Glycolysis Pathway, Glycolysis Definition,Glycolysis Steps, Cycle and Diagram

Photo of Glycolysis Pathway, Glycolysis Definition,Glycolysis Steps, Cycle and Diagram

Glycolysis is also called Embden-Meyerhof-Parnas Pathway (EMP Pathway) because it was discovered by these three scientists: Embden, Meyerhof and Parnas. It is sometimes shortened as Embden Meyerhof Pathway. Glycolysis was formed from a Greek word: Glykys which means Sweet and Lysis– which means Splitting. Hence, Glycolysis is the splitting of glucose molecule that produces energy; it is the major pathway for glucose utilization in all cells. The process of Glycolysis is described below in a simple and easy way that even a dummy can understand.

Glycolysis Definition

Glycolysis is the process of breaking glucose into 2 molecules of which each is made of 3-carbon molecules with the production of ATP and NADH. Glycolysis takes place in the Cytoplasm of the cell. Glycolysis can also be defined as the conversion of glucose or glycogen to pyruvate and lactate depending on the presence or absence of oxygen because it can occur in both aerobic and anaerobic conditions. In glycolysis, one molecule of glucose is oxidized to two molecules of pyruvate with the process giving rise to a net gain of two ATP molecules (two are consumed while four are produced) – in addition, two molecules of NADH and Hydrogen are generated which are then oxidized through the electron transport chain in order to generate 6 molecules of ATP. The total yield of glycolysis under aerobic conditions is therefore 8 molecules of ATP (when further oxidized in the Mitochondria).

What is Glycolysis

Glycolysis is a series of chemical reactions which involves the breaking down of glucose to a 3-carbon molecule called pyruvic acid (Pyruvate) and Glycolysis takes place in the cytoplasm of the cells with no oxygen being necessary for this process. Very little energy is produced or generated during glycolysis. A net formation of 2ATPs are produced from complete oxidation of one glucose molecule during glycolysis but when further oxidized in the mitochondria, more energy molecules are produced.

Glycolysis location and site in the body cells.
Glycolysis location and site in the body cells.

 

Where does Glycolysis occur? – Glycolysis Location (Site of Glycolysis)

Glycolysis occurs in the cytoplasm of all cells of the body especially in the nerve cells and red blood cells (Erythrocytes).

Diagram of Glycolysis pathway showing the enzymes in Glycolysis with the associated reactions they catalyze
Diagram of Glycolysis pathway showing the enzymes in Glycolysis with the associated reactions they catalyze

 

Glycolysis Pathway

The pathway of glycolysis can be seen as consisting of 2 separate phases. The first phase is the chemical priming phase requiring the input of energy in the form of ATP whereas the second phase is considered the energy-yielding phase. In the first phase of glycolysis, 2 equivalents of ATP are used to convert glucose to fructose 1,6-bisphosphate while the second phase of glycolysis involves the sequential oxidization of fructose 1,6-bisphosphate to pyruvate, with the production of 4 equivalents of ATP and 2 equivalents of NADH. The Glycolysis reactions are described below.

The Hexokinase Reaction of Glycolysis

This is the first reaction of glycolysis and it is a process that involves phosphorylation of glucose to form glucose 6-phosphate and it requires ATP (ATP dependent process). This reaction is catalyzed by tissue-specific isoenzymes called Hexokinases. This phosphorylation is useful in 2 ways: the hexokinase reaction converts nonionic glucose into an anion that is trapped in the cell, since cells lack transport systems for phosphorylated sugars while the second use is that the otherwise biologically inert glucose becomes activated into a labile form capable of being further metabolized. Four mammalian types of hexokinase (Isozymes) are known (Types I, II, III and IV), with the Type IV isozyme often referred to as Glucokinase. Glucokinase is the form of Hexokinase found in liver cells (hepatocytes) and pancreatic -cells. Glucokinase can phosphorylate other hexoses in vitro but is unable to do so at any physiologically relevant concentration of these other sugars. After meals, when postprandial blood glucose levels are high, liver glucokinase is significantly active, which causes the liver to preferentially trap and store circulating glucose.

Phosphohexose Isomerase reaction of Glycolysis

The second reaction of glycolysis is an isomerization reaction whereby glucose 6-phosphate is converted to fructose 6-phosphate by an enzyme known as Phosphohexose isomerase (also known as Phosphoglucose isomerase). The reaction is freely reversible at normal cellular concentrations of the 2 hexose phosphates and thus catalyzes this inter-conversion during glycolysis and during gluconeogenesis.

