Glycolysis. Aerobic and anaerobic glycolysis
In this article, we will take a closer look at aerobic glycolysis, its processes, and analyze the stages and stages. Let's get acquainted with the anaerobic, learn about the evolutionary modifications of this process and determine its biological significance.
What is glycolysis
Glycolysis is one of the three forms of glucose oxidation, in which the oxidation process itself is accompanied by the release of energy, which is stored in NADH and ATP. In the process of glycolysis from a molecule two molecules of pyruvic acid.
Glycolysis is a process that occurs under the influence of various biological catalysts - enzymes. The main oxidizing agent is oxygen - O 2, however, the processes of glycolysis can proceed in its absence. This type of glycolysis is called anaerobic glycolysis.
The process of glycolysis in the absence of oxygen
Anaerobic glycolysis is a stepwise process of glucose oxidation in which glucose is not completely oxidized. One molecule of pyruvic acid is formed. And from an energy point of view, glycolysis without the participation of oxygen (anaerobic) is less beneficial. However, when oxygen enters the cell, the anaerobic oxidation process can turn into an aerobic one and proceed in a full-fledged form.
Mechanisms of glycolysis
The process of glycolysis is the decomposition of six-carbon glucose into three-carbon pyruvate in the form of two molecules. The process itself is divided into 5 stages of preparation and 5 stages in which energy is stored in ATP.
The process of glycolysis of 2 stages and 10 stages is as follows:
- Stage 1, stage 1 - phosphorylation of glucose. At the sixth carbon atom in glucose, the saccharide itself is activated via phosphorylation.
- Stage 2 - isomerization of glucose-6-phosphate. At this stage, phosphoglucoseimerase catalytically converts glucose into fructose-6-phosphate.
- Stage 3 - Fructose-6-phosphate and its phosphorylation. This step consists in the formation of fructose-1,6-diphosphate (aldolase) by the action of phosphofructokinase-1, which accompanies the phosphoryl group from adenosine triphosphoric acid to the fructose molecule.
- Step 4 is the process of cleavage of aldolase to form two molecules of triose phosphate, namely eldose and ketose.
- Stage 5 - triose phosphates and their isomerization. At this stage, glyceraldehyde-3-phosphate is sent to subsequent steps in the breakdown of glucose, and dihydroxyacetone phosphate is converted to the form of glyceraldehyde-3-phosphate under the influence of the enzyme.
- Stage 2, stage 6 (1) - Glyceraldehyde-3-phosphate and its oxidation - the stage in which this molecule is oxidized and phosphorylated to diphosphoglycerate-1,3.
- Step 7 (2) - aims to transfer the phosphate group to ADP from 1,3-diphosphoglycerate. The end products of this stage are the formation of 3-phosphoglycerate and ATP.
- Step 8 (3) - transition from 3-phosphoglycerate to 2-phosphoglycerate. This process occurs under the influence of the enzyme phosphoglycerate mutase. A prerequisite for the occurrence of a chemical reaction is the presence of magnesium (Mg).
- Stage 9 (4) - 2 phosphoglycerta is dehydrated.
- Stage 10 (5) - phosphates obtained as a result of the passage of the previous stages are transferred to ADP and PEP. Energy from phosphoenulpyrovate is transferred to ADP. The reaction requires the presence of potassium (K) and magnesium (Mg) ions.
Modified forms of glycolysis
The process of glycolysis can be accompanied by additional production of 1,3 and 2,3-biphosphoglycerates. 2,3-phosphoglycerate under the influence of biological catalysts is able to return to glycolysis and pass into the form of 3-phosphoglycerate. The role of these enzymes is diverse, for example, 2,3-biphosphoglycerate, being in hemoglobin, causes oxygen to pass into tissues, promoting dissociation and lowering the affinity of O 2 and red blood cells.
