Glycolysis pathway

The metabolic pathway that converts glucose (C6H12O6) to pyruvic acid (CH3COCOOH) is called glycolysis or glycolysis pathway.

The free energy released during this process is used to create the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NAD) (NADH).

The EMP pathway (Embden–Meyerhof–Parnas) was discovered in the early nineteenth century by three German biochemists, Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas.

Glycolysis is an enzyme-catalyzed series of ten reactions.

Glycolysis is a non-oxygen-requiring metabolic pathway.

Glycolysis is the first stage of cellular respiration in organisms. Though glycolysis does not require oxygen,  many anaerobic organisms (organisms that do not use oxygen) have this pathway as well.

The presence of glycolysis in a wide range of other species suggests that it is an ancient metabolic pathway.

The reactions that comprise glycolysis and its parallel pathway, the pentose phosphate pathway, occur in the oxygen-free conditions of the Archean oceans, as well as in the absence of metal-catalysed enzymes. 

Glycolysis occurs in the liquid part of cells, known as the cytosol, in the vast majority of organisms.

Glycolysis phases 

Glycolysis occurs in the cytosol of a cell and is divided into two phases: the energy-requiring phase and the energy-releasing phase.

The energy-requiring phase 

  • The starting molecule of glucose is rearranged in this phase, and two phosphate groups are attached to it. 
  • The phosphate groups render the modified sugar, now known as fructose-1,6-bisphosphate, unstable, allowing it to split in half and form two phosphate-containing three-carbon sugars.
  • When the unstable sugar degrades, three-carbon sugars are formed that are distinct from one another. 
  • Only one can enter the next step: glyceraldehyde-3-phosphate. 
  • However, the unfavourable sugar, D H A P can be easily converted into the favourable one, allowing both to complete the pathway.

The energy-releasing phase 

  • During this phase, each three-carbon sugar is converted into another three-carbon molecule, pyruvate, via a series of reactions. 
  • Two ATP molecules and one NADH molecule are formed in these reactions. Because this phase occurs twice, once for each of the two three-carbon sugars, it generates four ATP and two NADH in total.
  • Overall, glycolysis converts one six-carbon glucose molecule into two three-carbon pyruvate molecules. 
  • The end result of this process is two molecules of ATP. 4 ATP molecules were produced – 2 ATP molecules were depleted – and 2 molecules of NADH were consumed.

Steps of glycolysis

  1. Step 1 – Glucose-6-phosphate is formed when a phosphate group is transferred from ATP to glucose. Glucose-6-phosphate is more reactive than glucose, and the addition of the phosphate traps glucose inside the cell because glucose with a phosphate cannot cross the membrane easily.
  2. Step 2 – The isomer of glucose-6-phosphate, fructose-6-phosphate, is formed.
  3. Step 3 – Fructose-1,6-bisphosphate is formed when a phosphate group is transferred from ATP to fructose-6-phosphate. The enzyme phosphofructokinase catalyses this step, which can be controlled to speed up or slow down the glycolysis pathway.
  4. Step 4 – Fructose-1,6-bisphosphate is broken down into two three-carbon sugars: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate. They are isomers of each other, but only one—glyceraldehyde-3-phosphate—can proceed directly through glycolysis’s next steps.
  5. Step 5 – Glyceraldehyde-3-phosphate is formed by the conversion of DHAP. The two molecules are in equilibrium, but in the scheme of the diagram above, the equilibrium is “pulled” strongly downward as glyceraldehyde-3-phosphate is depleted. As a result, all of the DHAP is converted eventually.
  6. Step 6 – Two half-reactions occur at the same time:
    1. One of the three-carbon sugars formed in the initial phase, glyceraldehyde-3-phosphate, is oxidised, and
    2. NAD + is reduced to NADH and H +.
    3. The reaction as a whole is exergonic, releasing energy that is then used to phosphorylate the molecule, resulting in 1,3-bisphosphoglycerate.
  7. Step 7 – 1,3-bisphosphoglycerate donates one of its phosphate groups to ADP, resulting in the formation of an ATP molecule and the transformation of 3-phosphoglycerate.
  8. Step 8 – The isomer of 3-phosphoglycerate, 2-phosphoglycerate, is formed.
  9. Step 9 – PEP is formed when 2-phosphoglycerate loses a water molecule. PEP is an unstable molecule that is on the verge of losing its phosphate group in the final step of glycolysis.
  10. Step 10 – PEP readily donates its phosphate group to ADP, resulting in the formation of a second molecule of ATP. PEP is converted to pyruvate, the end product of glycolysis, as it loses its phosphate.

Conclusion

The glycolysis or EMP pathway   is the first stage of respiration. It is the breakdown of glucose into pyruvic acid.

Through aerobic respiration, pyruvic acid generates water and carbon dioxide.

Anaerobic respiration results in the production of ethanol in plants and lactic acid in animals. It takes place in the cytoplasm.

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