Home » Glycolysis Pathway-Definition, Diagram, Steps, Regulation

Glycolysis Pathway-Definition, Diagram, Steps, Regulation

The term “glycolysis” originates from the Greek (glycose-sweet or sugar; lysis-dissolution). This pathway can be found in every single type of live cell. In 1940, the full glycolysis pathway was mapped out. This pathway is sometimes called the Embden-Meyerhof pathway (E.M. pathway) after the two biochemists who did a lot to learn about glycolysis. Glycolysis is the series of chemical reactions that turn glucose (or glycogen) into pyruvate or lactate and ATP.

Glycolysis Definition

Glycolysis is defined as the sequence of reactions converting glucose (or glycogen) to pyruvate or lactate, with the production of ATP.

Features of Glycolysis

The first is that glycolysis occurs in every single cell. Enzymes involved in this pathway are found in the cytosol. Glycolysis can happen without oxygen (anaerobic) or with oxygen present (aerobic). When there is no oxygen, the end product is lactate. In an aerobic environment, pyruvate is made, which is then broken down into CO2 and H2O.

Glycolysis is one of the main ways that ATP is made in tissues that don’t have mitochondria, like erythrocytes, the cornea, the lens, and so on. Glycolysis is crucial for the brain, which derives its energy from glucose. Before glucose is converted to CO2 and H2O, it must undergo glycolysis in the brain.

Glycolysis (anaerobic) may be summarized by the net reaction

Glucose + 2ADP + 2Pi -> 2Lactate + 2ATP

Glycolysis is a key part of the metabolic process, and many of its intermediates lead to other processes. This means that the intermediates of glycolysis can be used to make amino acids and fat. Reversal of glycolysis along with the alternate arrangements at the irreversible steps, will result in the synthesis of glucose (gluconeogenesis).

Glycolysis enzymes

In most types of cells, the enzymes that speed up glycolytic reactions are in the cytosol, which is the part of the cell outside of the mitochondria. Almost all of the enzymes that are involved in glycolysis need Mg2+. This is something they all have in common. During the process of glycolysis, the following enzymes help speed up different steps: 

  1. Hexokinase
  2. Phosphoglucoisomerase
  3. Phosphofructokinase
  4. Aldolase
  5. Phosphotriose isomerase
  6. Glyceraldehyde 3-phosphate dehydrogenase
  7. Phosphoglycerate kinase
  8. Phosphoglycerate mutase
  9. Enolase
  10. Pyruvate kinase

Steps of Glycolysis

The sequence of reactions of glycolysis pathway with enzymes is given in Fig.1. The pathway can be divided into three distinct phases: 1. Energy investment phase or priming stage, 2. Splitting phase, 3. Energy generation phase. The sequence of glycolysis pathway with enzymes reactions are discussed below.

Glycolysis 10 steps Diagram

Figure 1: Glycolysis 10 steps Diagram. Image Source: Quizlet Inc.

A. Energy investment phase

Step 1

glycolysis pathway Step 1 Phosphorylation of glucose

Figure 2: Step 1: Phosphorylation of glucose. Image Source: Lehninger Principles of Biochemistry.

The glucose is initiated or primed for the subsequent steps of glycolysis in the first step by phosphorylation at the C6 carbon. In the presence of the enzymes hexokinase and glucokinase, phosphate is transferred from ATP to glucose, forming Glucose-6-phosphate (in animals and microbes). This step also results in a significant loss of energy as heat.

Step 2

glycolysis pathway Step 2 Isomerization of Glucose-6-phosphate

Figure 3: Step 2 – Isomerization of Glucose-6-phosphate. Image Source: Lehninger Principles of Biochemistry.

Glucose 6-phosphate undergoes isomerization to give fructose 6-phosphate in the presence of the enzyme phosphohexose isomerase and Mg2+.

