Glycolysis is a metabolic pathway that breaks down glucose into pyruvate and occurs in two main stages: a priming stage that invests energy and an energy generation stage. Key reactions include the phosphorylation of glucose, the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate, and the conversion of phosphoenolpyruvate to pyruvate, with the latter producing ATP. The process can occur anaerobically yielding lactate or aerobically producing pyruvate, which further enters the citric acid cycle for complete oxidation.

1. LECTURE 1
GLYCOLYSIS
Likando Chababa, B.Sc, M.Sc (Biochemistry)
BIOCHEMISTRY II

2. GLYCOLYSIS
 The glycolytic pathway is a pathway used by all tissues for the breakdown
of glucose.
 Glucose is metabolized to pyruvate by the pathway of glycolysis, which can
occur aerobically.
 When it occurs anaerobically, the end product is lactate.
 Aerobic tissues metabolize pyruvate to acetyl-CoA, which can enter the citric
acid cycle for complete oxidation to CO2 and H2O, linked to the formation of
ATP in the process of oxidative phosphorylation.
 Glucose is the major fuel of most tissues

3. Reactions of Glycolytic Pathway
 All reactions occur in the cytoplasm.
 The conversion of glucose to pyruvate occur in two stages:
1. The first being the priming stage and
2. The second being energy generation stage.
 The priming stage corresponds to energy investment in the form of
ATP where glucose and other hexose sugars are converted to the
common product called glyceraldehyde-3-phosphate.
 In the energy generation state, a net of two ATP molecules are formed
per glucose molecule metabolised.
 In addition to ATP, two molecules of NADH + H+
are formed if the end
product is pyruvate.
 If the end product is lactate, then NADH + H+
are converted to NAD+
.

4. Reactions of Glycolytic Pathway
1. Phosphorylation of glucose:
 In this reaction, glucose is first phosphorylated at the hydroxyl group
on C-6 and thus glucose 6-phosphate thus formed in the presence of
ATP.
 The reaction is catalyzed by the specific enzyme glucokinase in liver
cells and by non-specific Hexokinase in liver and extrahepatic tissues.
 Glucokinase activity in hepatocytes is also increased by insulin.
 As blood glucose levels rise following a meal, the cells of the pancreas
are stimulated to release insulin into the portal circulation.
Glucose + ATP Glu 6-P + ADP

5. Reactions of Glycolytic Pathway
2. Isomerization of glucose 6-phosphate:
 The isomerization of glucose 6-phosphate to fructose 6-
phosphate is catalyzed by phosphohexose isomerase.
 The reaction is readily reversible and is not a rate-limiting or
regulated step.
Glu 6-P Fru 6-P

6. Reactions of Glycolytic Pathway
3. Phosphorylation of fructose 6-phosphate:
 The irreversible phosphorylation reaction catalyzed by
phosphofructokinase 1(PFK-1) is the most important control point and
the rate-limiting step of glycolysis.
 PFK-1 is controlled by the available concentrations of the substrates
ATP and fructose 6-phosphate and fructose 2,6-bisphosphate activates
PFK-1 of glycolysis.
 Fructose 6-phosphate is phosphorylated to Fructose 1,6-bisphosphate
through an ATP input.
Fru 6-P + ATP Fru 1,6-bisp + ADP

7. Reactions of Glycolytic Pathway
4. Cleavage of Fructose 1,6-bisphosphate:
 Fructose 1, 6-bisphosphate is split by the enzyme aldolase into two
molecules of triose phosphates:
 An aldotriose, glyceraldehyde 3-phosphate (GAP) and
 A Ketotriose, Dihydroxyacetone phosphate (DHAP).
Fru 1,6-bisp GAP + DHAP

8. Reactions of Glycolytic Pathway
5. Isomerization of dihydroxyacetone phosphate:
 Triose phosphate isomerase interconverts dihydroxyacetone
phosphate and glyceraldehyde 3-phosphate.
 Dihydroxyacetone phosphate must be isomerized to
glyceraldehyde3-phosphate for further metabolism by the glycolytic
pathway.
 This isomerization results in the net production of two molecules of
glyceraldehyde 3-phosphate from the cleavage products of
fructose 1,6-bisphosphate.
DHAP GAP