6-Phosphofructo-1-Kinase (Phosphofructokinase-1) reaction in glycolysis

The third reaction of glycolysis involves the utilization of a second ATP to convert Fructose-6-phosphate to fructose 1,6-bisphosphate and it is catalyzed by 6-phosphofructo-1-kinase enzyme which is commonly called phosphofructokinase-1. This reaction is not readily reversible because of its large positive free energy in the reverse direction. The activity of phosphofructokinase-1 is highly regulated and as such the enzyme is considered to be the rate-limiting enzyme of glycolysis.

Aldolase A reaction in glycolysis

Aldolase A is also called fructose-1,6-bisphosphate aldolase and it is an enzyme that catalyzes the 4th reaction of glycolysis which is the hydrolysis of fructose 1,6-bisphosphate into two 3-carbon products: Dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. This reaction is reversible and can utilized in glycolysis and gluconeogenesis.

Triose Phosphate Isomerase reaction in glycolysis

This reaction ends the energy investment phase of glycolysis. The 2 products of the aldolase reaction equilibrate readily in a reaction catalyzed by triose phosphate isomerase. Succeeding reactions of glycolysis utilize glyceraldehyde-3-phosphate as a substrate.

Glyceraldehyde-3-Phosphate Dehydrogenase reaction of glycolysis

The second phase of glucose catabolism features the energy-yielding glycolytic reactions that produce ATP and NADH. In the first of these reactions, glyceraldehyde-3-phosphate dehydrogenase catalyzes the NAD+ dependent oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate and NADH it is a freely reversible reaction.

Phosphoglycerate Kinase reaction of glycolysis

The high-energy phosphate of 1,3-bisphosphoglycerate is used to form ATP and 3-phosphoglycerate by the enzyme phosphoglycerate kinase. This reaction is the only reaction of glycolysis and gluconeogenesis that involves ATP and yet is reversible under normal physiological conditions.

Phosphoglycerate Mutase reaction in glycolysis

Phosphoglycerate mutases is a family of related enzymes where Phosphoglycerate mutase 1 is the major glycolytic enzyme and is responsible for converting the relatively low-energy phosphoacyl-ester of 3-phosphoglycerate to 2-phosphoglycerate (a higher energy form).

Enolase reaction in glycolysis

Enolase enzyme is also known as phosphopyruvate hydratase and it catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate which is the final high-energy intermediate in glycolysis.

Pyruvate Kinase reaction

This is the final reaction of aerobic glycolysis and it is catalyzed by the highly regulated enzyme pyruvate kinase. It is a strongly exergonic reaction in which the high-energy phosphate of phosphoenolpyruvate is transferred to ADP-yielding ATP.

Glycolytic pathway showing the energy investment phase
Glycolytic pathway showing the energy investment phase

 

Two Phases of Glycolysis

  1. Energy investment phase of glycolysis
  2. Energy generation phase of glycolysis

The energy-investment phase of Glycolysis

This phase starts from the first reaction catalyzed by Hexokinase and ends with the fifth reaction catalyzed by Triosephosphate isomerase.

Glycolytic pathway showing the energy generation phase
Glycolytic pathway showing the energy generation phase

 

The energy-generation phase of Glycolysis

This starts with Glyceraldehyde-3-phosphate conversion to 1,3-Bisphosphoglycerate (catalyzed by Glyceraldehyde-3-phosphate Dehydrogenase) and ends with formation of Pyruvate with Pyruvate kinase as the enzyme.

Aerobic and Anaerobic Glycolysis

  1. Aerobic Glycolysis: Oxidation is carried out by dehydrogenation and reducing equivalent is transferred to NAD+. Reduced NAD in presence of Oxygen is oxidized in electron-transport chain producing ATP.
  2. Anaerobic Glycolysis: NADH cannot be oxidized in electron transport chain, so no ATP is produced in electron transport chain. But the NADH is oxidized to NAD+ by conversion of pyruvate to lactate, without producing ATP. Anaerobic phase limits the amount of energy per mol. of glucose oxidized. Hence, to provide a given amount of energy, more glucose must undergo glycolysis under anaerobic as compared to aerobic.

Regulation of Glycolysis (Control of Glycolysis)

Although most of the reactions of glycolysis are reversible, three are markedly exergonic and must therefore be considered physiologically irreversible. These reactions include those catalyzed by Hexokinase (and Glucokinase), Phosphofructokinase, and Pyruvate kinase. They are the major sites of regulation of glycolysis. Cells that are capable of reversing the glycolytic pathway (gluconeogenesis) have different enzymes that catalyze reactions which effectively reverse these irreversible reactions.

