Simplified 10 stages of Glycolysis

Stages of GlycolysisDefinition of Glycolysis

Glycolysis is of the most ancient metabolic pathways in living organisms. In studying the processes involved in the breakdown of food materials to liberate energy, Glycolysis is the frontier in the process.
  • Glycolysis, which translates to “splitting sugars”, is the metabolic process of releasing energy within sugars. The term is coined from two words viz glyco (sugars; this process mainly runs with glucose which is a monomer of carbohydrate), and lysis (to split).
In glycolysis, a six-carbon sugar known as glucose is split into two molecules of a three-carbon sugar (triose sugar) called pyruvate.
This metabolic processes is piloted with the aid of different enzymes. These enzymes do not start the reactions of glycolysis but speeds up the reaction rate.  In the other way round, we can say that it lowers the activation energy of reactions.
On completion of this multistep process, it yields two ATP molecules containing free energy, two pyruvate molecules, two high energy, electron-carrying molecules of NADH, and two molecules of water. Pyruvate molecules from this pathway further undergoes degradation. It normally depends on the need of the organism and the presence of oxygen.
Under aerobic condition, it is decarboxylated to water and carbon dioxide. Under anaerobic condition in man, it converted to lactate. Other life forms may deviate from the above. Such deviation maybe seen in fungi (yeast), which converts it to ethanol

Summary Hints on Glycolysis

  • Glycolysis is the process of breaking down glucose to pyruvate.
  • Glycolysis can take place with or without oxygen depending on the organism and the need of the organism.
  • Glycolysis produces two molecules of pyruvate, two molecules of ATP (energy currency of living organisms), two molecules of NADH2 (high energy molecules), and two molecules of water.
  • Glycolysis takes place in the cytoplasm of cells.
  • There are 10 enzymes involved in breaking down sugar. The 10 steps of glycolysis are organized by the order in which specific enzymes act upon the system.

Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. which is also known as aerobic respiration. In the absence of oxygen (anaerobic respiration), glycolysis allows cells to make small amounts of ATP through a process of fermentation. Anaerobic process is energy consuming.

Glycolysis takes place in the cytosol of the cell’s cytoplasm. A net of two ATP molecules are produced through glycolysis (two are used during the process and four are produced.) Learn more about the 10 steps of glycolysis below

 

Stages of Glycolysis

  • Stage 1 – Activation of Glucose
  • Stage 2 – conversion of glucose-6-phospate to fructose-6-phosphate
  • Stage 3 – Phosphorylation of fructose-6-phosphate
  • Stage 4 – Splitting of fructose-1,6-bisphosphate
  • Stage 5 – Isomerization of Dihydroxyacetone phosphate to Glyceraldehyde-3-phospate
  • Stage 6 – Dehydration of Glyceraldehyde-3-phosphate
  • Stage 7 – Dephosphorylation of 1,3-bisphophoglycerate to yield ATP
  • Stage 8 – Mutation of 3-bisphosphoglycerate to 2-bisphosphoglycerate
  • Stage 9 – Dehydration of 2-phosphoglycerate
  • Stage 10 – Formation of pyruvate
  • Stage 1 – Activation of Glucose

The enzyme hexokinase phosphorylates or adds a phosphate group to glucose in a cell’s cytoplasm. In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate or G6P. One molecule of ATP is consumed during this phase. This process of activation enables the subsequent reactions to occur.  If Glucose molecules are not activated, the cells won’t have access to the for catabolism. This will lead to the accumulation of Glucose in the body.

 

Stage 2 – conversion of glucose-6-phospate to fructose-6-phosphate

The enzyme phosphoglucomutase isomerizes G6P into its isomer fructose 6-phosphate or F6P. Isomers have the same molecular formula as each other but different atomic arrangements. The arrangements is different following the position of double bond. Another point necessary here is; the isomerization reaction changes the former molecule from an aldose sugar, to a ketose sugar.

 

Stage 3 – Phosphorylation of fructose-6-phosphate

The enzyme phosphofructokinase uses another ATP molecule to transfer a phosphate group to F6P in order to form fructose 1,6-bisphosphate or FBP. The transfer or addition of phosphate to a molecule is known as phosphorylation. Two ATP molecules have been used so far. The position of the phosphate at the alpha and omega carbon atoms enables the next reaction.  Hence the actualization of two molecules.

 

Stage 4 – Splitting of fructose-1,6-bisphosphate

The enzyme aldolase splits fructose 1,6-bisphosphate into a ketone and an aldehyde molecule. These sugars, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), are isomers of each other. The two molecules formed contains 3 carbon chains. The two are triose sugars. They have same chemical formula, but different structural formula.

 

Stage 5 – Isomerization of Dihydroxyacetone phosphate to Glyceraldehyde-3-phospate

The enzyme triose-phosphate isomerase rapidly converts DHAP into GAP (these isomers can inter-convert). GAP is the substrate needed for the next step of glycolysis. This step accounts for the production of two pyruvate molecules at the end of the metabolic process of glycolysis.

Stage 6 – Dehydration of Glyceraldehyde-3-phosphate

The enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) serves two functions in this reaction. First, it dehydrogenates GAP by transferring one of its hydrogen (H+) molecules to the oxidizing agent nicotinamide adenine dinucleotide (NAD⁺) to form NADH + H+.

Next, GAPDH adds a phosphate from the cytosol to the oxidized GAP to form 1,3-bisphosphoglycerate (BPG). Both molecules of GAP produced in the previous step undergo this process of dehydrogenation and phosphorylation.

 

Stage 7 – Dephosphorylation of 1,3-bisphophoglycerate to yield ATP

The enzyme phosphoglycerokinase transfers a phosphate from BPG to a molecule of ADP to form ATP. This happens to each molecule of BPG. This reaction yields two 3-phosphoglycerate (3 PGA) molecules and two ATP molecules. This is one of the stages of Glycolysis that yields adenosine triphosphate (ATP). This form of ATP generation is known as substrate phosphorylation. The inorganic phosphate from the substrate is transferred to the pyrol ring of ADP to form ATP.

 

Stage 8 – Mutation of 3-bisphosphoglycerate to 2-bisphosphoglycerate

The enzyme phosphoglyceromutase relocates the Phosphate of the two 3 PGA molecules from the third to the second carbon to form two 2-phosphoglycerate (2 PGA) molecules. The rearrangement will enable the action the next stage of Glycolysis.

 

Stage 9 – Dehydration of 2-phosphoglycerate

The enzyme enolase removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvate (PEP). This happens for each molecule of 2 PGA from Step 8.

 

Stage 10 – Formation of pyruvate

The enzyme pyruvate kinase transfers a P from PEP to ADP to form pyruvate and ATP. This happens for each molecule of PEP. This reaction yields two molecules of pyruvate and two ATP molecules. This last stage of Glycolysis produces the end product of Glycolysis, pyruvate. It also yields two molecules of ATP.

 

 

Summary

Glycolysis is one of the first evolved pathway for metabolism of carbohydrates. It converts assimilated Glucose (also a product of Photosynthesis) in the blood to pyruvate, through the action of 10 different enzymes. In every Glucose molecule metabolized, 4 ATP are produced. 2 ATP are consumed in the process, leaving a net of 2 ATP. 2 pyruvate molecules are produced and NADH2. The pyruvate will undergo other metabolic pathways such as TCA cycle to yield more energy. NADH2 undergoes oxidative phosphorylation to also yield more ATPs.

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