Cyclic reactions of tricarboxylic acids proceed. tricarboxylic acid cycle

The acetyl-SCoA formed in the PVC-dehydrogenase reaction then enters into tricarboxylic acid cycle(CTC, citric acid cycle, Krebs cycle). In addition to pyruvate, keto acids coming from catabolism are involved in the cycle. amino acids or any other substances.

Tricarboxylic acid cycle

The cycle runs in mitochondrial matrix and represents oxidation molecules acetyl-SCoA in eight consecutive reactions.

In the first reaction, they bind acetyl And oxaloacetate(oxaloacetic acid) to form citrate(citric acid), then citric acid isomerizes to isocitrate and two dehydrogenation reactions with concomitant release of CO 2 and reduction of NAD.

In the fifth reaction, GTP is formed, this is the reaction substrate phosphorylation. Next, FAD-dependent dehydrogenation occurs sequentially succinate(succinic acid), hydration fumaric acid up malate(malic acid), then NAD-dependent dehydrogenation with the formation of oxaloacetate.

As a result, after eight reactions of the cycle again oxaloacetate is formed .

The last three reactions make up the so-called biochemical motif (FAD-dependent dehydrogenation, hydration and NAD-dependent dehydrogenation, it is used to introduce a keto group into the succinate structure. This motif is also present in fatty acid β-oxidation reactions. In reverse order (reduction, de hydration and recovery) this motif is observed in fatty acid synthesis reactions.

DTC functions

1. Energy

generation hydrogen atoms for the operation of the respiratory chain, namely three NADH molecules and one FADH2 molecule,
single molecule synthesis GTP(equivalent to ATP).

2. Anabolic. In the CTC are formed

heme precursor succinyl-SCoA,
keto acids that can be converted into amino acids - α-ketoglutarate for glutamic acid, oxaloacetate for aspartic,
lemon acid, used for the synthesis of fatty acids,
oxaloacetate, used for glucose synthesis.

Anabolic reactions of the TCA

Regulation of the tricarboxylic acid cycle

Allosteric regulation

Enzymes catalyzing the 1st, 3rd and 4th reactions of TCA are sensitive to allosteric regulation metabolites:

Regulation of oxaloacetate availability

chief And basic the regulator of the TCA is oxaloacetate, or rather its availability. The presence of oxaloacetate involves acetyl-SCoA in the TCA cycle and starts the process.

Usually the cell has balance between the formation of acetyl-SCoA (from glucose, fatty acids or amino acids) and the amount of oxaloacetate. The source of oxaloacetate is pyruvate, (formed from glucose or alanine), derived from aspartic acid as a result of transamination or the AMP-IMF cycle, and also from fruit acids the cycle itself (succinic, α-ketoglutaric, malic, citric), which can be formed during the catabolism of amino acids or come from other processes.

Synthesis of oxaloacetate from pyruvate

Regulation of enzyme activity pyruvate carboxylase carried out with the participation acetyl-SCoA. It is allosteric activator enzyme, and without it, pyruvate carboxylase is practically inactive. When acetyl-SCoA accumulates, the enzyme starts to work and oxaloacetate is formed, but, of course, only in the presence of pyruvate.

Also most amino acids during their catabolism, they are able to turn into metabolites of TCA, which then go to oxaloacetate, which also maintains the activity of the cycle.

Replenishment of the pool of TCA metabolites from amino acids

Cycle replenishment reactions with new metabolites (oxaloacetate, citrate, α-ketoglutarate, etc.) are called anaplerotic.

The role of oxaloacetate in metabolism

An example of a significant role oxaloacetate serves to activate the synthesis of ketone bodies and ketoacidosis blood plasma at inadequate the amount of oxaloacetate in the liver. This condition is observed during decompensation of insulin-dependent diabetes mellitus (type 1 diabetes) and during starvation. With these disorders, the process of gluconeogenesis is activated in the liver, i.e. the formation of glucose from oxaloacetate and other metabolites, which entails a decrease in the amount of oxaloacetate. Simultaneous activation of fatty acid oxidation and accumulation of acetyl-SCoA triggers a backup pathway for the utilization of the acetyl group - synthesis of ketone bodies. In this case, the body develops acidification of the blood ( ketoacidosis) with a characteristic clinical picture: weakness, headache, drowsiness, decreased muscle tone, body temperature and blood pressure.