Step 3

Glycolysis Pathway Step 3 Phosphorylation of fructose-6-phosphate

Figure 4: Step 3- Phosphorylation of fructose-6-phosphate. Image Source: Lehninger Principles of Biochemistry.

Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate by phosphofructokinase (PFK). This is an irreversible and a regulatory step in glycolysis.

B. Carbon Splitting phase

Step 4

Glycolysis Pathway Step 4 - Cleavage of fructose 1, 6-diphosphate

Figure 5: Step 4 – Cleavage of fructose 1, 6-diphosphate. Image Source: Lehninger Principles of Biochemistry.

The six carbon fructose 1,6-bisphosphate is split to two 3 carbon compounds, glyceraldehyde 3 – phosphate and dihydroxyacetone phosphate by the enzyme aldolase (fructose 1,6-bisphosphate aldolase).

Step 5

Step 5- Isomerization of dihydroxyacetone phosphate

Figure 6: Step 5- Isomerization of dihydroxyacetone phosphate. Image Source: Lehninger Principles of Biochemistry.

The enzyme phosphotriose isomerase catalyses the reversible interconversion of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Thus, two molecules of glyceraldehyde 3-phosphate are obtained from one molecule of glucose.

C. Energy generation phase

Step 6

Glycolysis Pathway Step 6- Oxidative Phosphorylation of Glyceraldehyde 3-phosphate

Figure 7: Glycolysis Pathway Step 6- Oxidative Phosphorylation of Glyceraldehyde 3-phosphate. Image Source: Lehninger Principles of Biochemistry.

Glyceraldehyde 3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. This step is important as it is involved in the formation of NADH + H+ and a high energy compound 1,3-bisphosphoglycerate. Iodoacetate and arsenate inhibit the enzyme glyceraldehyde 3-phosphate dehydrogenase. In aerobic condition, NADH passes through the electron transport chain and 6 ATP (2 u 3 ATP) are synthesized by oxidative phosphorylation.

Step 7

Transfer of phosphate from 1, 3-diphosphoglycerate to ADP

Figure 8: Glycolysis Pathway Step 7- Transfer of phosphate from 1, 3-diphosphoglycerate to ADP. Image Source: Lehninger Principles of Biochemistry.

The enzyme phosphoglycerate kinase acts on 1,3-bisphosphoglycerate resulting in the synthesis of ATP and formation of 3-phosphoglycerate. This step is a good example of substrate level phosphorylation, since ATP is synthesized from the substrate without the involvement of electron transport chain. Phosphoglycerate kinase reaction is reversible, a rare example among the kinase reactions.

Step 8

Glycolysis Pathway Step 8- Isomerization of 3-phosphoglycerate

Figure 9: Glycolysis Pathway Step 8- Isomerization of 3-phosphoglycerate. Image Source: Lehninger Principles of Biochemistry.

3-Phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase. This is an isomerization reaction.

Step 9

Glycolysis Pathway Step 9- Dehydration 2-phosphoglycerate

Figure 10: Glycolysis Pathway Step 9- Dehydration 2-phosphoglycerate. Image Source: Lehninger Principles of Biochemistry.

The high energy compound phosphoenol pyruvate is generated from 2-phosphoglycerate by the enzyme enolase. This enzyme requires Mg2+ or Mn2+ and is inhibited by fluoride. For blood glucose estimation in the laboratory, fluoride is added to the blood to prevent glycolysis by the cells, so that blood glucose is correctly estimated. (Fluoride combines with Mg2+ and phosphate to form a complex that binds with active site of enolase and blocks access of substrate. Thus, fluoride is an unusual competitive inhibitor).

Step 10

Glycolysis Pathway Step 10- Transfer of phosphate from phosphoenolpyruvate

Figure 11: Glycolysis Pathway Step 10- Transfer of phosphate from phosphoenolpyruvate.  Image Source: Lehninger Principles of Biochemistry.

