The enzyme requires the presence of manganese (or magnesium) ions. Being an allosteric protein by its nature, isocitrate dehydrogenase requires a specific activator - ADP.

4th reaction - oxidative decarboxylation of α-ketoglutaric acid. The process is catalyzed by α-ketoglutarate dehydrogenase - an enzyme complex similar in structure and mechanism of action to the pyruvate dehydrogenase complex. It contains the same coenzymes: TPP, LA and FAD - the complex’s own coenzymes; CoA-SH and NAD + are external coenzymes.

5th reaction - substrate phosphorylation. The essence of the reaction is the transfer of the energy-rich succinyl-CoA bond (a high-energy compound) to HDF with the participation of phosphoric acid - this forms GTP, the molecule of which enters into the reaction rephosphorylation with ADP - ATP is formed.

6th reaction - dehydrogenation of succinic acid with succinate dehydrogenase. The enzyme directly transfers hydrogen from the substrate (succinate) to ubiquinone in the inner mitochondrial membrane. Succinate dehydrogenase - complex II of the mitochondrial respiratory chain. The coenzyme in this reaction is FAD.

7th reaction - the formation of malic acid by the enzyme fumarase. Fumarase (fumarate hydratase) hydrates fumaric acid - this produces malic acid, and its L-form, since the enzyme is stereospecific.

8th reaction - formation of oxalacetate. The reaction is catalyzed malate dehydrogenase , the coenzyme of which is NAD +. The oxalacetate formed under the action of the enzyme is again included in the Krebs cycle and the entire cyclic process is repeated.

The last three reactions are reversible, but since NADH?H + is captured by the respiratory chain, the equilibrium of the reaction shifts to the right, i.e. towards the formation of oxalacetate. As you can see, during one revolution of the cycle, complete oxidation, “combustion,” of the acetyl-CoA molecule occurs. During the cycle, reduced forms of nicotinamide and flavin coenzymes are formed, which are oxidized in the mitochondrial respiratory chain. Thus, the Krebs cycle is closely related to the process of cellular respiration.

The functions of the tricarboxylic acid cycle are diverse:

· Integrative - the Krebs cycle is a central metabolic pathway that combines the processes of breakdown and synthesis of the most important components of the cell.

· Anabolic - cycle substrates are used for the synthesis of many other compounds: oxalacetate is used for the synthesis of glucose (gluconeogenesis) and the synthesis of aspartic acid, acetyl-CoA - for the synthesis of heme, α-ketoglutarate - for the synthesis of glutamic acid, acetyl-CoA - for the synthesis of fatty acids, cholesterol , steroid hormones, acetone bodies, etc.

· Catabolic - in this cycle, the breakdown products of glucose, fatty acids, and ketogenic amino acids complete their journey - all of them are converted into acetyl-CoA; glutamic acid - into α-ketoglutaric acid; aspartic - into oxaloacetate, etc.

· Actually energy - one of the reactions of the cycle (decomposition of succinyl-CoA) is a substrate phosphorylation reaction. During this reaction, one molecule of GTP is formed (the rephosphorylation reaction leads to the formation of ATP).

· Hydrogen donor - with the participation of three NAD + -dependent dehydrogenases (isocitrate, α-ketoglutarate and malate dehydrogenases) and FAD-dependent succinate dehydrogenase, 3 NADH?H + and 1 FADH 2 are formed. These reduced coenzymes are hydrogen donors for the mitochondrial respiratory chain; the energy of hydrogen transfer is used for the synthesis of ATP.

· Anaplerotic - replenishing. Significant amounts of Krebs cycle substrates are used for the synthesis of various compounds and leave the cycle. One of the reactions that compensate for these losses is the reaction catalyzed by pyruvate carboxylase.

