Glycolysis- Definition, Equation, Enzymes, 10 Steps, Diagram

Glycolysis Definition

  • The primary mechanism for the breakdown of glucose is known as glycolysis, and it consists of a series of 10 stages that transforms glucose (6-carbon component) into pyruvate (3-carbon compound).
  • The first stage of the metabolism of glucose is known as glycolysis, it is present in both aerobic as well as anaerobic organisms.
  • The manner of regulation and subsequent metabolic fate of the pyruvate generated at the completion of the glycolytic cascade varies across species.
  • In aerobic organisms, glycolysis precedes the citric acid cycle and also the electron transport chain, that together liberate the bulk of glucose’s energy.
  • This route is also known as Embden-Meyerhof-Parnas,or EMP pathway, in recognition of the early researchers in the area.

Glycolysis equation

A summary of the process of glycolysis cab be written as follows:

C6H12O6 + 2ADP + 2Pi + 2NAD+   →   2C3H4O3 + 2H2O + 2ATP + 2NADH + 2H+

In words, the equation is written as:

Glucose + Adenosine diphosphate + Phosphate  +  Nicotinamide adenine dinucleotide

Pyruvate + Water + Adenosine triphosphate + Nicotinamide adenine dinucleotide + Hydrogen ions

Glycolysis enzymes

In the majority of cell types, glycolytic process-catalyzing enzymes are located in the cytosol outside of mitochondria. The fact that almost all of the glycolysis-related enzymes need Mg2+ is one thing they have in common. The following list of enzymes includes those that catalyse various stages of the glycolysis process:

  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

Glycolysis steps

  • During glycolysis, a series of 10 consecutive enzyme-catalyzed events converts a single mole of 6-carbon glucose into two moles of 3-carbon pyruvate. Phase I and Phase II are the categories used to organise these responses.
  • Stage I consists of “preparatory” reactions that aren’t redox reactions, don’t release energy, but instead create a vital route intermediate.
  • The first five stages of glycolysis are included in Stage I.
  • Similar to Stage I, Stage II involves redox processes, ATP-based energy conservation, and the formation of two pyruvate molecules.
  • Phase II of glycolysis is composed of the last five processes.

The order in which the 10 stages of glycolysis take place is as follows:

Step 1- Phosphorylation of glucose

  • The phosphorylation of the glucose at the C6 carbon initiates or primes it for the succeeding stages of glycolysis in the first phase.
  • In the process, the enzymes hexokinase and glucokinase combine to transfer phosphate from ATP to glucose, creating glucose-6-phosphate (in animals and microbes).
  • Additionally, a significant amount of energy is lost as heat during this process.

Step 2- Isomerization of Glucose-6-phosphate

  • The enzyme phosphohexoisomerase/phosphoglucoisomerase reversibly isomerizes glucose 6-phosphate to fructose 6-phosphate.
  • In this reaction, the carbonyl oxygen is moved from carbon position C1 to carbon position C2, turning an aldose into a ketose.

Step 3- Phosphorylation of fructose-6-phosphate

  • In the presence of the enzyme phosphofructokinase, fructose-6-phosphate is changed into fructose-1,6-bisphosphate in this stage, which is the second priming step of glycolysis.
  • Similar to Step 1, energy is also lost in the form of heat when the phosphate is transferred from ATP.

Step 4- Cleavage of fructose 1, 6-diphosphate

  • This process involves a special cleavage of the fructose 1, 6-bisphosphate C-C link.
  • The cleavage of fructose 1,6-bisphosphate between C3 and C4 by the enzyme fructose diphosphate aldolase produces two distinct triose phosphates: glyceraldehyde 3-phosphate (an aldose) and dihydroxyacetone phosphate (a ketose).
  • Instead of using six carbon units, the subsequent stages of glycolysis use three carbon units.

Step 5- Isomerization of dihydroxyacetone phosphate

  • In the latter stages of glycolysis, glyceraldehyde 3-phosphate may be easily broken down, but dihydroxyacetone phosphate cannot. As a result, glyceraldehyde 3-phosphate is produced as the isomer.
  • In this phase, triose phosphate isomerase is used to isomerize dihydroxyacetone phosphate into glyceraldehyde 3-phosphate.
  • The first stage of glycolysis is finished by this reaction.

