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Glycolysis: Definition, Equation, 10 Steps, Enzymes, Product

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Glycolysis Definition

Glycolysis is the main pathway for glucose catabolism, in which glucose (6-carbon component) is transformed to pyruvate (3-carbon compound) in a ten-step process.

  • Glycolysis is the initial stage in the glucose metabolism process in both aerobic and anaerobic organisms.
  • The mechanism of control and the following metabolic destiny of the pyruvate generated at the end of the glycolytic chain of events changes from one species to the next.
  • Glycolysis is the precursor of the citric acid cycle and the electron transport chain in aerobic organisms, which together liberate the majority of the energy stored in glucose.
  • In recognition of the pioneer workers in the field, it is also known as the Embden-Meyerhof-Parnas or EMP trail.


Glycolysis Equation

The following is a brief overview of the glycolysis process:

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

Glycolysis enzymes are enzymes that break down carbohydrates into sugars.

The enzymes that catalyse glycolytic processes are found in the extra-mitochondrial part of the cell’s cytoplasm in most types of cells. Nearly all of the enzymes involved in glycolysis require Mg2+, which is a common feature among them. The enzymes that catalyse different phases in the glycolysis process are as follows:

  1. Hexokinase
  2. Phosphoglucoisomerase
  3. Phosphofructokinase
  4. Aldolase
  5. Isomerase for phosphotriose
  6. Dehydrogenase of glyceraldehyde 3-phosphate
  7. Phosphoglycerate kinase (PGK) is a phosphoglycerate kinase enzyme
  8. Mutase of phosphoglycerate
  9. Enolase
  10. Pyruvate kinase

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Steps in Glycolysis

  • A set of 10 enzyme-catalyzed sequential events breaks down a single mole of 6-carbon glucose into two moles of 3-carbon pyruvate during glycolysis. These reactions are divided into two categories: phase I and phase II.
  • Stage I consists of “preparatory” reactions that are not redox reactions and do not release energy, but instead result in the formation of a route crucial intermediate.
  • The first five steps of the glycolysis process make up Stage I.
  • Redox processes occur in Stage II, energy is preserved in the form of ATP, and two molecules of pyruvate are produced.
  • Phase II of glycolysis is made up of the last five processes.

The ten steps of glycolysis take place in this order:

Step 1: Glucose phosphorylation

  • Phosphorylation of the C6 carbon in the first stage of glycolysis initiates or primes the glucose for the succeeding steps.
  • In the presence of the enzymes hexokinase and glucokinase, phosphate from ATP is transferred to glucose, creating glucose-6-phosphate (in animals and microbes).
  • This process also results in a significant loss of energy in the form of heat.

Step 2: Glucose-6-phosphate Isomerization

  • The enzyme phosphohexoisomerase/phosphoglucoisomerase reversibly isomerizes glucose 6-phosphate to fructose 6-phosphate.
  • The carbonyl oxygen is shifted from C1 to C2, resulting in the conversion of an aldose to a ketose.

Step 3: Fructose-6-phosphate phosphorylation

  • In the presence of the enzyme phosphofructokinase, fructose-6-phosphate is transformed into fructose-1,6-bisphosphate in the second priming stage of glycolysis.
  • Phosphate is transferred from ATP, and some energy is lost in the form of heat, just as it was in Step 1.

Step 4: Fructose 1, 6-diphosphate cleavage

  • The C-C bond in fructose 1, 6-bisphosphate is cleaved in this step for the first time.
  • The enzyme fructose diphosphate aldolase catalyses the cleavage of fructose 1,6-bisphosphate between C3 and C4 to produce two triose phosphates: glyceraldehyde 3-phosphate (an aldose) and dihydroxyacetone phosphate(a kerosene)
  • In glycolysis, the remaining steps use three-carbon units rather than six-carbon units.

Step 5 – Isomerization of dihydroxyacetone phosphate

  • Glyceraldehyde 3-phosphate is easily destroyed in glycolysis’ latter phases, whereas dihydroxyacetone phosphate is not. As a result, it is isomerized to glyceraldehyde 3-phosphate.
  • Dihydroxyacetone phosphate is isomerized into glyceraldehyde 3-phosphate in the presence of the enzyme triose phosphate isomerase.
  • The first phase of glycolysis is completed with this reaction.

