GLYCOLYSIS
Manoj Sigdel

GLYCOLYSIS
It is defined as a sequence of reactions converting
glucose to pyruvate or lactate, with the
production of ATP.
Greek: glykys = sweet; lysis = splitting

SALIENT FEATURES OF GLYCOLYSIS
• takes place in all cells of the body
• the enzymes of this pathway are present in cytosol of
cells
• in absence of oxygen -anaerobic glycolysis takes place ,
lactate is the end product.
• in presence of oxygen -aerobic glycolysis, pyruvate is the
end product.
•it is also known as Embden-Meyerhof (e.m.) pathway. (
Gustav Embden; Otto Meyerhof; - elucided the
whole pathway in muscle. )

• glycolysis is the major pathway for ATP synthesis in
tissues lacking mitochondria.
• glycolysis is very essential for brain.
• the intermediates of glycolysis is used in formation
of non-essential amino acids and glycerol.

• The sequence of reactions of glycolysis can be
divided into three distinct phases
1) Energy investment phase or priming stage
(reactions 1,2 and 3)
2) Splitting phase (reaction 4)
3) Energy generation phase

Hexokinase
Phosphofructokinase
glucose Glycolysis
ATP
ADP
glucose-6-phosphate
Phosphoglucose Isomerase
fructose-6-phosphate
ATP
ADP
fructose-1,6-bisphosphate
Aldolase
glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate
Triosephosphate
Isomerase
Glycolysis continued
Mg++
Mg++
Mg++
H O
OH
H
OH
H
OH
CH2OH
H
OH
H H O
OH
H
OH
H
OH
CH2OPO3
2
H
OH
H
2
3
4
5
6
1 1
6
5
4
3 2
ATP ADP
Mg2+
glucose glucose-6-phosphate
Hexokinase
H O
OH
H
OH
H
OH
CH2OPO3
2
H
OH
H
1
6
5
4
3 2
CH2OPO3
2
OH
CH2OH
H
OH H
H HO
O
6
5
4 3
2
1
glucose-6-phosphate fructose-6-phosphate
Phosphoglucose Isomerase

Glyceraldehyde-3-phosphate
Dehydrogenase
Phosphoglycerate Kinase
Enolase
Pyruvate Kinase
glyceraldehyde-3-phosphate
NAD+
+ Pi
NADH + H+
1,3-bisphosphoglycerate
ADP
ATP
3-phosphoglycerate
Phosphoglycerate Mutase
2-phosphoglycerate
H2O
phosphoenolpyruvate
ADP
ATP
pyruvate
Mg++
Mg++
Mg++

Phosphofructo kinase reaction is the rate-limiting step of
glycolysis. This is an irreversible and regulatory step in
glycolysis. PFK is an allosteric enzyme, the activity of
which is controlled by several allosteric molecules.
Phosphoglycerate kinase step is the good example of
substrate level of phosphorylation since ATP is
synthesized without ETC. It is reversible, a rare example
of kinase reactions
Enolase is inhibited by Fluoride

ENERGY PRODUCTION AND UTILIZATION
 2 ATP invested
 4 ATP produced (2 from each of two 3C fragments
from glucose)
 Net production of 2 ~P bonds of ATP per glucose.
Glycolysis - total pathway,
glucose + 2 NAD+ + 2 ADP + 2 Pi 
2 pyruvate + 2 NADH + 2 ATP
In aerobic organisms:
 pyruvate produced in Glycolysis is oxidized to CO2 via
Krebs Cycle
 NADH produced in Glycolysis & Krebs Cycle is
reoxidized via the respiratory chain, with production
of much additional ATP.

Glycolysis,
glucose + 2 NAD+ + 2 ADP + 2 Pi 
2 pyruvate + 2 NADH + 2 ATP
Fermentation, from glucose to lactate:
glucose + 2 ADP + 2 Pi  2 lactate + 2 ATP
Anaerobic catabolism of glucose yields only 2 “high
energy” bonds of ATP.