Many bacteria change the forms of glycolysis at various stages, reducing their total number or modifying them under the influence of various enzymes. A small part of anaerobes have other methods of carbohydrate decomposition. Many thermophiles have only 2 glycolytic enzymes at all, these are enolase and pyruvate kinase.
Glycogen and starch, disaccharides and other types of monosaccharides
Aerobic glycolysis is a process that is also characteristic of other types of carbohydrates, and specifically, it is inherent in starch, glycogen, and most disaccharides (manose, galactose, fructose, sucrose, and others). The functions of all types of carbohydrates are generally aimed at obtaining energy, but may differ in the specifics of their purpose, use, etc. For example, glycogen lends itself to glycogenesis, which in fact is a phospholytic mechanism aimed at obtaining energy from the breakdown of glycogen. Glycogen itself can be stored in the body as a reserve source of energy. So, for example, glucose obtained during a meal, but not absorbed by the brain, accumulates in the liver and will be used when there is a lack of glucose in the body in order to protect the individual from serious disruptions in homeostasis.
Significance of glycolysis
Glycolysis is a unique, but not the only type of glucose oxidation in the body, the cell of both prokaryotes and eukaryotes. Glycolysis enzymes are water soluble. The glycolysis reaction in some tissues and cells can only occur in this way, for example, in the brain and liver nephron cells. Other ways of oxidizing glucose in these organs are not used. However, the functions of glycolysis are not the same everywhere. For example, adipose tissue and liver in the process of digestion extract the necessary substrates from glucose for the synthesis of fats. Many plants use glycolysis as a way to extract the bulk of their energy.
Glucose catabolism 1 is accompanied by the consumption of 2 ATP molecules for substrate phosphorylation of hexoses, the formation of 4 ATP molecules in substrate phosphorylation reactions, the reduction of 2 NADH 2 molecules and the synthesis of 2 PVC molecules. 2 cytoplasmic molecules of NADH 2, depending on the shuttle mechanism, give from 4 to 6 ATP molecules in the respiratory chain of mitochondria.
Thus, the final energy effect of aerobic glycolysis, depending on the shuttle mechanism, is from 6 to 8 ATP molecules.
Anaerobic glycolysis
Under anaerobic conditions, PVC, like O 2 in the respiratory chain, ensures the regeneration of NAD + from NADH 2, which is necessary for the continuation of glycolysis reactions. PVC is then converted to lactic acid. The reaction takes place in the cytoplasm with the participation of lactate dehydrogenase.
11. Lactate dehydrogenase(lactate: NAD + oxidoreductase). Consists of 4 subunits, has 5 isoforms.
Lactate is not a metabolic end product that is removed from the body. This substance from the tissue enters the bloodstream and is utilized, turning into glucose in the liver (Cori cycle), or, when oxygen is available, it turns into PVC, which enters the general path of catabolism, oxidizing to CO 2 and H 2 O.
ATP output during anaerobic glycolysis
Anaerobic glycolysis is less efficient than aerobic glycolysis. The catabolism of 1 glucose is accompanied by the consumption of 2 ATP molecules for substrate phosphorylation, the formation of 4 ATP molecules in reactions of substrate phosphorylation, and the synthesis of 2 lactate molecules. Thus, the final energy effect of anaerobic glycolysis is equal to 2 ATP molecules.
The plastic significance of glucose catabolism
During catabolism, glucose can perform plastic functions. Glycolysis metabolites are used to synthesize new compounds. Thus, fructose-6f and 3-PGA are involved in the formation of ribose-5-f (a component of nucleotides); 3-phosphoglycerate can be included in the synthesis of amino acids such as serine, glycine, cysteine. In the liver and adipose tissue, Acetyl-CoA is used in the biosynthesis of fatty acids, cholesterol, and DAP is used for the synthesis of glycerol-3p.
Regulation of glycolysis
Pasteur effect– decrease in the rate of glucose consumption and lactate accumulation in the presence of oxygen.
