GLYCOLYSIS
(Embden-Meyerhof Pathway or EMP Pathway)
Importance of the Pathway
Glycolysis is derived from the Greek word, glykys = sweet; and lysis splitting. In this =
pathway glucose is converted to pyruvate (aerobic condition) or lactate (anaerobic
condition), along with production of a small quantity of energy
All the reaction steps take place in the cytoplasm. It is the only pathway that is taking
place in all the cells of the body.
Glycolysis is the only source of energy in erythrocytes.
In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis
forms the major source of energy for muscles.
The glycolytic pathway may be considered as the preliminary step before complete
oxidation.
The glycolytic pathway also provides carbon skeletons for synthesis of certain
nonessential amino acids as well as glycerol part of fat.
Most of the reactions of the glycolytic pathway are reversible, which are also used for
gluconeogenesis.
Step 0: Glucose Entry into Cells
Glucose transporter-4 (Glu T4) transports glucose from extracellular fluid to muscle cells and
adipocytes. This translocase is under the influence of insulin. So, in diabetes mellitus, insulin
deficiency hinders the entry of glucose into the peripheral cells. But GluT2 is the transporter in
liver cells; it is not under the control of insulin.
Step 1 of Glycolysis
o The metabolic fates of glucose-6-phosphate Glucose-6-phosphate
Glycolysis to
pyruvate
Glucose
Glucose-6-
Glucose phosphate
Glycogen
Shunt pathway
o Glucose is activated by phosphorylation to glucose-6-phosphate. The enzyme is
Hexokinase (HK), which splits the ATP into ADP, and the Pi is added on to the glucose. The
energy released by the hydrolysis of ATP is utilized for the forward reaction. Hexokinase
, and glucokinase may be considered as isoenzymes. Glucokinase is under the influence of
insulin; but hexokinase is not.
o The kinase reaction is irreversible; the same enzyme cannot produce glucose. But this
irreversibility is circumvented by another enzyme glucose-6-phosphatase .Hexokinase is
a key glycolytic enzyme, while glucose-6-phosphatase is a key gluconeogenic enzyme .
Step 2 of Glycolysis
Glucose-6-phosphate is isomerized to fructose-6 phosphate by an isomerase. This is readily
reversible .
Step 3 of Glycolysis
o Fructose-6-phosphate is further phosphorylated to fructose-1,6-bisphosphate .The
enzyme is phosphofructokinase (PFK). It needs ATP. PFK is an allosteric and inducible
enzyme. It is an important key enzyme of this pathway.
o This reaction is an irreversible step in glycolysis. However, this difficulty is circum
vented by another enzyme, fructose-1,6 bisphosphatase
Step 4 of Glycolysis
The fructose-1,6-bisphosphate is cleaved into two halves; one molecule of glyceraldehyde-3-
phosphate and another molecule of dihydroxyacetone phosphate. The enzyme is called
aldolase. This reaction is reversible.
Step 4-A of Glycolysis
Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by the enzyme
phosphotriose isomerase. Thus, net result is that glucose is now cleaved into 2 molecules of
glyceraldehyde-3-phosphate. For synthesis of neutral fat, glycerol is required which is derived
from glucose through dihydroxyacetone phosphate.
Step 5 of Glycolysis
In this step, glyceraldehyde-3-phosphate is dehydrogenated and simultaneously phospho
rylated to 1,3-bisphosphoglycerate (1,3-BPG) with the help of NAD+ . The enzyme is
glyceraldehyde-3-phosphate dehydrogenase. This is a reversible reaction. The product contains
a high energy bond.
Step 6 of Glycolysis
The energy of 1,3-BPG is trapped to synthesise one ATP molecule with the help of a kinase. This
is an example of substrate level phosphorylation, where energy is trapped directly from the.
(Embden-Meyerhof Pathway or EMP Pathway)
Importance of the Pathway
Glycolysis is derived from the Greek word, glykys = sweet; and lysis splitting. In this =
pathway glucose is converted to pyruvate (aerobic condition) or lactate (anaerobic
condition), along with production of a small quantity of energy
All the reaction steps take place in the cytoplasm. It is the only pathway that is taking
place in all the cells of the body.
Glycolysis is the only source of energy in erythrocytes.
In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis
forms the major source of energy for muscles.
The glycolytic pathway may be considered as the preliminary step before complete
oxidation.
The glycolytic pathway also provides carbon skeletons for synthesis of certain
nonessential amino acids as well as glycerol part of fat.
Most of the reactions of the glycolytic pathway are reversible, which are also used for
gluconeogenesis.
Step 0: Glucose Entry into Cells
Glucose transporter-4 (Glu T4) transports glucose from extracellular fluid to muscle cells and
adipocytes. This translocase is under the influence of insulin. So, in diabetes mellitus, insulin
deficiency hinders the entry of glucose into the peripheral cells. But GluT2 is the transporter in
liver cells; it is not under the control of insulin.
Step 1 of Glycolysis
o The metabolic fates of glucose-6-phosphate Glucose-6-phosphate
Glycolysis to
pyruvate
Glucose
Glucose-6-
Glucose phosphate
Glycogen
Shunt pathway
o Glucose is activated by phosphorylation to glucose-6-phosphate. The enzyme is
Hexokinase (HK), which splits the ATP into ADP, and the Pi is added on to the glucose. The
energy released by the hydrolysis of ATP is utilized for the forward reaction. Hexokinase
, and glucokinase may be considered as isoenzymes. Glucokinase is under the influence of
insulin; but hexokinase is not.
o The kinase reaction is irreversible; the same enzyme cannot produce glucose. But this
irreversibility is circumvented by another enzyme glucose-6-phosphatase .Hexokinase is
a key glycolytic enzyme, while glucose-6-phosphatase is a key gluconeogenic enzyme .
Step 2 of Glycolysis
Glucose-6-phosphate is isomerized to fructose-6 phosphate by an isomerase. This is readily
reversible .
Step 3 of Glycolysis
o Fructose-6-phosphate is further phosphorylated to fructose-1,6-bisphosphate .The
enzyme is phosphofructokinase (PFK). It needs ATP. PFK is an allosteric and inducible
enzyme. It is an important key enzyme of this pathway.
o This reaction is an irreversible step in glycolysis. However, this difficulty is circum
vented by another enzyme, fructose-1,6 bisphosphatase
Step 4 of Glycolysis
The fructose-1,6-bisphosphate is cleaved into two halves; one molecule of glyceraldehyde-3-
phosphate and another molecule of dihydroxyacetone phosphate. The enzyme is called
aldolase. This reaction is reversible.
Step 4-A of Glycolysis
Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by the enzyme
phosphotriose isomerase. Thus, net result is that glucose is now cleaved into 2 molecules of
glyceraldehyde-3-phosphate. For synthesis of neutral fat, glycerol is required which is derived
from glucose through dihydroxyacetone phosphate.
Step 5 of Glycolysis
In this step, glyceraldehyde-3-phosphate is dehydrogenated and simultaneously phospho
rylated to 1,3-bisphosphoglycerate (1,3-BPG) with the help of NAD+ . The enzyme is
glyceraldehyde-3-phosphate dehydrogenase. This is a reversible reaction. The product contains
a high energy bond.
Step 6 of Glycolysis
The energy of 1,3-BPG is trapped to synthesise one ATP molecule with the help of a kinase. This
is an example of substrate level phosphorylation, where energy is trapped directly from the.
