What is glycolysis, its types, complete metabolic pathway step by step

1. Glycolysis, ATP
Calculation and Regulation
Dr. Umar Hamid
Lecturer
Allied Health Sciences
Superior University, Sargodha
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2. Glycolysis
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3. “Oxidation of glucose or glycogen to pyruvate and lactate is called glycolysis.”
• This was described by Embden, Meyerhof and Parnas. Hence, it is also called as Embden
Meyerhof pathway.
• It occurs virtually in all tissues.
• Erythrocytes and nervous tissues derive its energy mainly from glycolysis.
• This pathway is unique in the sense that it can utilise O2 if available (aerobic) and it can
function in absence of O2 also (anaerobic).
• Enzymes: Enzymes involved in glycolysis are extramitochondrial.
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4. Glycolysis, ATP Calculation, Regulation; Dr Umar Hamid 4

5. Reactions of glycolytic pathway
Stage I: (Preparatory / conversion phase)
• This is a preparatory stage.
• No splitting of glucose molecule.
• Conversion of glucose molecule to fructose 1,6- bisphosphate.
- Donation of 2 PO4 groups from ATP.
1. Phosphorylation of glucose:
2. Conversion of G-6-P to fructose-6-P:
3. Phosphorylation of fructose-6-P to fructose-1, 6-bi-P:
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6. 1. Uptake of glucose by cells and its phosphorylation: (Phosphorylation of glucose)
• Glucose is freely permeable to Liver cells.
• Insulin facilitates the uptake of glucose in skeletal muscles, cardiac muscle, diaphragm
and adipose tissue.
• Glucose is then phosphorylated to form glucose-6-P.
• The reaction is catalyzed by the specific enzyme glucosidase in liver cells and by non-
specific hexokinase in liver and extrahepatic tissues.
• Note:
- Reaction is irreversible – ATP acts as PO4 donor.
- One ATP is utilised for phosphorylation.
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7. 2. Conversion of G-6-P to fructose-6-P:
- Conversion of G-6-P to fructose-6-P by phosphohexose isomerase.
3. Conversion of fructose-6-P to fructose-1, 6-bi-P: (Phosphorylation of F-6-P to F-1,6-
bi-P)
• Phosphorylation of F-6-P to F-1,6-bi-P catalysed by the enzyme phosphofructokinase-1.
• Note:
- The reaction is irreversible.
- One ATP is utilised for phosphorylation.
- Phosphofructokinase-1 is the key enzyme in glycolysis which regulates breakdown of
glucose.
- At this stage glucose oxidation does not yield any useful energy rather there is
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8. Stage II: (Splitting phase)
1. Splitting of Fructose-1-6-bi-P to two triose-phosphates:
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9. 1. Splitting of Fructose-1-6-bi-P to two triose-phosphates:
• Splitting of Fructose-1-6-bi-P to two triose-phosphates by the enzyme aldolase.
• These two trioses are; an aldotriose–glyceraldehyde-3-P and one ketotriose, Dihydroxy
acetone-P
• Note:
- The reaction is reversible.
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10. Stage III: (Energy yielding phase)
• It is the energy-yielding reaction.
• Aldehyde group is oxidized to an acid are accompanied by liberation of large amounts of
potentially useful energy.
• This stage consists of the following two reactions:
1. Oxidation of glyceraldehyde-3-P to 1,3-bi-phosphoglycerate:
2. Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate:
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11. 1. Oxidation of glyceraldehyde-3-P to 1,3-bi-phosphoglycerate:
• Oxidation of glyceraldehyde-3-P to 1,3-bi-phosphoglycerate, glyceraldehyde-3-P
dehydrogenase
• Dihydroxyacetone-P also form 1,3-bi-phosphoglycerate via glyceraldehyde-3-P.
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12. 2. Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate:
• Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate, by the enzyme
phosphoglycerate kinase.
• This Step involves formation of ATP.
- The high energy PO4 bond at position1 can donate the PO4 to ADP and forms ATP
molecule.
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13. Energetics:
• Each NADH produces 3 ATP molecule.
• NADH is produced in the presence of O2.
• Since, 2 molecules of triose are formed per glucose molecule, hence 2 NADH are
produced and eventually generating 6 molecules of ATP.
+ 6 ATP
• The second reaction will produce one ATP. Two molecules of substrate will produce 2
ATP.
+ 2 ATP
• Net gain at this stage per molecule of glucose oxidised is
+ 8 ATP
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14. Stage IV: (Recovery phase)
• It is the recovery of the PO4 group from 3-Phosphoglycerate.