Step 6: Glyceraldehyde 3-phosphate Oxidative Phosphorylation

  • Step 6 is one of glycolysis’ three energy-conserving or forming processes.
  • The enzyme glyceraldehyde 3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate (phosphoglyceraldehyde dehydrogenase).
  • The H– from glyceraldehydes 3-phosphate reduces NAD+ to coenzyme NADH in this mechanism.
  • Two NADH are generated in this stage because one mole of glucose yields two moles of glyceraldehyde 3-phosphate.

Step 7: Phosphate transfer from 1, 3-diphosphoglycerate to AD

  • The ATP-generating stage of glycolysis is this step.
  • Phosphoglycerate kinase is an enzyme that converts 1, 3-bisphosphoglycerate to ATP and 3-phosphoglycerate by transferring the phosphate group from 1, 3-bisphosphoglycerate to ADP.
  • Two ATPs are created in this stage because two moles of 1, 3-bisphosphoglycerate are synthesised from one mole of glucose.

Step 8: Isomerization of 3-phosphoglycerate

  • The enzyme phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate by shifting the phosphoryl group from C3 to C2.
  • This is an isomerization reaction that can be reversed.

Step 9: 2-phosphoglycerate dehydration

  • The action of enolase (phosphopyruvate hydratase) converts 2-phosphoglycerate to phosphoenolpyruvate in this stage.
  • This is likewise an irreversible process that results in the loss of two moles of water.

Step 10: Phosphate transfer from phosphoenolpyruvate

  • This is glycolysis’ second energy-generating phase.
  • The enzyme pyruvate kinase converts phosphoenolpyruvate to an enol form of pyruvate.
  • The enol pyruvate, on the other hand, undergoes a fast and non-enzymatic rearrangement to generate the keto form of pyruvate (i.e. ketopyruvate). At pH 7.0, the keto form prevails.
  • The enzyme catalyses the transfer of a phosphoryl group from phosphoenolpyruvate to ADP, resulting in the production of ATP.

Glycolysis’s Outcome

The following events occur as a result of the glycolysis process as a whole:

  • Pyruvate is formed when glucose is oxidised.
  • NAD+ is broken down into NADH.
  • Phosphorylation of ADP results in the formation of ATP.

Pyruvate’s Fate

Pyruvate travels through one of three key routes, depending on the organism and metabolic conditions:

  1. Pyruvate oxidation
  • The pyruvate is subsequently transported to the mitochondria and metabolised into the acetyl group of acetyl-coenzyme A in aerobic organisms (acetyl Co-A).
  • One mole of CO2 is released during this process.
  • By entering the citric acid cycle, the acetyl CoA is totally oxidised into CO2 and H2O.
  • In aerobic species and plants, this route is followed by glycolysis.
  1. Fermentation of lactic acid
  • Due to a lack of oxygen, pyruvate cannot be metabolised in settings where oxygen is scarce, such as in skeletal muscle cells.
  • Anaerobic glycolysis reduces pyruvate to lactate under these conditions.
  • Lactic acid fermentation is a mechanism that produces lactate from glucose in other anaerobic organisms.
  1. Alcoholic Fermentation is the third step in the fermentation process.
  • The pyruvate generated from glucose is transformed anaerobically into ethanol and CO2 in some microorganisms, such as brewer’s yeast.
  • This is the most primitive form of glucose metabolism, which occurs when oxygen levels are low.


Glycolysis Most Commonly Asked Questions (FAQs)

Does glycolysis require oxygen?

No, Glycolysis requires no oxygen. It is an anaerobic respiration performed by all cells, including anaerobic cells that are killed by oxygen.

What is aerobic glycolysis, and how does it work?

Aerobic glycolysis is defined as the process of converting glucose to pyruvate, then converting pyruvate to CO2 and H2O in the presence of adequate oxygen.

What is anaerobic glycolysis, and how does it work?

In the absence of sufficient oxygen, anaerobic glycolysis occurs, resulting in the conversion of pyruvate to lactate and the reoxidation of NADH to NAD+.

Where does glycolysis occur?

Glycolysis takes place in the cytosol of the cell’s extramitochondrial component.

 What are glycolysis’s byproducts?

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

How much NADH is generated during glycolysis?

Glycolysis generates two moles of NADH.

In glycolysis, how many ATPs are produced?

In glycolysis, a total of four moles of ATP are produced. Because two ATPs are used during the preparation phase of glycolysis, the net gain of ATP in glycolysis is only two ATPs.

What are the advantages and disadvantages of glycolysis?

Glycolysis’ major role is to produce energy in the form of ATP. Similarly, glycolysis generates pyruvate, which is then further oxidised to produce more ATPs.

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