The enzyme pyruvate kinase catalyses the transfer of high energy phosphate from phosphoenol pyruvate to ADP, leading to the formation of ATP. This step also is a substrate level phosphorylation. This reaction is irreversible.

Result of Glycolysis

The overall process of glycolysis results in the following events: 1. Glucose is oxidized into pyruvate. 2. NAD+ is reduced to NADH. 3. ADP is phosphorylated into ATP.

Regulation of glycolysis

The three enzymes namely hexokinase (and glucokinase), phosphofructokinase and pyruvate kinase, catalysing the irreversible reactions regulate glycolysis.

Hexokinase is inhibited by glucose 6-phosphate. This enzyme prevents the accumulation of glucose 6-phosphate due to product inhibition. Glucokinase, which specifically phosphorylates glucose, is an inducible enzyme. The substrate glucose, probably through the involvement of insulin, induces glucokinase.

Phosphofructokinase (PFK) is the most important regulatory enzyme in glycolysis. This enzyme catalyses the rate limiting committed step. PFK is an allosteric enzyme regulated by allosteric effectors. ATP, citrate and H+ ions (low pH) are the most important allosteric inhibitors, whereas, fructose 2,6-bisphosphate, ADP, AMP and Pi are the allosteric activators.

Fates of Pyruvate

Depending on the organism and metabolic conditions, pyruvate follows one of the three essential routes listed below:

Glycolysis Pathway Fates of Pyruvate

Figure 11: Glycolysis Pathway Fates of Pyruvate. Image Source: Lehninger Principles of Biochemistry.

1. Oxidation of pyruvate

The pyruvate is subsequently transported to the mitochondria in aerobic species, where it is oxidised into the acetyl group of acetyl-coenzyme A. (acetyl Co-A). This procedure releases one mole of carbon dioxide. By entering the citric acid cycle, the acetyl CoA is subsequently fully oxidised into CO2 and H2O. This pathway follows glycolysis in aerobic organisms and plants.

2. Lactic acid fermentation

Pyruvate cannot be oxidised in conditions where there is insufficient oxygen, such as in skeletal muscle cells. The anaerobic glycolysis process reduces pyruvate to lactate under these conditions. Lactate is produced from glucose in other anaerobic organisms via the lactic acid fermentation process.

3. Alcoholic Fermentation

In certain microorganisms, such as brewer’s yeast, glucose-derived pyruvate is transformed anaerobically into ethanol and CO2. This is considered the most ancient form of glucose metabolism, as demonstrated in environments with low oxygen levels.

FAQ

FAQs on Glycolysis

The cytoplasm of a cell is where glycolysis takes place.

Glycolysis takes place in the cytoplasm of a cell.

No, glycolysis does not require oxygen. It is an anaerobic process, meaning it can occur in the absence of oxygen.

Glycolysis is an anaerobic process, meaning it can occur in the absence of oxygen.

The products of glycolysis are: 2 molecules of pyruvate, 2 molecules of ATP (or other energy-rich molecules such as GTP), and 2 molecules of NADH (nicotinamide adenine dinucleotide in its reduced form).

The reactants in glycolysis are: 1 molecule of glucose (or other 6-carbon sugars such as fructose) and 2 molecules of ATP (or other energy-rich molecules such as GTP) and 2 molecules of NAD+ (nicotinamide adenine dinucleotide in its oxidized form)

2 ATP, 2 NADH, and 2 pyruvate molecules

During glycolysis, glucose ultimately breaks down into pyruvate and energy.

The byproducts of glycolysis are: 2 molecules of pyruvate, 2 molecules of ATP (or other energy-rich molecules such as GTP), and 2 molecules of NADH (nicotinamide adenine dinucleotide in its reduced form), and 2 molecules of water.

ATP is used as an energy source in glycolysis to drive the initial phosphorylation of glucose, resulting in a net gain of two ATP molecules per glucose molecule degraded through glycolysis.