• The two molecules of 3-phosphoglycerate, the end-product of the previous stage, still
retains the PO4 group originally derived from ATP in stage 1.
• Body wants back the two ATP spent in first stage for two phosphorylations.
• This is achieved by the following three reactions:
1. Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate:
2. Conversion of 2-Phosphoglycerate to Phosphoenol pyruvate:
3. Conversion of Phosphoenol Pyruvate to Pyruvate:
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15. 1. Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate:
• Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate by the enzyme
Phosphoglycerate mutase.
2. Conversion of 2-Phosphoglycerate to Phosphoenol pyruvate:
• Conversion of 2-Phosphoglycerate to Phosphoenol pyruvate by the enzyme Enolase.
• The reaction involves dehydration.
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16. 3. Conversion of Phosphoenol Pyruvate to Pyruvate:
• Conversion of Phosphoenol Pyruvate to Pyruvate by the enzyme Pyruvate kinase.
• The high energy PO4 of phosphoenol pyruvate is directly transferred to ADP producing
ATP.
• Note:
- Reaction is irreversible.
- This step involves ATP formation.
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17. Clinical importance:
• Sodium fluoride is used along with K-oxalate for collection of blood for glucose
estimation.
• If K-oxalate is used alone, then in vitro glycolysis will reduce the glucose value in the
sample.
• Functions of Fluoride:
- Inhibits in vitro glycolysis by inhibiting enzyme enolase.
- Also acts as anticoagulant.
- Act. as an antiseptic
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18. Energetics:
• In this stage, 2 molecules of ATP are produced, per molecule of glucose oxidised.
+ 2 ATP
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19. ATP Calculation
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20. Two Phases of Glycolysis:
Aerobic phase:
• Oxidation is carried out by dehydrogenation and reducing equivalent is transferred to
NAD+.
• Reduced NAD in presence of O2 is oxidised in electron-transport chain producing ATP.
Anaerobic phase:
• NADH cannot be oxidised in electron transport chain, so no ATP is produced in electron
transport chain.
• But the NADH is oxidised to NAD+ by conversion of pyruvate to lactate, without
producing ATP.
• Anaerobic phase limits the amount of energy per mol. of glucose oxidised. Hence, to
provide a given amount of energy, more glucose must undergo glycolysis under anaerobic
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21. Aerobic phase
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22. Anaerobic phase
• In absence of O2, re-oxidation of NADH at glyceraldehyde-3-P-dehydrogenase stage
cannot take place in electron-transport chain.
• It is to be noted that in the reaction catalyzed by glyceraldehyde-3-P-dehydrogenase,
therefore, no ATP is produced. In anaerobic phase per molecule of glucose oxidation 4 – 2
= 2 ATP will be produced. + 2 ATP
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23. Regulation Of Glycolysis
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24. Regulation of glycolysis achieved by three types of mechanisms:
1. Changes in the rate of enzyme synthesis, Induction/ repression.
2. Covalent modification by reversible phosphorylation.
3. Allosteric modification.
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25. 1. Induction and repression of key enzymes:
This is not rapid and takes several hours to come into operation.
• Glucose:
- When there is increased substrate, i.e. glucose, the enzymes involved in utilization of
glucose are activated.
- On the other hand, enzymes responsible for producing glucose (gluconeogenesis) are
inhibited.
- Glucose also increases the activity of the key enzymes glucokinase, phosphofructokinase-
1 and pyruvate kinase.
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26. • Insulin:
- The secretion of insulin which is responsive to blood glucose concentration enhances the
synthesis of the key enzymes responsible for glycolysis.
- On the other hand, it antagonises the effects of glucocorticoids.
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27. 2. Covalent modification by reversible phosphorylation:
• Hormones like epinephrine and glucagon which increase protein kinase which can
phosphorylate and inactivate the Key enzyme Pyruvate kinase and, thus, inhibit
glycolysis.
• This is a rapid process and occurs quickly.
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28. 3. Allosteric modification:
Phosphofructokinase-1 is the Key regulatory enzyme and is subject to “feedback” control.
• Inhibition of the enzyme:
- The enzyme is inhibited by citrate and by ATP.
• Activator of the enzyme:
- The enzyme is activated by AMP.
In hypoxia: The concentration of ATP in the cells decreases and there is increase in
concentration of AMP which explains why glycolysis should increase in absence of O2.
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