The Pasteur effect is explained by the presence of competition between the enzymes of the aerobic (DG malate, glycerol-6f DG, PVA DG) and anaerobic (LDG) oxidation pathways for the common metabolite of PVA and the coenzyme NADH 2 . Without oxygen, mitochondria do not consume PVK and NADH 2, as a result, their concentration in the cytoplasm increases and they go to the formation of lactate. In the presence of oxygen, mitochondria pump out PVC and NADH2 from the cytoplasm, interrupting the lactate formation reaction. Since anaerobic glycolysis produces little ATP, there may be an excess of AMP (ADP + ADP = AMP + ATP), which, via phosphofructokinase 1, stimulates glycolysis. During aerobic catabolism of glucose, ATP is formed a lot, a possible excess of ATP through phosphofructokinase 1 and pyruvate kinase, on the contrary, inhibits glycolysis. The accumulation of glucose-6p inhibits hexokinase, which reduces the consumption of glucose by cells.
FRUCTOSE AND GALACTOSE METABOLISM
Fructose and galactose, along with glucose, are used to obtain energy or synthesize substances: glycogen, TG, GAG, lactose, etc.
Fructose metabolism
A significant amount of fructose, formed during the breakdown of sucrose, is converted into glucose already in the intestinal cells. Part of the fructose goes to the liver.
To understand what glycolysis is, you will have to turn to Greek terminology, because this term comes from the Greek words: glycos - sweet and lysis - splitting. From the word Glycos comes the name of glucose. Thus, this term refers to the process of saturation of glucose with oxygen, as a result of which one molecule of a sweet substance breaks down into two microparticles of pyruvic acid. Glycolysis is a biochemical reaction that occurs in living cells and is aimed at breaking down glucose. There are three types of glucose breakdown, and aerobic glycolysis is one of them.
This process consists of a number of intermediate chemical reactions accompanied by the release of energy. This is the essence of glycolysis. The released energy is spent on the general vital activity of a living organism. The general formula for glucose breakdown looks like this:
Glucose + 2NAD + + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2H + + 2ATP + 2H2O
Aerobic oxidation of glucose, followed by cleavage of its six-carbon molecule, is carried out through 10 intermediate reactions. The first 5 reactions are combined by the preparatory phase of preparation, and subsequent reactions are aimed at the formation of ATP. During the reactions, stereoscopic isomers of sugars and their derivatives are formed. The main accumulation of energy by cells occurs in the second phase associated with the formation of ATP.
Stages of oxidative glycolysis. Phase 1
In aerobic glycolysis, 2 phases are distinguished.
The first phase is preparatory. In it, glucose reacts with 2 ATP molecules. This phase consists of 5 consecutive steps of biochemical reactions.
1st step. Phosphorylation of glucose
Phosphorylation, that is, the process of transferring phosphoric acid residues in the first and subsequent reactions, is carried out at the expense of adesine triphosphoric acid molecules.
In the first step, phosphoric acid residues from adesine triphosphate molecules are transferred to the molecular structure of glucose. The process produces glucose-6-phosphate. Hexokinase acts as a catalyst in the process, accelerating the process with the help of magnesium ions, acting as a cofactor. Magnesium ions are also involved in other reactions of glycolysis.
2nd stage. Formation of the glucose-6-phosphate isomer
At the 2nd stage, the isomerization of glucose-6-phosphate to fructose-6-phosphate occurs.
Isomerization is the formation of substances that have the same weight, composition of chemical elements, but have different properties due to the different arrangement of atoms in the molecule. The isomerization of substances is carried out under the influence of external conditions: pressure, temperature, catalysts.
In this case, the process is carried out under the action of a phosphoglucose isomerase catalyst with the participation of Mg + ions.
3rd step. Phosphorylation of fructose-6-phosphate
At this stage, the addition of a phosphoryl group occurs due to ATP. The process is carried out with the participation of the enzyme phosphofructokinase-1. This enzyme is intended only for participation in hydrolysis. As a result of the reaction, fructose-1,6-bisphosphate and nucleotide adesine triphosphate are obtained.