The speed of the Krebs cycle reaction is determined by the energy needs of the cell

The rate of reactions of the Krebs cycle correlates with the intensity of the process of tissue respiration and associated oxidative phosphorylation - respiratory control. All metabolites that reflect sufficient energy supply to the cell are inhibitors of the Krebs cycle. An increase in the ATP/ADP ratio is an indicator of sufficient energy supply to the cell and reduces the activity of the cycle. An increase in the ratio of NAD + / NADH, FAD / FADH 2 indicates energy deficiency and is a signal of acceleration of oxidation processes in the Krebs cycle.

The main action of the regulators is aimed at the activity of three key enzymes: citrate synthase, isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. Allosteric inhibitors of citrate synthase are ATP and fatty acids. In some cells, citrate and NADH play the role of its inhibitors. Isocitrate dehydrogenase is allosterically activated by ADP and inhibited by increasing NADH+H + levels.

Rice. 5.15. Tricarboxylic acid cycle (Krebs cycle)

The latter is also an inhibitor of a-ketoglutarate dehydrogenase, the activity of which also decreases with an increase in the level of succinyl-CoA.

The activity of the Krebs cycle largely depends on the supply of substrates. The constant “leakage” of substrates from the cycle (for example, during ammonia poisoning) can cause significant disturbances in the energy supply of cells.

The pentose phosphate pathway of glucose oxidation serves reductive synthesis in the cell.

As the name implies, this pathway produces pentose phosphates, which are much needed by the cell. Since the formation of pentoses is accompanied by oxidation and elimination of the first carbon atom of glucose, this pathway is also called apotomic (apex- top).

The pentose phosphate pathway can be divided into two parts: oxidative and non-oxidative. In the oxidative part, which includes three reactions, NADPH?H + and ribulose-5-phosphate are formed. In the non-oxidative part, ribulose 5-phosphate is converted into various monosaccharides with 3, 4, 5, 6, 7 and 8 carbon atoms; the end products are fructose 6-phosphate and 3-PHA.

· Oxidative part . First reaction-dehydrogenation of glucose-6-phosphate by glucose-6-phosphate dehydrogenase with the formation of δ-lactone 6-phosphogluconic acid and NADPH?H + (NADP + - coenzyme glucose-6-phosphate dehydrogenase).

Second reaction- hydrolysis of 6-phosphogluconolactone by gluconolactone hydrolase. The reaction product is 6-phosphogluconate.

Third reaction- dehydrogenation and decarboxylation of 6-phosphogluconolactone by the enzyme 6-phosphogluconate dehydrogenase, the coenzyme of which is NADP +. During the reaction, the coenzyme is restored and C-1 glucose is cleaved to form ribulose-5-phosphate.

· Non-oxidizing part . Unlike the first, oxidative, all reactions of this part of the pentose phosphate pathway are reversible (Fig. 5.16)

Fig. 5.16. Oxidative part of the pentose phosphate pathway (F-variant)

Ribulose 5-phosphate can isomerize (enzyme - ketoisomerase ) into ribose-5-phosphate and epimerize (enzyme - epimerase ) to xylulose-5-phosphate. This is followed by two types of reactions: transketolase and transaldolase.

Transketolase(coenzyme - thiamine pyrophosphate) splits off a two-carbon fragment and transfers it to other sugars (see diagram). Transaldolase transports three-carbon fragments.

Ribose 5-phosphate and xylulose 5-phosphate react first. This is a transketolase reaction: the 2C fragment is transferred from xylulose-5-phosphate to ribose-5-phosphate.

The two resulting compounds then react with each other in a transaldolase reaction; in this case, as a result of the transfer of the 3C fragment from sedoheptulose-7-phosphate to 3-PHA, erythrose-4-phosphate and fructose-6-phosphate are formed. This is the F-variant of the pentose phosphate pathway. It is characteristic of adipose tissue.

However, reactions can also take a different path (Fig. 5.17). This path is designated as the L-variant. It occurs in the liver and other organs. In this case, octulose-1,8-diphosphate is formed in the transaldolase reaction.