3 regulatory enzymes of glycolysis

  1. Hexokinase
  2. Phosphofructokinase (The main rate limiting enzyme)
  3. Pyruvate kinase

Glycolysis Cycle

Glycolysis diagram showing the reactions and their enzymes and the different pathways (Aerobic and Anaerobic) of Glucose in the Cycle
Glycolysis diagram showing the reactions and their enzymes and the different pathways (Aerobic and Anaerobic) of Glucose in the Cycle

 

Glycolysis Reaction

What happens in Glycolysis is that one molecule of glucose is oxidized to produce two molecules of pyruvate. This process gives rise to a net gain of two ATP molecules (two are consumed while four are produced) with additional two molecules of NADH + H+ generated. These are oxidized through the electron transport chain (ETC) in order to generate 6 molecules of ATP.

The total yield of glycolysis under oxygen (aerobic conditions) is therefore the production of 8 ATP molecules.

Glucose + 2ATP 2 Pyruvate + 2NADH + H + (= 6ATP) + 4ATP

The pyruvate molecule has two possible courses:

In aerobic conditions, it most times diffuses into the mitochondria, is decarboxylated into acetyl CoA, and enters the Krebs cycle.

Pyruvate + NAD + + CoA Acetyl-CoA + NADH + + + CO2(In Aerobic condition)

In anaerobic conditions, the NADH + H+ produced by glycolysis reduces the pyruvate to lactic acid. The reduction of pyruvate regenerates NAD+ from NADH + H+ allowing the glycolysis to continue. However, as the two molecules of NADH + H+ are spent in the reduction of two molecules of pyruvate and not in ATP production, the net yield of glycolysis per molecule of glucose is only two ATP molecules.

Pyruvate + NADH + H+ Lactate + NAD (In Anaerobic condition)

Glycolysis Diagram showing the 10 steps of Glycolysis

Diagram of the 10 steps of glycolysis
Diagram of the 10 steps of glycolysis

 

Glycolysis Steps

  1. Step 1: Phosphorylation of glucose to give glucose-6-phosphate (ATP is the source of the phosphate group)
  2. Step 2: Isomerization of glucose-6-phosphate to give fructose-6-phosphate
  3. Step 3: Phosphorylation of fructose-6-phosphate to give fructose-1,6-bisphosphate (ATP is the source of the phosphate group)
  4. Step 4: Cleavage of fructose-1,6-bisphosphate to give two 3-carbon fragments, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
  5. Step 5: Isomerization of dihydroxyacetone phosphate to give glyceraldehyde-3-phosphate.
  6. Step 6: Oxidation (and phosphorylation) of glyceraldehyde-3-phosphate to give 1,3-bisphosphoglycerate.
  7. Step 7: Transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP (phosphorylation of ADP to ATP) to give 3-phosphoglycerate.
  8. Step 8: Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate.
  9. Step 9: Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate.
  10. Step 10: Transfer of a phosphate group from phosphoenolpyruvate to ADP (phosphorylation of ADP to ATP) to give pyruvate.

10 steps of glycolysis with enzymes

  1. Conversion of Glucose to Glucose-6-phosphate catalyzed by Hexokinase enzyme
  2. Glucose-6-phosphate to Fructose-6-phosphate catalyzed by Phosphoglucoisomerase enzyme
  3. Fructose-6-phosphate to Fructose-1,6-bisphosphate catalyzed by Phosphofructokinase enzyme (This is the rate limiting step of glycolysis)
  4. Fructose-1,6-bisphosphate to Dihydroxyacetone phosphate catalyzed by Aldolase enzyme
  5. Dihydroxyacetone phosphate to Glyceraldehyde-3-phosphate catalyzed by Triosephosphate isomerase enzyme
  6. Glyceraldehyde-3-phosphate to 1,3-Bisphosphoglycerate catalyzed by Glyceraldehyde-3-phosphate dehydrogenase enzyme
  7. 1,3-Bisphosphoglycerate to 3-Phosphoglycerate catalyzed by Phosphoglycerate kinase enzyme
  8. 3-Phosphoglycerate to 2-Phosphoglycerate catalyzed by Phosphoglycerate mutase enzyme
  9. 2-Phosphoglycerate to Phosphoenolpyruvate catalyzed by Enolase enzyme
  10. Phosphoenolpyruvate to Pyruvate catalyzed by Pyruvate kinase enzyme

Rate limiting Step in Glycolysis

The major rate-limiting step in glycolysis is the reaction catalyzed by phosphofructokinase (The rate limiting enzyme of glycolysis). The major sites for regulation of glycolysis and gluconeogenesis are the phosphofructokinase-1 and fructose-1,6-bisphosphatase catalyzed reactions.

