Glycolysis is the pathway that breaks down glucose into pyruvate, generating ATP and NADH. It occurs in two phases: the preparatory phase prepares glucose for breakdown, while the payoff phase harvests energy through substrate-level phosphorylation to form ATP and NADH. Glycolysis ultimately yields two molecules of pyruvate, two ATP, and two NADH from each molecule of glucose. Pyruvate can then be further metabolized through aerobic or anaerobic pathways.

1. Glycolysis and
its regulation.
By Deepali
PGD20213384
M.Sc. Biotechnology (research)

2. “
A common pathway for the catabolism of glucose is glycolysis,
which breaks down glucose into pyruvate. Glycolysis is also
called the Embden-Meyerhof-Parnas pathway for its major
discoverers. Whether glucose is fermented or respired, it
travels through this pathway.
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3. Brief about the reaction
◉ The three-carbon, energy-conserving phase begins when the enzyme fructose 1,6-bisphosphate
aldolase catalyzes the cleavage of fructose 1,6-bisphosphate into two halves, each with a
phosphate group. One of the products, dihydroxyacetone phosphate, is immediately converted to
glyceraldehyde 3-phosphate. This yields two molecules of glyceraldhyde 3-phosphate, which are
then converted to pyruvate in a five-step process. Because dihydroxyacetone phosphate can be
easily changed to glyceraldehyde 3-phosphate, both halves of fructose 1,6-bisphosphate are used
in the three-carbon phase. First, glyceraldehyde 3-phosphate is oxidized with NAD+ as the electron
acceptor (to form NADH), and a phosphate (Pi ) is simultaneously incorporated to give a high
energy molecule called 1,3-bisphosphoglycerate. The high-energy phosphate on carbon one is
subsequently donated to ADP to produce ATP.
◉ This synthesis of ATP is called substrate-level phosphorylation because ADP phosphorylation is
coupled with the exergonic breakdown of a high-energy bond.
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4. Glycolysis has two phases
Preparatory phase
The breakdown of the six-carbon glucose into two molecules of the three-carbon pyruvate occurs in 10 steps, the
first 5 of which constitute the preparatory phase.
Step 1- In these reactions, glucose is first phosphorylated at the hydroxyl group on C-6.
Step 2- The D-glucose 6-phosphate thus formed is converted to D-fructose 6-phosphate.
Step 3- D-fructose 6-phosphate is again phosphorylated, this time at C-1, to yield D-fructose 1,6-bisphosphate.
(For both phosphorylations, ATP is the phosphoryl group donor. As all sugar derivatives in glycolysis are the D
isomers).
Step 4- Fructose 1,6-bisphosphate is split to yield two three-carbon molecules, dihydroxyacetone phosphate and
glyceraldehyde 3-phosphate, this is the “lysis” step that gives the pathway its name. The dihydroxyacetone
phosphate is isomerized to a second molecule of glyceraldehyde 3-phosphate, ending the first phase of glycolysis.
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5. Glycolysis has two phases
Payoff phase
The energy gain comes in the payoff phase of glycolysis.
Step 6- Each molecule of glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate
(not by ATP) to form 1,3-bisphosphoglycerate.
Steps 7 to 10- Energy is then released as the two molecules of 1,3-bisphosphoglycerate are converted to two
molecules of pyruvate.
Much of this energy is conserved by the coupled phosphorylation of four molecules of ADP to ATP. The net yield is
two molecules of ATP per molecule of glucose used, because two molecules of ATP were invested in the
preparatory phase. Energy is also conserved in the payoff phase in the formation of two molecules of the electron
carrier NADH per molecule of glucose.
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6. In the sequential reactions of glycolysis,
three types of chemical transformations
are particularly noteworthy:
(1)
Degradation of the
carbon skeleton of
glucose to yield
pyruvate.
(3)
Transfer of a hydride
ion to NAD+, forming
NADH.
(2)
Phosphorylation of
ADP to ATP by
compounds with high
phosphoryl group
transfer potential,
formed during
glycolysis.
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8. Fates of Pyruvate
◉ Under aerobic conditions, glycolysis is only the first stage in
the complete degradation of glucose. Pyruvate is oxidized,
with loss of its carboxyl group as CO2, to yield the acetyl
group of acetyl-coenzyme A; the acetyl group is then
oxidized completely to CO2 by the citric acid cycle.
◉ The second route for pyruvate is its reduction to lactate via
lactic acid fermentation. Lactate is also the product of
glycolysis under anaerobic conditions in some
microorganisms.
◉ The third major route of pyruvate catabolism leads to
ethanol by a process called ethanol (alcohol) fermentation.
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9. Glucose+2NAD++2ADP+Pi= 2Pyruvate+2NADH+2H++2ATP+2H2O
ATP and NADH Formation Coupled to Glycolysis
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During glycolysis some of the energy of the glucose molecule is
conserved in ATP, while much remains in the product, pyruvate. The
overall equation for glycolysis is:

10. Energetics of Glycolysis
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11. Bibliography
◉ Lehninger Principles of Biochemistry by David
L. Nelson and Michael M. Cox (Sixth edition).
◉ Prescott, Harley and Klein’s Microbiology
(Seventh edition).
◉ Brock Biology of Microorganisms by Madigan,
Martinko, Stahl and Clark (Thirteenth edition).
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12. Any questions ?
Thanks!
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