Glycolysis: Explained.

Glycolysis.

• Glycolysis the term is derived from two words, Glyco (Glucose) and Lysis (Breakdown), and hence glycolysis is nothing but the breakdown of a glucose molecule to gain the energy.


• A body gets its energy from the breakdown of many biomolecules, carbs, fats, and proteins, chiefly from carbs.


• The body uses its energy in the form of ATP (Adenosine tri Phosphate), in glycolysis, the glucose molecule is broken down to get ATPs.


• Glycolysis involves nine reactions that convert glucose into pyruvate.


• All of the glycolytic enzymes are found in the cytosol.

• Step 1: Conversion of glucose to glucose-6-phosphate.

• In the first step of glycolysis, the glucose ring is phosphorylated by addition of a phosphate to the glucose ring.
  


• The reaction is catalyzed by enzyme "Hexokinase" with Mg ion.
  


• The "Kinase" are a group of enzymes that catalyze the "phosphorylation" reactions, the Hexokinase catalyzes phosphorylation of many six membraned rings.
  


• One ATP is consumed in this process.
  


• The reaction results in the formation of Glucose-6-Phosphate.
  


• Glucose (C₆H₁₂O₆) + hexokinase + ATP → ADP + Glucose 6-phosphate (C₆H₁₃O₉P)
  
  
• 
  

  Step 2: Conversion of Glucose-6-Phosphate to Fructose-6-Phosphate.

  
  
• In the second step, the formed Glucose-6-Phosphate is get converted into its isomer i.e. Fructose-6-Phosphate.


• This reaction is called as isomerization is catalyzed by enzyme "Phosphoglucose Isomerase (Phosphofructo isomerase)".


• This reaction pulls out one carbon out from the ring.


• The reaction results in the formation of fructose-6-Phosphate.


• Glucose 6-phosphate (C₆H₁₃O₉P) + Phosphoglucoisomerase → Fructose 6-phosphate (C₆H₁₃O₉P)
  

• Step 3: Conversion of Fructose-6-Phosphate to Fructose 1-6- biphosphate.

• In the third step of glycolysis, fructose-6-phosphate is converted to fructose- 1,6-biphosphate.
  


• In this reaction, one molecule of phosphate is added.


• The reaction is catalyzed by "Phosphofructo Kinase" with Mg ion.


• One ATP is consumed in this step.


• This reaction results in the formation of Fructose-1-6-biphosphate.


• Fructose 6-phosphate (C₆H₁₃O₉P) + phosphofructokinase + ATP → ADP + Fructose 1, 6-bisphosphate (C₆H₁₄O₁₂P₂)
  



• Step 4: Cleavage of Fructose-1-6-biphosphate to Glyceraldehyde-3-phosphate and Dihydroxyacetone Phosphate.

• In this reaction, the Fructose 1-6-biphosphate a six carbon compound is get cleaved into 2 three carbon compounds namely glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
  


• The reaction is catalyzed by enzyme "Aldolase"
  


• The formed dihydroxyacetone phosphate is unstable and gets reorganized to glyceraldehyde-3-phosphate immediately by the action of the enzyme "triphosphate isomerase".
  


• Hence the glyceraldehyde-3-phosphate continues the cycle.
  


• Fructose 1, 6-bisphosphate (C₆H₁₄O₁₂P₂) + aldolase → Dihydroxyacetone phosphate (C₃H₇O₆P) + Glyceraldehyde phosphate (C₃H₇O₆P)
  


• Dihydroxyacetone phosphate (C₃H₇O₆P) → Glyceraldehyde 3-phosphate (C₃H₇O₆P)
  
  
  
  



• Step 5: Conversion of glyceraldehyde-3-phosphate to 1-3-biphospho glycerate.

• In this step, two main events take place: 
  


• 1) Glyceraldehyde-3-phosphate is oxidized by the coenzyme nicotinamide adenine dinucleotide (NAD); to give NADH+H this NADH+H after entering ETC (Electron Transport Chain) produces 3 ATPs.
  


• 2) The molecule is phosphorylated by the addition of a free phosphate group.
  


• The enzyme that catalyzes this reaction is glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
  


• This reaction produces 6 ATPs. (Two Glyceraldehyde-3-phosphate molecules)
  


• The reaction results in the formation of 1-3-biphospho glycerate.
  


• 
  
  A. Triose phosphate dehydrogenase + 2 H^- + 2 NAD⁺ → 2 NADH + 2 H⁺
  B. Triose phosphate dehydrogenase + 2 P + 2 glyceraldehyde 3-phosphate (C₃H₇O₆P) → 2 molecules of 1,3-bisphosphoglycerate (C₃H₈O₁₀P₂) 
  
  
  

  Step 6: Conversion of 1-3 biphosphoglycerate to 3-phosphoglycerate.