Change in the rate of TCA reactions and the reasons for the accumulation of ketone bodies under certain conditions

The described method of regulation with the participation of oxaloacetate is an illustration of the beautiful formulation " Fats burn in the flame of carbohydrates". It implies that the "burning flame" of glucose leads to the appearance of pyruvate, and pyruvate is converted not only into acetyl-SCoA, but also into oxaloacetate. The presence of oxaloacetate guarantees the inclusion of an acetyl group formed from fatty acids in the form of acetyl-SCoA, in the first reaction of the TCA.

In the case of a large-scale "burning" of fatty acids, which is observed in the muscles during physical work and in the liver fasting, the rate of entry of acetyl-SCoA in the TCA reaction will directly depend on the amount of oxaloacetate (or oxidized glucose).

If the amount of oxaloacetate in hepatocyte not enough (no glucose or it is not oxidized to pyruvate), then the acetyl group will go to the synthesis of ketone bodies. This happens when prolonged fasting And type 1 diabetes.

The Krebs cycle is also called the citric acid cycle, or cellular respiration. The cycle of citric acid transformation in living cells was discovered and studied by the German biochemist Hans Krebs, for this work he (together with F. Lipman) was awarded the Nobel Prize (1953). Many scientists took part in deciphering the individual reactions of this process: A. Szent-Gyorgyi, A. Lehninger, S. E. Severin and others.

The Krebs cycle is the final pathway for the oxidation of acetyl groups, into which most of the organic molecules that play the role of "cellular fuel" - carbohydrates, fatty acids and amino acids, are converted during catabolism.

In one turn of the cycle, consisting of eight enzymatic reactions, one molecule is completely oxidized. At the level of the Krebs cycle, the breakdown pathways of carbohydrates, lipids and proteins are combined. Metabolites of the Krebs cycle are used to synthesize other substances (oxaloacetic acid → glucose, aspartic acid ). This cycle occurs in the mitochondrial matrix.

The Krebs cycle is the main hydrogen supply system for the mitochondrial respiratory chain.

All paths of catabolism are reduced to the formation of a three-carbon compound - pyruvic acid, which then, by oxidative decarboxylation in the presence of the coenzyme - thiamine pyrophosphate, undergoes decarboxylation with the formation of acetyl-CoA. Acetyl coenzyme A “burns out” in the Krebs cycle to two CO 2 molecules.

1. The first reaction of the Krebs cycle is the formation of citrate - citric acid

2. In the second reaction, through the stage of dehydration and the formation of cis-aconitic acid, isocitric acid is formed.

Note that adding a water molecule to cis-aconitic acid goes against Markovnikov's rule.

3. In the third reaction, which seems to limit the rate of the Krebs cycle, isocitric acid is dehydrogenated in the presence of NAD-dependent isocitrate dehydrogenase:

4. In the fourth reaction, oxidative decarboxylation of α-ketoglutaric acid to succinyl-CoA occurs. The mechanism of this reaction is similar to the reaction of oxidative decarboxylation of pyruvate to acetyl-CoA.

5. 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 thioether bond of succinyl-CoA:

6. 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:

7. 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 has stereospecificity - during this reaction, L-malic acid (malate) is formed:

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

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 via 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 conjugated oxidative phosphorylation), and therefore, in total, 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, during the oxidation of acetyl-CoA in the Krebs cycle, 12 ATP molecules are formed.

Schematic representation of the Krebs cycle:

There are two key enzymes in TCA:

1) citrate synthase (1st reaction)

2) isocitrate dehydrogenase (3rd reaction)

Both enzymes are allosterically inhibited by excess ATP and NADH 2 . Isocitrate dehydrogenase is strongly activated by ADP. If there is no ADP, then this enzyme is inactive. Under conditions of energy rest, the ATP concentration increases, and the rate of TCA reactions is low - ATP synthesis decreases. Isocitrate dehydrogenase is inhibited by ATP much more strongly than citrate synthase; therefore, under conditions of energy rest, the concentration of citrate increases, and it enters the cytoplasm along the concentration gradient by facilitated diffusion. In the cytoplasm, citrate is converted to Acetyl-CoA, which is involved in the synthesis of fatty acids. Intermediate products of the metabolism of the Krebs cycle go to the synthesis of other substances. From α-ketoglutarate and oxaloacetic acid (oxaloacetate) amino acids are synthesized, from oxaloacetic acid - carbohydrates, from succinyl-CoA → synthesis of heme hemoglobin. The resulting reduced coenzymes NADH 2 and FADH 2 in the respiratory chain are oxidized to form water, ATP and a by-product, hydrogen peroxide.