ATP - adesine triphosphate, a unique source of energy in a living organism. It is a rather complex and bulky molecule consisting of hydrocarbon, hydroxyl groups, nitrogen and phosphoric acid groups with one free bond, assembled in several cyclic and linear structures. The release of energy occurs as a result of the interaction of phosphoric acid residues with water. Hydrolysis of ATP is accompanied by the formation of phosphoric acid and the release of 40-60 J of energy that the body spends on its vital activity.
But first, phosphorylation of glucose must occur due to the Adesine triphosphate molecule, that is, the transfer of the phosphoric acid residue to glucose.
4th step. The breakdown of fructose-1,6-diphosphate
In the fourth reaction, fructose-1,6-diphosphate decomposes into two new substances.
- dihydroxyacetone phosphate,
- Glyceraldehyde-3-phosphate.
In this chemical process, aldolase acts as a catalyst, an enzyme involved in energy metabolism and necessary for diagnosing a number of diseases.
5th step. Formation of triose phosphate isomers
And finally, the last process is the isomerization of triose phosphates.
Glycerald-3-phosphate will continue to participate in the process of aerobic hydrolysis. And the second component, dihydroxyacetone phosphate, with the participation of the enzyme triose phosphate isomerase, is converted into glyceraldehyde-3-phosphate. But this transformation is reversible.
Phase 2. Synthesis of adesine triphosphate
In this phase of glycolysis, biochemical energy will be accumulated in the form of ATP. Adesine triphosphate is formed from adesine diphosphate by phosphorylation. It also produces NADH.
The abbreviation NADH has a very complex and difficult-to-remember decoding for a non-specialist - Nicotinamide adenine dinucleotide. NADH is a coenzyme, a non-protein compound involved in the chemical processes of a living cell. It exists in two forms:
- oxidized (NAD + , NADox);
- restored (NADH, NADred).
In metabolism, NAD takes part in redox reactions by transporting electrons from one chemical process to another. By donating or accepting an electron, the molecule is converted from NAD + to NADH, and vice versa. In a living organism, NAD is produced from tryptophan or amino acid aspartate.
Two microparticles of glyceraldehyde-3-phosphate undergo reactions during which pyruvate is formed, and 4 ATP molecules. But the final output of adesine triphosphate will be 2 molecules, since two are spent in the preparatory phase. The process continues.
6th step - oxidation of glyceraldehyde-3-phosphate
In this reaction, the oxidation and phosphorylation of glyceraldehyde-3-phosphate occurs. The result is 1,3-diphosphoglyceric acid. Glyceraldehyde-3-phosphate dehydrogenase is involved in the acceleration of the reaction
The reaction occurs with the participation of energy received from outside, therefore it is called endergonic. Such reactions proceed in parallel with exergonic, that is, releasing, giving off energy. In this case, such a reaction is the following process.
7th step. Transfer of the phosphate group from 1,3-diphosphoglycerate to adesine diphosphate
In this intermediate reaction, a phosphoryl group is transferred by phosphoglycerate kinase from 1,3-diphosphoglycerate to adesine diphosphate. The result is 3-phosphoglycerate and ATP.
The enzyme phosphoglycerate kinase gets its name from its ability to catalyze reactions in both directions. This enzyme also transports a phosphate residue from adesine triphosphate to 3-phosphoglycerate.
The 6th and 7th reactions are often considered as a single process. 1,3-diphosphoglycerate in it is considered as an intermediate product. Together, the 6th and 7th reactions look like this:
Glyceraldehyde-3-phosphate + ADP + Pi + NAD + ⇌3 -phosphoglycerate + ATP + NADH + H +, ΔG'o \u003d -12.2 kJ / mol.
And in total these 2 processes release part of the energy.
8th step. Transfer of a phosphoryl group from 3-phosphoglycerate.
