GLYCOLYSIS — Quiz time


1.  Which of the enzyme(s) are irreversible in the prepatory phase?

1..Phosphohexose Isomerase
2. Hexokinase
3. Aldose
4. Phosohofructo-kinase 1

(A) 1 only

(B)  1, 2

(C)  1, 3

(D)  2, 4

(E)   3, 4

2. In glycolysis process, the step(s) that produces ATP is called…

(A)  First priming reaction

(B)  Oxidative phosphorylation

(C)  Substrate level phosphorylation

(D)  Oxidation

(E)  Reductive phosphorylation


Fate of Pyruvate

  • NADH is formed from NAD+ during glycolysis.
  • The redox balance of the cell has to be maintained for further cycles of glycolysis to continue.
  • NAD+ can be regenerated by one of the following reactions /pathways:
  • Pyruvate is converted to lactate
  • Pyruvate is converted to ethanol
  • In the presence of O2, NAD+ is regenerated by ETC.
  • Pyruvate is converted to acetyl CoA which enters the TCA cycle and gets completely oxidized to CO2.


Entry into the TCA cycle/Link Reaction


Lactate Fermentation

  • Occurs in erythrocytes, depends solely on glycolysis for energy
  • Pyruvat is converted to lactate to regenerate NAD+
  • Formation of lactate catalyzed by lactate dehydrogenase:


  • In highly active muscle, there is anaerobic glycolysis because the supply of O2 cannot keep up with the demand for ATP.
  • Lactate builds up causing a drop in pH which inactivates glycolytic enzymes. End result is energy deprivation and cell death; the symptoms being pain and fatigue of the muscle.
  • Lactate is transported to the liver where it can be reconverted to pyruvate by the LDH reverse reaction

The best-conditioned athletes can sprint at top speed for no more than a minute.



Ethanol fermentation

  • Formation of ethanol catalyzed by 2 enzymes
  • Pyruvate decarboxylase catalyzes the first irreversible reaction to form acetaldehyde using TPP and Mg2+ cofactors:


  • Acetaldehyde is reduced by alcohol dehydogenase is a reversible reaction:

CH3-CHO + NADH + H+ <–> CH3CH2OH + NAD+

  • Ethanol fermentation is used during wine-making

Glycolysis – Energy Generation Phase

Summary Continued:

Energy  Generation phase


  • GAP is dehydrogenated by the enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH). In the process, NAD+ is reduced to NADH + H+ from NAD.
  • Oxidation is coupled to the phosphorylation of the C1 carbon. The product is 1,3-bisphosphoglycerate.



  • BPG has a mixed anhydride, a high energy bond, at C1. This high energy bond is hydrolyzed to a carboxylic acid and the energy released is used to generate ATP from ADP.
  • Product: 3-phosphoglycerate.
  • Enzyme: phosphoglycerate kinase.



  •  The phosphate shifts from C3 to C2 to form 2-phosphoglycerate.
  • Enzyme: phosphoglycerate mutase.



  • Dehydration catalyzed by enolase (a lyase). A water molecule is removed to form phosphoenolpyruvate which has a double bond between C2 and C3.



  • Enolphosphate is a high energy bond. It is hydrolyzed to form the enolic form of pyruvate with the synthesis of ATP.
  • The irreversible reaction is catalyzed by the enzyme pyruvate kinase.
  • Enol pyruvate quickly changes to keto pyruvate which is far more stable.


Glycolysis – Energy Investment Phase

This is basically what the podcast says:
Untitled 1


    • Phosphorylation reaction i.e. ATP –> ADP, terminal phosphate from the ATP is transferred to the glucose molecule.  This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
    • The first reaction of the energy investment phase is called FIRST PRIMING REACTION
    • Enzyme: hexokinase. It has a low Km for glucose; thus, once glucose enters the cell, it gets phosphorylated.
        • Hexokinase require a cofactor, Mg2+, which stabilizes the negative charge on the ATP.
    • This step is irreversible. So the glucose gets trapped inside the cell. (Glucose transporters transport only free glucose, not  phosphorylated glucose).
        • The forward reaction of glucose –> glucose-6-phosphate is highly negative and requires additional energy therefore if the reaction is to be reversed, it would require much more energy due to its positive nature hence the reaction is not energetically feasible.


    • Isomerization of glucose-6-phosphate to fructose 6-phosphate. The aldose sugar is converted into the keto isoform.
    • The enzyme phosphohexose kinase belongs to the isomerase class (click to see more)

Aldose sugar —-> Ketose sugar

    • This is a reversible reaction. The fructose-6-phosphate is quickly consumed and the forward reaction is favored.


    • This reaction is called the SECOND PRIMING REACTION and requires energy from the ATP
    • This is another kinase reaction. Phosphorylation of the hydroxyl group on C1 forming fructose-1,6- bisphosphate.
    • Enzyme: phosphofructokinase. This allosteric enzyme regulates the pace of glycolysis.
    • Phosphofructokinase-1 (PFK-1) is the most regulated enzyme of glycolysis.
    • Reaction is coupled to the hydrolysis of an ATP to ADP and Pi.
    • This is the second irreversible reaction of the glycolytic pathway.


    • The six-carbon fructose-1,6-bisphosphate is slit in to 2 three-carbon sugar phosphates… glyceraldehyde-3-phosphate (G3P) and dihydroxyactetone (DHAP).
    • This reaction is catalyzed by Aldolase.
    • The DHAP molecule does not take part the the glycolysis reactions.


    • DHAP and GAP are isomers of each other and can readily inter-convert by the action of the enzyme triose-phosphate isomerase.
    • Since DHAP does not part take in glycolysis, it undergoes an enzyme catalyzed reaction, triose phosphate isomerae, and is converted to G3P
    • GAP is a substrate for the next step in glycolysis so all of the DHAP is eventually depleted. So, 2 molecules of GAP are formed from each molecule of glucose 

Up to this step, 2 molecules of ATP were required for each molecule of glucose being oxidized

The remaining steps release enough energy to shift the balance sheet to the positive side. This part of the glycolytic pathway is called as the payoff or harvest stage.


What is Glycolysis? 

It is series of reactions where one glucose molecule is broken down to form two molecules of pyruvic acid.


Where does it occur?

It is universal … i.e. it occurs in the CYTOPLASM of animal cells, plant cells, and the cells of microorganisms.

How does Glycolysis work?

There are 10 reactions in glycolysis that are broken in to two stages:

1.Energy Investment Phase / Prepatory Phase – Phosphorylation of glucose and its conversion to glyceraldehyde-3-phosphate

    • The first 5 steps of glycolysis using:
        •  2 irreversible and 3 reversible enzyme catalyzed reactions
        • And 2 ATP

2. Energy Generation Phase / Pay off Phase – oxidative conversion of glyceraldehyde-3-phosphate to pyruvate and coupled formation of ATP and NADH.

    • The second 5 steps of glycolysis using:
        • 1 irreversible and 4 reversible enzyme catalyzed reactions
        • 4 ADP and 2 NAD+

End Result

  • The net gain at the end of glycolysis is 2 ATP.