Glycolysis Process

The process of Glycolysis involves the conversion of one molecule of glucose (a six-carbon compound) is converted to fructose-1,6-bisphosphate (also a six-carbon compound), which eventually gives rise to two molecules of pyruvate (a three-carbon compound)

Under aerobic conditions, pyruvate is oxidized to CO2 and H2O by the citric acid cycle and oxidative phosphorylation whereas under anaerobic conditions, lactate is produced, especially in muscle tissue. Alcoholic fermentation occurs in yeast. The NADH produced in the conversion of glucose to pyruvate is reoxidized to NAD+ in the subsequent reactions of pyruvate.

Glycolysis Equation

Glucose + 2NAD+ + 2ADP + 2Pi 2pyruvate + 2NADH + 2H+ + 2ATP

Glycolysis Enzymes

  1. Hexokinase
  2. Phosphoglucoisomerase
  3. Phosphofructokinase (Rate limiting enzyme of glycolysis)
  4. Aldolase
  5. Triosephosphate isomerase
  6. Glyceraldehyde-3-phosphate dehydrogenase
  7. Phosphoglycerate kinase
  8. Phosphoglycerate mutase
  9. Enolase
  10. Pyruvate kinase

Importance of Glycolysis

  1. The Glycolytic pathway is a source of energy to cells
  2. Glycolysis provides ATP even in the absence of oxygen in skeletal muscle, thereby helping the muscles to survive anoxic conditions.
  3. Although glycolysis is useful to cells because it produces some ATP, its primary importance derives from the fact that it sets the stage for subsequent events that yield even more ATP.
  4. The NADH that is produced in glycolysis will eventually give up its electrons, thereby releasing energy that will be used to synthesize more ATP by oxidative phosphorylation.
  5. Also, the pyruvate that is generated will eventually be catabolized in the next stage of glucose oxidation, the Krebs cycle.

Glycolysis Summary

The process of glycolysis begins with the phosphorylation of glucose to glucose-6-phosphate and so on till pyruvic acid is produced. What happens to the pyruvic acid depends on whether oxygen is present or not. Therefore, one can conclude that glycolysis takes place in both aerobic and anaerobic organisms yielding only a net gain of 2ATP molecules.

In anaerobes (absence of oxygen), the pyruvic acid is converted to alcohol in plant cells and lactic acid in animal cells.

If oxygen is present (aerobes), the pyruvic acid will enter the mitochondria where it is converted to acetyl co-enzyme A (acetyl CoA) which is an important intermediate in the breaking down of sugar. It links glycolysis to the Krebs Cycle. It is also formed in the breaking-down of fats and proteins.

Questions and Answers on Glycolysis

Does Glycolysis require oxygen?

Glycolysis does not require oxygen for the process to occur; in fact the main process of Glycolysis is does not require oxygen this is known as Anaerobic Glycolysis; but it can still occur in the presence of oxygen and it is called Aerobic Glycolysis.

Is Glycolysis Aerobic or Anaerobic?

Glycolysis is both an aerobic and anaerobic process but it is mainly an anaerobic process.

How many ATPs are produced in Glycolysis?

The net yield from the oxidation of 1 mole of glucose to 2 moles of pyruvate is either 6 or 8 moles of ATP. This comprises 2 moles of ATP synthesized via substrate-level phosphorylation during the glycolytic reactions and 4 to 6 moles of ATP generated via oxidative phosphorylation from the reoxidation of cytoplasmic NADH, dependent upon which shuttle mechanism is utilized.

Complete oxidation of the 2 moles of pyruvate, through the TCA cycle, yields an additional 30 moles of ATP; the total yield, therefore being either 36 or 38 moles of ATP from the complete oxidation of 1 mole of glucose to CO2 and H2O.

What is the end product of glycolysis?

The end product of glycolysis is pyruvate or lactate dependent upon the availability of oxygen. Under aerobic conditions, the dominant end product in most tissues of the body is pyruvate and the pathway is known as aerobic glycolysis. When oxygen is depleted, as for instance during prolonged vigorous exercise, the dominant glycolytic product in skeletal muscle is lactate and the process is known as anaerobic glycolysis.

What are the Products of Glycolysis (Products of Glycolysis)

  1. The total yield of glycolysis under oxygen (aerobic conditions) is the production of 8 ATP molecules.
  2. Under anaerobic conditions, glycolysis yields 2 ATP molecules and Lactic acid

 

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