Obtaining 2-phosphoglycerate is a reversible process, occurs under the catalytic action of the enzyme phosphoglycerate mutase. The phosphoryl group is transferred from the divalent carbon atom of 3-phosphoglycerate to the trivalent atom of 2-phosphoglycerate, resulting in the formation of 2-phosphoglyceric acid. The reaction takes place with the participation of positively charged magnesium ions.
9th step. Isolation of water from 2-phosphoglycerate
This reaction is essentially the second reaction of glucose breakdown (the first was the reaction of the 6th step). In it, the enzyme phosphopyruvate hydratase stimulates the elimination of water from the C atom, that is, the process of elimination from the 2-phosphoglycerate molecule and the formation of phosphoenolpyruvate (phosphoenolpyruvic acid).
10th and last step. Transfer of a phosphate residue from PEP to ADP
The final reaction of glycolysis involves coenzymes - potassium, magnesium and manganese, the enzyme pyruvate kinase acts as a catalyst.
The conversion of the enol form of pyruvic acid to the keto form is a reversible process, and both isomers are present in cells. The process of transition of isometric substances from one to another is called tautomerization.
What is anaerobic glycolysis?
Along with aerobic glycolysis, that is, the breakdown of glucose with the participation of O2, there is also the so-called anaerobic breakdown of glucose, in which oxygen does not participate. It also consists of ten consecutive reactions. But where does the anaerobic stage of glycolysis take place, is it associated with the processes of oxygen breakdown of glucose, or is it an independent biochemical process, let's try to figure it out.
Anaerobic glycolysis is the breakdown of glucose in the absence of oxygen to form lactate. But in the process of formation of lactic acid, NADH does not accumulate in the cell. This process is carried out in those tissues and cells that function under conditions of oxygen starvation - hypoxia. These tissues primarily include skeletal muscles. In red blood cells, despite the presence of oxygen, lactate is also formed during glycolysis, because there are no mitochondria in blood cells.
Anaerobic hydrolysis occurs in the cytosol (liquid part of the cytoplasm) of cells and is the only act that produces and supplies ATP, since in this case oxidative phosphorylation does not work. Oxygen is needed for oxidative processes, but it is not present in anaerobic glycolysis.
Both pyruvic and lactic acids serve as sources of energy for the muscles to perform certain tasks. Excess acids enter the liver, where, under the action of enzymes, they are again converted into glycogen and glucose. And the process starts again. The lack of glucose is replenished by nutrition - the use of sugar, sweet fruits, and other sweets. So you can’t completely refuse sweets for the sake of the figure. The body needs sucrose, but in moderation.
Before studying cellular respiration in detail, it is useful to familiarize yourself with it in general terms. The figure shows the pathways of aerobic and anaerobic respiration. Note that there is only one aerobic pathway, while there are two anaerobic pathways. Note also that the first stage of all these paths is common. This step is glycolysis.
glycolysis
Glycolysis The oxidation of glucose to pyruvic acid is called. As can be seen from the figure, one molecule of glucose (6-carbon compound, 6C) produces two molecules of pyruvic acid (beta-carbon compound, 3C). The process takes place not in the mitochondria, but in the cytoplasm of the cell, and oxygen is not required for it. The process can be subdivided into three stages:
1. Phosphorylation of sugar. As a result of this reaction, the sugar is “activated”, i.e., its reactivity increases. When activated, some ATP is consumed, and since the whole point of breathing is to supply ATP, it may seem counterproductive to expend it. This should, however, be seen as a kind of "investment" by which the reactions leading to the formation of ATP can later occur.
2. Cleavage of phosphorylated 3C sugar into two 3C-saccharophosphates. The origin of the name "glycolysis" (from the Greek lysis - decomposition, decay) is also connected with this. The two sugar phosphates formed are isomers. Before undergoing further transformation, one of them passes into the other, so that two identical 3C-saccharophosphates are obtained.