Fig.5.17. Pentose phosphate (apotomic) pathway of glucose metabolism (octulose, or L-variant)

Erythrose 4-phosphate and fructose 6-phosphate can enter into a transketolase reaction, resulting in the formation of fructose 6-phosphate and 3-PHA.

The general equation for the oxidative and non-oxidative parts of the pentose phosphate pathway can be represented as follows:

Glucose-6-P + 7H 2 O + 12NADP + 5 Pentoso-5-P + 6CO 2 + 12 NADPH?H + + Fn.

TRICARBOXYLIC ACIDS CYCLE (KREBS CYCLE)

Glycolysis converts glucose into pyruvate and produces two ATP molecules from a glucose molecule—a small fraction of that molecule's potential energy.

Under aerobic conditions, pyruvate is converted from glycolysis to acetyl-CoA and oxidized to CO2 in the tricarboxylic acid cycle (citric acid cycle). In this case, the electrons released in the reactions of this cycle pass NADH and FADH 2 to 0 2 - the final acceptor. Electron transport is associated with the creation of a proton gradient in the mitochondrial membrane, the energy of which is then used for the synthesis of ATP as a result of oxidative phosphorylation. Let's consider these reactions.

Under aerobic conditions, pyruvic acid (1st stage) undergoes oxidative decarboxylation, more efficient than transformation into lactic acid, with the formation of acetyl-CoA (2nd stage), which can be oxidized to the final products of glucose breakdown - CO 2 and H 2 0 (3rd stage). G. Krebs (1900-1981), a German biochemist, having studied the oxidation of individual organic acids, combined their reactions into a single cycle. Therefore, the tricarboxylic acid cycle is often called the Krebs cycle in his honor.

The oxidation of pyruvic acid to acetyl-CoA occurs in mitochondria with the participation of three enzymes (pyruvate dehydrogenase, lipoamide dehydrogenase, lipoyl acetyltransferase) and five coenzymes (NAD, FAD, thiamine pyrophosphate, lipoic acid amide, coenzyme A). These four coenzymes contain B vitamins (B x, B 2, B 3, B 5), which indicates the need for these vitamins for the normal oxidation of carbohydrates. Under the influence of this complex enzyme system, pyruvate is converted in an oxidative decarboxylation reaction into the active form of acetic acid - acetyl coenzyme A:

Under physiological conditions, pyruvate dehydrogenase is an exclusively irreversible enzyme, which explains the impossibility of converting fatty acids into carbohydrates.

The presence of a high-energy bond in the acetyl-CoA molecule indicates the high reactivity of this compound. In particular, acetyl-CoA can act in mitochondria to generate energy; in the liver, excess acetyl-CoA is used for the synthesis of ketone bodies; in the cytosol it participates in the synthesis of complex molecules such as steroids and fatty acids.

Acetyl-CoA obtained in the reaction of oxidative decarboxylation of pyruvic acid enters the tricarboxylic acid cycle (Krebs cycle). The Krebs cycle, the final catabolic pathway for the oxidation of carbohydrates, fats, and amino acids, is essentially a “metabolic cauldron.” The reactions of the Krebs cycle, which occur exclusively in mitochondria, are also called the citric acid cycle or the tricarboxylic acid cycle (TCA cycle).

One of the most important functions of the tricarboxylic acid cycle is the generation of reduced coenzymes (3 molecules of NADH + H + and 1 molecule of FADH 2) followed by the transfer of hydrogen atoms or their electrons to the final acceptor - molecular oxygen. This transport is accompanied by a large decrease in free energy, part of which is used in the process of oxidative phosphorylation for storage in the form of ATP. It is clear that the tricarboxylic acid cycle is aerobic, oxygen dependent.

1. The initial reaction of the tricarboxylic acid cycle is the condensation of acetyl-CoA and oxaloacetic acid with the participation of the mitochondrial matrix enzyme citrate synthase to form citric acid.

2. Under the influence of the enzyme aconitase, which catalyzes the removal of a water molecule from citrate, the latter turns

to cis-aconitic acid. Water combines with cis-aconitic acid, turning into isocitric acid.

3. The enzyme isocitrate dehydrogenase then catalyzes the first dehydrogenase reaction of the citric acid cycle, when isocitric acid is converted by oxidative decarboxylation to α-ketoglutaric acid:

In this reaction, the first molecule of CO 2 and the first molecule of NADH 4- H + cycle are formed.

4. Further conversion of α-ketoglutaric acid to succinyl-CoA is catalyzed by the multienzyme complex of α-ketoglutaric dehydrogenase. This reaction is chemically analogous to the pyruvate dehydrogenase reaction. It involves lipoic acid, thiamine pyrophosphate, HS-KoA, NAD +, FAD.

As a result of this reaction, a NADH + H + and CO 2 molecule is again formed.

5. The succinyl-CoA molecule has a high-energy bond, the energy of which is stored in the next reaction in the form of GTP. Under the influence of the enzyme succinyl-CoA synthetase, succinyl-CoA is converted into free succinic acid. Note that succinic acid can also be obtained from methylmalonyl-CoA by oxidation of fatty acids with an odd number of carbon atoms.

This reaction is an example of substrate phosphorylation, since the high-energy GTP molecule in this case is formed without the participation of the electron and oxygen transport chain.

6. Succinic acid is oxidized to fumaric acid in the succinate dehydrogenase reaction. Succinate dehydrogenase, a typical iron-sulfur-containing enzyme, the coenzyme of which is FAD. Succinate dehydrogenase is the only enzyme anchored to the inner mitochondrial membrane, while all other cycle enzymes are located in the mitochondrial matrix.

7. This is followed by the hydration of fumaric acid into malic acid under the influence of the enzyme fumarase in a reversible reaction under physiological conditions:

8. The final reaction of the tricarboxylic acid cycle is the malate dehydrogenase reaction with the participation of the active enzyme mitochondrial NAD~-dependent malate dehydrogenase, in which the third molecule of reduced NADH + H + is formed:

The formation of oxaloacetic acid (oxaloacetate) completes one revolution of the tricarboxylic acid cycle. Oxalacetic acid can be used in the oxidation of a second molecule of acetyl-CoA, and this cycle of reactions can be repeated many times, constantly leading to the production of oxaloacetic acid.

Thus, the oxidation of one molecule of acetyl-CoA in the TCA cycle as a substrate of the cycle leads to the production of one molecule of GTP, three molecules of NADP + H + and one molecule of FADH 2. Oxidation of these reducing agents in the biological oxidation chain

lenition leads to the synthesis of 12 ATP molecules. This calculation is clear from the topic “Biological oxidation”: the inclusion of one NAD + molecule in the electron transport system is ultimately accompanied by the formation of 3 ATP molecules, the inclusion of a FADH 2 molecule ensures the formation of 2 ATP molecules, and one GTP molecule is equivalent to 1 ATP molecule.

Note that two carbon atoms of adetyl-CoA enter the tricarboxylic acid cycle and two carbon atoms leave the cycle as CO 2 in decarboxylation reactions catalyzed by isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

With the complete oxidation of a glucose molecule under aerobic conditions to C0 2 and H 2 0, the formation of energy in the form of ATP is:

• 4 molecules of ATP during the conversion of a glucose molecule into 2 molecules of pyruvic acid (glycolysis);
• 6 ATP molecules formed in the 3-phosphoglyceraldehyde dehydrogenase reaction (glycolysis);
• 30 ATP molecules formed during the oxidation of two molecules of pyruvic acid in the pyruvate dehydrogenase reaction and in the subsequent transformations of two molecules of acetyl-CoA to CO 2 and H 2 0 in the tricarboxylic acid cycle. Therefore, the total energy output from complete oxidation of a glucose molecule can be 40 ATP molecules. However, it should be taken into account that during the oxidation of glucose, two ATP molecules are consumed at the stage of converting glucose into glucose-6-phosphate and at the stage of converting fructose-6-phosphate into fructose-1,6-diphosphate. Therefore, the “net” energy output from the oxidation of a glucose molecule is 38 ATP molecules.

You can compare the energetics of anaerobic glycolysis and aerobic catabolism of glucose. Of the 688 kcal of energy theoretically contained in 1 gram molecule of glucose (180 g), 20 kcal is in two molecules of ATP formed in the reactions of anaerobic glycolysis, and 628 kcal theoretically remains in the form of lactic acid.

Under aerobic conditions, from 688 kcal of a gram molecule of glucose in 38 ATP molecules, 380 kcal are obtained. Thus, the efficiency of glucose use under aerobic conditions is approximately 19 times higher than in anaerobic glycolysis.

It should be noted that all oxidation reactions (oxidation of triose phosphate, pyruvic acid, four oxidation reactions of the tricarboxylic acid cycle) compete in the synthesis of ATP from ADP and phosphorus (Pasteur effect). This means that the resulting molecule NADH + H + in oxidation reactions has a choice between the reactions of the respiratory system, transferring hydrogen to oxygen, and the enzyme LDH, transferring hydrogen to pyruvic acid.

In the early stages of the tricarboxylic acid cycle, its acids can leave the cycle to participate in the synthesis of other cell compounds without disrupting the functioning of the cycle itself. Various factors are involved in the regulation of tricarboxylic acid cycle activity. Among them, primarily the supply of acetyl-CoA molecules, the activity of the pyruvate dehydrogenase complex, the activity of the components of the respiratory chain and associated oxidative phosphorylation, as well as the level of oxaloacetic acid should be mentioned.

Molecular oxygen is not directly involved in the tricarboxylic acid cycle, but its reactions are carried out only under aerobic conditions, since NAD ~ and FAD can be regenerated in mitochondria only by transferring electrons to molecular oxygen. It should be emphasized that glycolysis, in contrast to the tricarboxylic acid cycle, is also possible under anaerobic conditions, since NAD~ is regenerated during the transition of pyruvic acid to lactic acid.

In addition to the formation of ATP, the tricarboxylic acid cycle has another important meaning: the cycle provides intermediary structures for various biosyntheses of the body. For example, most of the atoms of porphyrins come from succinyl-CoA, many amino acids are derivatives of α-ketoglutaric and oxaloacetic acids, and fumaric acid occurs in the process of urea synthesis. This demonstrates the integrity of the tricarboxylic acid cycle in the metabolism of carbohydrates, fats, and proteins.

As the reactions of glycolysis show, the ability of most cells to generate energy lies in their mitochondria. The number of mitochondria in various tissues is associated with the physiological functions of the tissues and reflects their ability to participate in aerobic conditions. For example, red blood cells do not have mitochondria and therefore do not have the ability to generate energy using oxygen as the final electron acceptor. However, in cardiac muscle functioning under aerobic conditions, half the volume of the cell cytoplasm is represented by mitochondria. The liver also depends on aerobic conditions for its various functions, and mammalian hepatocytes contain up to 2 thousand mitochondria per cell.

Mitochondria include two membranes - outer and inner. The outer membrane is simpler, consisting of 50% fats and 50% proteins, and has relatively few functions. The inner membrane is structurally and functionally more complex. Approximately 80% of its volume is proteins. It contains most of the enzymes involved in electron transport and oxidative phosphorylation, metabolic intermediaries and adenine nucleotides between the cytosol and the mitochondrial matrix.

Various nucleotides involved in redox reactions, such as NAD +, NADH, NADP +, FAD and FADH 2, do not penetrate the inner mitochondrial membrane. Acetyl-CoA cannot move from the mitochondrial compartment to the cytosol, where it is required for the synthesis of fatty acids or sterols. Therefore, intramitochondrial acetyl-CoA is converted into the citrate synthase reaction of the tricarboxylic acid cycle and enters the cytosol in this form.