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Cellular respiration involves breaking the bonds of glucose to produce energy in the form of ATP (adenosine triphosphate). The total energy produced during glucose metabolism is described at Energetics of Cellular Respiration. Glycolysis is the most critical phase in glucose metabolism during cellular respiration. The term “glycolysis” literally means breakdown of glucose and sugars. Biochemically, it involves the breakdown of glucose to pyruvate (or pyruvic acid) via a series of enzymes. Glycolysis does not require molecular oxygen and is hence considered anaerobic. Therefore, it is a common pathway for all living organisms. Glycolysis is followed by Kreb’s cycle in the stages of cellular respiration.

Glycolysis is said to occur in two phases:

1. The Preparatory Phase: From glucose till formation of Glyceraldehyde 3-Phosphate (GADP)
2. The Pay-off Phase: From Glyceraldehyde-3-Phosphate (GADP) to the final product pyruvate

The animation below gives an outline of the entire pathway of glucose metabolism by glycolysis.

Note – The animation is best played in full screen. To go forward in the animation, press the Play button. To skip the whole section press the forward button. To go back press the rewind button.

The Preparatory Phase

In this stage of the cycle, ATP or energy is actually consumed and is hence also known as the investment phase of glycolysis.

Step 1, involves the conversion of glucose to glucose-6-phosphate (G6P) with the help of the enzyme hexokinase and the consumption of 1 molecule of ATP. This reaction helps keep the concentration of glucose low in the cell, allowing for more absorption of glucose into it. Additionally, G6P is not transported out of the cell as there are no G6P transporters on the cell.

Step 2 involves the rearrangement of glucose-6-phosphate to fructose-6-phosphate (F6P) with the help of the enzyme phosphohexose isomerase in a reversible manner. Fructose can directly enter the glycolysis pathway at this point. This isomerization to a keto-sugar such as fructose is essential for carbanion stabilization required for the next step.

Step 3 involves the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate (F1,6BP) by the use of 1 molecule of ATP and the enzyme phosphofructokinase-1 (PPK1). This phosphorylation step destabilizes the molecule and helps drive the next reaction which ensures breakdown of the molecule to a 3-carbon unit.

Step 4 involves the breakdown of fructose-1,6-biphosphate (6 carbons) to two molecules of 3-carbon units i.e. glyceralde 3-phosphate (GADP) and Dihydroxyacetone phosphate (DHAP). The GADP can be interconverted to DHAP by enzyme triose phosphate isomerase.

The Pay-Off Phase

In this stage of the cycle, ATP or energy is produced either in the form of ATP alone or in the form of NADH + H+ which can be later converted to ATP via the electron transport chain (ETS). In this since energy is restored it is known as the pay-off phase of glycolysis. All steps in this phase occur with 2 molecules of the substrates each as indicated in the brackets by the name of the molecules.

Step 1, involves the dehydrogenation of glyceraldehyde-3-phosphate (GADP) to 1,3-biphophoglycerate (1,3BPG) by the use of 2 molecules of inorganic phosphate (Pi) with the production of 2 molecules of NADH + H+ in the presence of the enzyme glyceraldehyde 3-phosphate dehydrogenase.

Step 2, in this step dephosphorylation of 1,3-biphosphoglycerate (1,3BPG) to 3-phospoglycerate (3PG) produces 2 molecules of ATP by the enzyme phosphoglycerate kinase.

Step 3, involves the isomerisation of 3-phosphoglycerate (3PG) to 2-phosphoglycerate (2PG) by the enzyme phosphoglycerate mutase in a reversible manner.

Step 4 involves the enolization of 2-phosphoglycerate (2PG) to phosphoenolpyruvate (PEP) with the loss of one molecule of water in the presence of enzyme enolase.

Step 5 is the final step of the glycolysis pathway and it involves the dephosphorylation of the phosphoenolpyruvate (PEP) to pyruvate by enzyme pyruvate kinase to produce 2 more molecules of ATP.

Net Yield of Glycolysis

1. The preparatory phase consumes 2 ATP
2. The pay-off phase produces 4 ATP.
3. The gross yield of glycolysis is therefore
   4 ATP – 2 ATP = 2 ATP
4. The pay-off phase also produces 2 molecules of NADH + H+ which can be further converted to a total of 5 molecules of ATP* by the electron transport chain (ETC) during oxidative phosphorylation.
5. Thus the net yield during glycolysis is 7 molecules of ATP.

* This is calculated assuming one NADH molecule gives 2.5 molecules of ATP during oxidative phosphorylation.

References

1. David L. Nelson and Michael M. Cox, Lehninger Principles of Biochemistry, 4th Ed.
2. Jeremy M. Berg, John L. Tymockzo and Luber Stryer, Biochemistry, 7th Ed.