9. Reactions of Glycolytic Pathway
6. Oxidation of glyceraldehyde 3-phosphate:
 The conversion of glyceraldehyde 3-phosphate to 1,3-
bisphosphoglycerate (1,3-BPG) by glyceraldehyde 3-phosphate
dehydrogenase is the first oxidation-reduction reaction of glycolysis.
2GAP + 2NAD + 2Pi 2 1,3-BPG + 2NADH + 2H+

10. Reactions of Glycolytic Pathway
7. Synthesis of 3-phosphoglycerate producing ATP:
 1,3-BPG is converted to 3-phosphoglycerate (3-PG), the high-energy
phosphate group of 1,3-BPG is used to synthesize ATP from ADP.
 This reaction is catalyzed by phosphoglycerate kinase, which, unlike
most other kinases, is physiologically reversible.
 Two molecules of 1,3-BPG are formed from each glucose molecule, this
kinase reaction replaces the two ATP molecules consumed by the
earlier formation of glucose 6-phosphate and fructose 6-Phosphate.
2 1,3-BPG + 2ADP 2 3-PG + 2ATP

11. Reactions of Glycolytic Pathway
8. Shift of the phosphate group from carbon 3 to carbon 2:
 The shift of the phosphate group from carbon 3 to carbon 2 of
phosphoglycerate by phosphoglycerate mutase is freely reversible
2 3-PG 2 2-PG

12. Reactions of Glycolytic Pathway
9. Dehydration of 2-PG:
 The dehydration of 2-phosphoglycerate by enolase redistributes the
energy within the 2-phosphoglycerate molecule, resulting in the
formation of phosphoenolpyruvate (PEP), which contains a high-
energy phosphate plus water.
 The reaction is reversible despite the high-energy nature of the
product.
2 2-PG 2 PEP + 2H2O

13. Reactions of Glycolytic Pathway
10. Formation of pyruvate producing ATP:
 The conversion of PEP to pyruvate is catalyzed by pyruvate kinase,
the third irreversible reaction of glycolysis.
 The equilibrium of the pyruvate kinase reaction favours the formation
of ATP.
 This is the third irreversible reaction of the glycolytic pathway.
2 PEP + 2 ADP 2 Pyruvate + 2 ATP

14. Reactions of Glycolytic Pathway
 Under anaerobic conditions, pyruvate is converted to lactate through
a redox reaction.
 The reaction helps to yield NAD+ from reaction 6.
 The enzyme Lactate dehydrogenase catalyses this reversible
reaction.
 Pyruvate + NADH + H+
Lactate + NAD+

16. Energy Yield of Glycolysis
1. Anaerobic:
Glu + 2ATP + 4ADP + 2NAD+
+ 2Pi + 2NADH + 2H+
2Lactate +
4ATP + 2ADP + 2NAD+
+ 2 NADH + 2H+
+ 2H2O
 Net Equation: Glu + 2ADP + 2Pi 2Lactate + 2ATP + 2H2O
2. Aerobic:
 Glu + 2ATP + 4ADP + 2NAD+
+ 2Pi 2Pyruvate + 4ATP +
2ADP + 2NAD+
+ 2 NADH + 2H+
+ 2H2O
 Net Eq:
 Glu + 2ADP + 2Pi + 2NAD+
2Pyruvate + 2ATP + 2H2O + 2NADH

17. Energy Yield of Glycolysis
 On-going aerobic glycolysis requires the oxidation of most of this
NADH by the electron transport chain, producing approximately
three ATP for each NADH molecule entering the chain.
 Therefore aerobic glycolysis results in the production of a total of 8
ATP molecules.
 Despite the production of some ATP during glycolysis, the end
products, pyruvate or lactate, still contain most of the energy
originally contained in glucose.
 The Tricarboxylic Acid (TCA) cycle is required to release that
energy completely.