3. Oxidation by elimination of hydrogen.
Each 3C-saccharophosphate converted to pyruvic acid. In this case, dehydrogenation occurs with the formation of one molecule of reduced NAD and two molecules of ATP. The total yield (from two molecules of 3C-saccharophosphate) is: two molecules of reduced NAD and four molecules of ATP.
So on first step of glycolysis in phosphorylation reactions, two ATP molecules are consumed, and in the third, four molecules are formed. Thus, the net yield of ATP from glycolysis is two molecules. In addition, during glycolysis, four hydrogen atoms are cleaved off and transferred to NAD. We will consider their fate later. The overall reaction of glycolysis can be written as follows:
Consumption and output of various substances in the process of glycolysis are indicated in the table.
When used in lipid respiration process glycerol is easily converted into 3C-saccharophosphate, which enters the path of glycolysis. In this case, one molecule of ATP is consumed and three molecules are formed.
ultimate destiny pyruvic acid depends on the presence of oxygen in the cell. If oxygen is available, then pyruvic acid passes into the mitochondria for complete oxidation to CO2 and water (aerobic respiration). If there is no oxygen, then it turns into either ethanol or lactic acid (anaerobic respiration).
glycolysis- a sequence of enzymatic reactions leading to the breakdown of glucose with the formation of PVC, accompanied by the formation of ATP (in the cytosol of the cell). There are two types of glycolysis - aerobic and anaerobic.
Aerobic glycolysis: PVK is formed and enters the mitochondria. Under aerobic conditions, PVA further, in the general path of catabolism, decomposes to CO 2 and H 2 O. Aerobic glycolysis is part of the aerobic breakdown of glucose.
Anaerobic glycolysis: PVC is formed, which is then converted to lactate. Anaerobic breakdown of glucose and anaerobic glycolysis are synonyms. Anaerobic glycolysis occurs in the first minutes of muscle work, in erythrocytes (no mitochondria), with insufficient oxygen supply.
Glycolysis reactions:
1). Phosphorylation of glucose. The reaction is catalyzed by hexokinase, in the parenchymal cells of the liver - by glucokinase. The formation of glucose-6-phosphate in the cell is a trap for glucose, because the membrane for phosphorylated glucose is impermeable. Glucose-6-phosphate is an allosteric inhibitor of the reaction.
2). Isomerization reaction with the participation of glucose-6-phosphate isomerase:
3) Limiting stage- phosphorylation reaction catalyzed by 6-phosphofructokinase, which is inhibited by ATP and citrate, activated by - AMP.
4). Aldol cleavage reaction with the participation of aldolase.
5). Isomerization of dihydroxyacetone phosphate, enzyme - triose phosphate isomerase:
1 molecule of glucose is converted into 2 molecules of glyceraldehyde-3-phosphate (reactions 4, 5).
6). Oxidation reaction, enzyme - glyceraldehyde phosphate dehydrogenase:
7). Substrate phosphorylation with the participation of phosphoglycerate kinase:
8). Intramolecular transfer of the phosphate group, the enzyme - phosphoglyceromutase:
9). Dehydration reaction with the participation of enolase:
10). Substrate phosphorylation, enzyme - pyruvate kinase:
11). Under anaerobic conditions, the reaction of reduction of pyruvate to lactate occurs under the action of the enzyme lactate dehydrogenase:
The overall equation for anaerobic glycolysis is:
Anaerobic glycolysis does not require a respiratory chain.
ATP output during anaerobic glycolysis: ATP is formed due to two reactions of substrate phosphorylation: from 1,3-bisphosphoglycerate - reaction 7, and from phosphoenolpyruvate - reaction 10. Considering that 1 molecule of glucose is split into 2 trioses and gives 2 molecules of glyceraldehyde phosphate, 4ATP is formed. 2ATP is used to activate glucose (reactions 1 and 3 of glycolysis). In total.