Tricarboxylic acid cycle

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

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

Functions

Integrative function- the cycle is the link between the reactions of anabolism and catabolism.
catabolic function- transformation of various substances into cycle substrates:
- Fatty acids, pyruvate, Leu, Phen - Acetyl-CoA.
- Arg, His, Glu - α-ketoglutarate.
- Hair dryer, shooting range - fumarate.
Anabolic function- the use of cycle substrates for the synthesis of organic substances:
- Oxalacetate - glucose, Asp, Asn.
- Succinyl-CoA - heme synthesis.
- CO 2 - carboxylation reactions.
Hydrogen donor function- the Krebs cycle supplies protons to the mitochondrial respiratory chain in the form of three NADH.H + and one FADH 2 .
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, 1 GTP is synthesized in the cycle by substrate phosphorylation, and then ATP is synthesized from it through transphosphorylation: GTP + ADP = ATP + GDP.

Mnemonic rules

For easier memorization of the acids involved in the Krebs cycle, there is a mnemonic rule:

A Whole Pineapple And A Slice Of Soufflé Today Is Actually My Lunch, 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 an assistant at the Department of Biochemistry of KSMU E. V. Parshkova):

pike at acetyl lemon silt, but nar cis With a con I was afraid He was above him isolimon But Alpha ketoglutar alas. Succinyl Xia coenzyme om, Amber silt fumar ovo, Yabloch ek stocked up for the winter, turned around pike oh again.

(oxaloacetic acid, citric acid, cis-aconitic acid, isocitric acid, α-ketoglutaric acid, succinyl-KoA, 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 having given NAD, it loses CO 2, this is immensely glad alpha-ketoglutarate, oxidation is coming - NAD has stolen hydrogen TDF, coenzyme A take CO 2 and the energy barely appeared in succinyl immediately GTP was born and succinate was left, so it got to FAD - hydrogens need water fumarate on I drank, and it turned into malate, then OVER came to malate, hydrogens acquired PIKE reappeared and quietly hid To watch for acetate ...

Notes

Links

tricarboxylic acid cycle

Tricarboxylic acid cycle (TsTK) or citric acid cycle or Krebs cycle – the path of oxidative transformations of di- and tricarboxylic acids, which are formed as intermediate products during the breakdown and synthesis of proteins, fats and carbohydrates.

The tricarboxylic acid cycle is present in the cells of all organisms: plants, animals and microorganisms.

This cycle is basis of metabolism and performs two important functions:

- supplying the body with energy ;

- integration of all major metabolic flows, both catabolic (biodegradation) and anabolic (biosynthesis) .

Let me remind you that the reactions of aerobic glycolysis localized in cytoplasm cells and lead to the formation pyruvate (PVC).

!!! Subsequent transformation pyruvate flow into mitochondrial matrix.

In the matrix, pyruvate is converted to acetyl-CoA – macroergic compound. The reaction is catalyzed by an enzyme NAD-dependent pyruvate decarboxylase:

Restored form NADH∙H + , formed as a result of this reaction, enters the respiratory chain and generates 6 ATP molecules(in terms of 1 molecule of glucose).

!!! TCA is a sequence of eight reactions occurring in the matrix mitochondria (Rice. 9.6):

Rice. 9.6. Scheme of the tricarboxylic acid cycle

1) Irreversible reaction condensation acetyl-CoA co oxaloacetic acid (oxaloacetate) catalyzed by the enzyme citrate synthetase, with education citric acid (citrate ).

2) Reversible reaction isomerization citric acid (citrate ) V isocitric acid (isocitrate ), during which transfer of a hydroxyl group to another carbon atom, catalyzed by an enzyme aconitase .

The reaction comes through education intermediate product
cis-acanitic acid (cis aconitate ).

3) Irreversible reaction oxidative decarboxylation isocitric acid (isocitrate ): hydroxy group isocitric acid oxidized to a carbonyl group using the oxidized form OVER + and at the same time the carboxyl group is cleaved off
β position with education α-ketoglutaric acid
(α-ketoglutarate ). The intermediate product of this reaction oxalosuccinic acid (oxalosuccinate ).

This is the first reaction of the cycle in which the oxidized form of NAD + -coenzyme is reduced to NADH ∙ H +, the enzyme isocitrate dehydrogenase.

Restored form NADH∙N goes to respiratory chain, there it oxidizes to OVER +, which leads to the formation 2 molecules ATP .

4) Reversible reaction oxidative decarboxylation
α-ketoglutaric acid before macroergic connections succinyl-CoA . The reaction is catalyzed by an enzyme 2-oxoglutarate dehydrogenase complex.

5) Reaction is the only reaction in the cycle; catalyzed by an enzyme succinyl-CoA synthetase. In this reaction succinyl-CoA starring guanodine diphosphate (GDP ) And inorganic phosphate (H3PO4 ) turns into succinic acid (succinate ).

!!! At the same time, the synthesis of the macroergic compound GTP at the expense macroergic connection thioether bond succinyl-CoA.

6) Reaction dehydrogenation succinic acid (succinate ) with education fumaric acid (fumarate).

The reaction is catalyzed by a complex enzyme succinate dehydrogenase, in the molecule of which the coenzyme FAD + covalently bound, but by the protein part of the enzyme. oxidized form FAD + as a result of the reaction is reduced to FAD∙H 2.

Restored form FAD∙H 2 goes to respiratory chain, there it regenerates to the oxidized form FAD +, which leads to the formation two molecules ATP.

7) Reaction hydration fumaric acid (fumarate ) before malic acid (malate fumarase.

8) Reaction dehydrogenation malic acid before oxalacetic acid (oxaloacetate ). The reaction is catalyzed by an enzyme NAD+-dependent-malate dehydrogenase.

As a result of the reaction, the oxidized form NAD is recovering to restored form NADH∙H +.

Restored form NADH∙N goes to respiratory chain, there it oxidizes to OVER +, which leads to the formation 2 ATP molecules.

Summary Equation The CTC can be written as follows:

Acetyl-CoA + 3NAD + + FAD + + GDP + H 3 PO 4 =

2CO 2 + H 2 O + HS-CoA + 3NADH ∙ H + FAD ∙ H 2 + GTP

As can be seen from the scheme of the total equation of the CTC in this process, the following are restored:

Three molecules NADH∙N(reactions 3, 4, 8);

One molecule FAD∙H 2(reaction 6).

During aerobic oxidation of these molecules in the electron transport chain in the process of oxidative phosphorylation, it is formed during oxidation:

- one molecules NADH∙N – 3 molecules ATP ;

- one molecules FAD∙H 2 – 2 molecules ATP.

- one molecule GTP formed in the reaction substrate phosphorylation (reaction 5).

All this will amount to: 9 (3x3) ATP + 2 ATP + 1 ATP (GTP ) = 12 ATP . Hence, energy balance oxidation acetyl-CoA (2 molecules pyruvate from aerobic glycolysis) V TsTK is 24 molecules ATP .

!!! Complete oxidation glucose :

8 molecules ATP glycolysis + 6 molecules ATP oxidative decarboxylation of pyruvate to cetyl-CoA + 24 molecules ATP CTK =
38 molecules ATP per glucose molecule.

Tricarboxylic acid cycle

The cycle runs in mitochondrial matrix and represents oxidation molecules acetyl-SCoA in eight consecutive reactions.

As a result, after eight reactions of the cycle again oxaloacetate is formed .

DTC functions

1. Energy

generation hydrogen atoms for the operation of the respiratory chain, namely three NADH molecules and one FADH2 molecule,
single molecule synthesis GTP(equivalent to ATP).

2. Anabolic. In the CTC are formed

heme precursor succinyl-SCoA,
keto acids that can be converted into amino acids - α-ketoglutarate for glutamic acid, oxaloacetate for aspartic,
lemon acid, used for the synthesis of fatty acids,
oxaloacetate, used for glucose synthesis.

Anabolic reactions of the TCA

Regulation of the tricarboxylic acid cycle

Allosteric regulation

Enzymes catalyzing the 1st, 3rd and 4th reactions of TCA are sensitive to allosteric regulation metabolites:

Regulation of oxaloacetate availability

chief And basic the regulator of the TCA is oxaloacetate, or rather its availability. The presence of oxaloacetate involves acetyl-SCoA in the TCA cycle and starts the process.

Synthesis of oxaloacetate from pyruvate

Also most amino acids during their catabolism, they are able to turn into metabolites of TCA, which then go to oxaloacetate, which also maintains the activity of the cycle.

Replenishment of the pool of TCA metabolites from amino acids

Cycle replenishment reactions with new metabolites (oxaloacetate, citrate, α-ketoglutarate, etc.) are called anaplerotic.