  
  
• In this step, 1,3 bisphoglycerate is get converted to 3-phosphoglycerate.
  


• The reaction is catalyzed by enzyme "phosphoglycerate kinase" with Mg ions.
  


• The enzyme family "Kinase' deals with the transfer of ATP, the Mg (Magnesium) ion helps to shield the negative charges.
  


• Two ATP is synthesized in this step. ( As Dihydroxyacetone phosphate is get converted immediately to Glyceraldehyde-3-phosphate, we get two molecules of Glyceraldehyde-3-phosphate)
  


•  The reaction results in the formation of 3-Phosphoglycerate.
  


• 2 molecules of 1,3-bisphoshoglycerate (C₃H₈O₁₀P₂) + phosphoglycerokinase + 2 ADP → 2 molecules of 3-phosphoglycerate (C₃H₇O₇P) + 2 ATP
  
  
  
  

  Step 7: Conversion of 3-phosphoglycerate to 2-phosphoglycerate.

  
  
• This reaction causes  structural rearrangement of  3-phosphoglycerate forming 
  
  
  2-phosphoglycerate.


• The enzyme which catalyzes the reaction is "Phosphoglycerate Mutase."
  


• The enzyme family "Mutase" carries out the function of transferring the functional groups from one position to another.
  


• The reactions result in the formation of 2-phosphoglycerate.
  


• 2 molecules of 3-Phosphoglycerate (C₃H₇O₇P) + phosphoglyceromutase → 2 molecules of 2-Phosphoglycerate (C₃H₇O₇P)
  
  
  
  

  Step 8: Conversion of 2-phosphoglycerate to Phosphoenolpyruvate. 

  
  
• This step involves the conversion of 2-phosphoglycerate to phosphoenolpyruvate. 
  


• The reaction is catalyzed by the enzyme enolase. 
  


• Enolase works by removing a water group or dehydrating the 2- phosphoglycerate.
  


• The reaction results in the formation of "Phosphoenolpyruvate"
  


• 2 molecules of 2-Phosphoglycerate (C₃H₇O₇P) + enolase → 2 molecules of phosphoenolpyruvate (PEP) (C₃H₅O₆P)
  
  
  
  

  Step 9: Conversion of Phosphoenolpyruvate to Pyruvate.

  
  
• In this step, phosphoenolpyruvate is get converted into pyruvate.
  


• The enzyme that catalyzes this reaction is "Pyruvate kinase".
  


• As the enzyme name suggests the reaction involves the transfer of ATP molecule, one ADP (Adenosine triphosphate) is getting converted to one ATP.
  


• The reaction results in the formation of Pyruvate and 2 ATPs.
  


• All above reactions take place in an "aerobic environment (In presence of atmospheric oxygen)".


• If the conditions become "anaerobic (In absence of atmospheric oxygen)" the reaction proceeds to the formation of the "lactic acid".


• 2 molecules of phosphoenolpyruvate (C₃H₅O₆P) + pyruvate kinase + 2 ADP → 2 molecules of pyruvate (C₃H₃O₃^-) + 2 ATP
  
  
  
  

  Step 10: Conversion of Pyruvate to Lactate.

• When anaerobic conditions continue as in strenuous exercise, the formed Pyruvate is get converted to Lactate.
  


• The reaction is catalyzed by the enzyme "Lactate Dehydrogenase.


• Two NADH+H are used in the process.


• The reaction cause loss of 6 ATP molecules.
  

AFTERMATH:

• We have seen different reactions causing ATPs in and out, now let's see how much ATPs we can gain from the glycolysis.

Outcome of Glycolysis.
Loss		Profit
Reaction	Outcome of ATP	Reaction	Outcome of ATP
Step 1: Conversion of glucose to glucose-6-phosphate.	1	Step 5: Conversion of glyceraldehyde-3-phosphate to 1-3-biphospho glycerate.	3x2= 6 (Two molecules of Glyceraldehyde-3-phosphate)
Step 3: Conversion of Fructose-6-Phosphate to Fructose 1-6- biphosphate.	1	Step 6: Conversion of 1-3 biphosphoglycerate to 3-phosphoglycerate.	1x2= 2 (Two molecules of Glyceraldehyde-3-phosphate)
Step 10: Conversion of Pyruvate to Lactate.	3x2= 6 (Two molecules of Glyceraldehyde-3-phosphate)	Step 9: Conversion of Phosphoenolpyruvate to Pyruvate.	1x2= 2 (Two molecules of Glyceraldehyde-3-phosphate)
Total	08	Total	10

In Aerobic Conditions Glycolysis yields 10-2= 8 ATPs.

In Anaerobic Conditions Glycolysis yields 10-8= 2 ATPs.

Cyclic Representation of Glycolysis: