Glycolysis is a metabolic pathway that converts glucose into pyruvate or lactate, producing ATP in the process. This pathway occurs in all cells, can function under both anaerobic and aerobic conditions, and is crucial for energy in tissues lacking mitochondria, particularly the brain. The reaction sequence of glycolysis is divided into three phases: energy investment, splitting, and energy generation, involving key enzymes that facilitate ATP production and metabolic intermediates for other biosynthetic pathways.

1. GLYCOLYSIS
Prepared by –
Zuli M. Shingala
IICP

2. INTRODUCTION
• Glycolysis word is derived from two Greek words :
• Glycose – sugar & lysis : breakdown
• Complete glycolysis pathway was elucidated in 1940.
• Glycolysis pathway also called as “Embden-Meyerhof
Pathway” or
“E. M. Pathway”
• Glycolysis is defined as the sequence of reactions
converting glucose to Pyruvate or Lactate with
production of ATP.

3. SALIENT FEATURES OF GLYCOLYSIS PATHWAY
1. Glycolysis takes place in all cells of the body. The
enzymes of this pathway are present in the cytosomal
fraction of cell.
2. Glycolysis occurs in absence of oxygen (Anaerobic) or
in the presence of oxygen (Aerobic).
• In Anaerobic condition Lactate is the end product and
in Aerobic condition Pyruvate is the end product.
3. Glycolysis is a major pathway for ATP synthesis in
tissues lacking Mitochondria.
4. Glycolysis is very essential for brain which is
dependent on glucose for energy. The glucose in brain
has to undergo glycolysis before it is oxidized to CO2
and H2O.

4. 5. Glycolysis in Anaerobic condition may be
summarized by the net reaction :
• Glucose + 2ADP + 2Pi 2 Lactate + 2 ATP
• Glycolysis in Aerobic condition :
• Glucose + 2ADP + 2Pi 2 Pyruvate + 8 ATP
6. Glycolysis is a central metabolic pathway with
many of its intermediates providing branch point
to other pathways. Thus, the intermediates of
glycolysis are useful for the synthesis of amino
acids and fats.
7. Reversal of glycolysis along with the alternate
arrangements at the irreversible steps, will result
in the synthesis of glucose. (Gluconeogenesis)

5. The sequence or reactions of
Glycolysis

9. REACTIONS OF GLYCOLYSIS
• The sequence of reactions of glycolysis can be
divided into three distinct phases :
A. Energy investment phase / priming stage
B. Splitting phase
C. Energy generation phase

10. A. Energy investment phase :
1. Glucose is phosphorylated to Glucose-6-phosphate
by Hexokinase or Glucokinase
• This is an irreversible reaction
• Dependent on ATP and Mg+2
2. Glucose-6-phosphate undergoes isomerization to
give Fructose-6-phosphate in the presence of the
enzyme Phosphohexose isomerase.
3. Fructose-6-phosphate is phosphorylated to
Fructose-1,6-biphosphate by phosphofructokinase
(PFK).

11. B. Splitting phase :
4. The six carbon Fructose-1,6-biphosphate is split into
two three-carbon compounds.
• Glyceraldehyde-3-phosphate &
• Dihydroxyacetone phosphate by the enzyme Aldolase.
5. The enzyme phosphotriose isomerase catalyses the
reversible interconversion of glyceraldehyde-3-
phosphate and dihydroxyacetone phosphate.
• Thus two molecules of glyceraldehyde-3-phosphate are
obtained from one molecule of glucose.

12. C. Energy generation phase :
6. Gyceraldehyde-3-phosphate dehydrogenase
converts glyceraldehyde-3-phosphate to 1,3-
biphosphoglycerate.
• This step is important as it is involved in the
formation of NADH + H+
• And a high energy compound 1,3-
biphosphoglycerate.
• In aerobic condition, NADH passes through the
electron transport chain (ETC) and 6 ATP (2Χ3 ATP)
are synthesized by oxidative phosphorylation.

13. 7. The enzyme phosphoglycerate kinase acts on 1,3-
biphosphoglycerate resulting in the synthesis of
ATP and formation of 3-phosphoglycerate.
• This step is example of substrate level
phosphorylation, since ATP is synthesized from the
substrate without the involvement of electron
transport chain.
8. 3-phosphoglycerate is converted to
2-phosphoglycerate by phosphoglycerate mutase.
This is an isomerization reaction.
9. The high energy compound phosphoenol pyruvate
is generated from 2-phosphoglycerate by the
enzyme Enolase.

14. 10. The enzyme pyruvate kinase catalyses the
transfer of high energy phosphate from
phosphoenol pyruvate to ADP and leading to
formation of ATP and Pyruvate.
• Reaction is irreversible.
• This step is substrate level phosphorylation.
• In anaerobic condition pyruvate is reduced by
NADH to Lactate in the presence of enzyme Lactate
dehydrogenase.