C
C
CH3
O
O
O
C
HC
CH3
O
OH
O
NADH + H+
NAD+
Lactate Dehydrogenase
pyruvate lactate
E.g., Lactate Dehydrogenase catalyzes reduction of the
keto in pyruvate to a hydroxyl, yielding lactate, as
NADH is oxidized to NAD+.

REGULATION OF GLYCOLYSIS
Glycolysis pathway is regulated by control of 3
enzymes :
1) Hexokinase
2) Phosphofructokinase
3) Pyruvate Kinase.

1) Hexokinase is inhibited by product glucose-6-
phosphate:
 by competition
 by allosteric interaction
 Has low KM (0.1mM)
 Glucokinase ( a variant of Hexokinase) is found in
liver. Glucokinase has a high KM (10mM) for
glucose.
It is active only at high [glucose].

2) Phosphofructokinase is usually the rate-limiting step of
the glycolysis pathway.
phosphofructokinase is allosterically inhibited by ATP,
citrate, H+
it is allosteric activated by fructose 2,6 bisphosphate,
AMP, Pi
3) Pyruvate Kinase, the last step Glycolysis
Inhibited by ATP
Activated by F1,6-BP

Feeder Pathways for Glycolysis
• Many carbohydrates besides glucose meet their
catabolic fate in glycolysis, after being
transformed into one of the glycolytic
intermediates.
• The most significant are the storage
polysaccharides glycogen and starch; the
disaccharides maltose, lactose, trehalose, and
sucrose; and the monosaccharides fructose,
mannose, and galactose

Fig: Feeder pathways of glycolysis

Glycogen and Starch Are Degraded by
Phosphorolysis
• Glycogen in animal tissues and in microorganisms
(and starch in plants) can be mobilized for use
within the same cell by a phosphorolytic reaction
catalyzed by glycogen phosphorylase (starch
phosphorylase in plants).

Fructose
• D-Fructose, present in free form in many fruits
and formed by hydrolysis of sucrose in the
small intestine of vertebrates, is
phosphorylated by hexokinase
• This is a major pathway of fructose entry into
glycolysis in the muscles and kidney.

• In the liver, however, fructose enters by a
different pathway. The liver enzyme
fructokinase catalyzes the phosphorylation of
fructose at C-1 rather than C-6
• The fructose 1-phosphate is then cleaved to
glyceraldehyde and dihydroxyacetone
phosphate by fructose 1-phosphate aldolase

Galactose
• The conversion proceeds through a sugar-
nucleotide derivative, UDPgalactose, which is
formed when galactose 1-phosphate
displaces glucose 1-phosphate from UDP-
glucose.
• UDP-galactose is then converted by UDP-
glucose 4-epimerase to UDP-glucose, in a
reaction that involves oxidation of C-4 (pink)
by NAD, then reduction of C-4 by NADH; the
result is inversion of the configuration at C-4.
• The UDPglucose is recycled through another
round of the same reaction. The net effect of
this cycle is the conversion of galactose 1-
phosphate to glucose 1-phosphate; there is
no net production or consumption of UDP-
galactose or UDP-glucose.
Fig: Conversion of galactose to
glucose 1-phosphate.

Pasteur effect
• The inhibition of glycolysis by oxygen (aerobic condition)
is known as Pasteur effect.
• Discovered by Louis Pasteur while studying fermentation
in yeast.
• He observed that when anaerobic yeast cultures were
exposed to air, the utilization of glucose decreased by 7 fold.
• The levels of glycolytic intermediates from fructose 1,6
bisphosphate onward decrease while the earlier
intermediates accumulate
• This is due to inhibition of Phosphofructokinase
• The inhibitory effect of citrate and ATP on
phosphofructokinase explains the Pasteur effect

Crabtree effect
• The phenomenon of inhibition of oxygen consumption
by the addition of glucose to tissues having high
aerobic glycolysis is known as Crabtree effect.
• It is due to increased competition of glycolysis for
inorganic phosphate (Pi) and NAD+ which limits their
availability for phosphorylation and oxidation

Thank You