Step 6- Oxidative Phosphorylation of Glyceraldehyde 3-phosphate

  • One of glycolysis’ three energy-preserving or energy-forming phases is step six.
  • The enzyme glyceraldehyde 3-phosphate dehydrogenase transforms the glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (phosphoglyceraldehyde dehydrogenase).
  • The H- from glyceraldehydes 3-phosphate reduces NAD+ to coenzyme NADH in this mechanism.
  • One mole of glucose is converted into two moles of glyceraldehyde 3-phosphate, which results in the production of two NADH.

Step 7- Transfer of phosphate from 1, 3-diphosphoglycerate to ADP

  • This procedure is where glycolysis produces ATP.
  • Phosphoglycerate kinase transfers the phosphate group from 1, 3-bisphosphoglycerate to ADP in this process, resulting in the production of ATP and 3-phosphoglycerate.
  • Two ATPs are produced in this phase because one mole of glucose yields two moles of 1, 3-bisphosphoglycerate.

Step 8- Isomerization of 3-phosphoglycerate

  • By moving the phosphoryl group from C3 to C2, the enzyme phosphoglycerate mutase transforms the 3-phosphoglycerate into 2-phosphoglycerate.
  • This is an isomerization process that is reversible.

Step 9- Dehydration 2-phosphoglycerate

  • Enolase (phosphopyruvate hydratase) converts phosphoenolpyruvate to 2-phosphoglycerate, which is then dehydrated.
  • Two moles of water are lost in this process, which is likewise irreversible.

Step 10- Transfer of phosphate from phosphoenolpyruvate

  • This is the second phase of glycolysis when energy is produced.
  • The enzyme pyruvate kinase changes phosphoenolpyruvate into an enol form of pyruvate.
  • However, the enol pyruvate quickly undergoes an unenzymatic rearrangement to produce the keto form of pyruvate (i.e. ketopyruvate). At pH 7.0, the keto form prevails.
  • In order to create ATP, the enzyme catalyses the transfer of a phosphoryl group from phosphoenolpyruvate to ADP.

Result of Glycolysis

The following things happen as a consequence of glycolysis as a whole:

  1. Oxidation converts glucose to pyruvate.
  2. NAD+ is reduced to NADH.
  3. ATP is created by phosphorylating ADP.

Fates of Pyruvate

The pyruvate travels one of the following three crucial pathways, depending on the organism and the metabolic circumstances:

  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).
  • One mole of CO2 is released during this process.
  • After entering the citric acid cycle, the acetyl CoA is totally oxidised into CO2 and water.
  • This route is the one that plants and aerobic creatures use to get energy.
  1. Lactic acid fermentation

  • The pyruvate cannot be oxidised when there is insufficient oxygen present, such as in the skeletal muscle cells.
  • Anaerobic glycolysis converts the pyruvate to lactate under these circumstances.
  • Other anaerobic organisms also produce lactate from glucose via the process of lactic acid fermentation.
  1. Alcoholic Fermentation

  • Brewer’s yeast is one microorganism that anaerobically transforms the pyruvate it produces from glucose into ethanol and CO2.
  • Given that it is seen in low oxygen environments, this is thought to be the oldest known type of glucose metabolism.
  • Glycolysis Frequently Asked Questions (FAQs)

What is aerobic glycolysis?

When there is enough oxygen present, the process of aerobic glycolysis involves the oxidation of glucose into pyruvate, which is then followed by the oxidation of pyruvate into CO2 and H2O.

What is anaerobic glycolysis?

Anaerobic glycolysis is the process that converts pyruvate into lactate and reoxidizes NADH into NAD+ when there is insufficient oxygen present.

Where does glycolysis occur?

The cytosol of the extramitochondrial portion of the cell is where glycolysis takes place.

What are the products of glycolysis?

Two moles of pyruvate, four moles of ATPs (a net gain of two ATPs), and one mole of NADH are the end products of glycolysis.

How many NADH are produced by glycolysis?

Glycolysis results in two moles of NADH being generated.

How many ATPs are formed in glycolysis?

In total, glycolysis produces four moles of ATPs. Two ATPs are used up during the glycolysis’s first stages, hence there is only a net gain of 2 ATPs.

What are the functions of glycolysis?

The production of ATP-based energy is the main goal of glycolysis. The pyruvate that is produced by glycolysis is also further oxidised to make additional ATPs.


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