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Glycolysis Reactions
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Introduction to Glycolysis:
The overall reaction of glycolysis which occurs in the cytoplasm
is
represented simply as:
C6H12O6 + 2 NAD+
+ 2 ADP + 2 P -----> 2 pyruvic acid, (CH3(C=O)COOH
+ 2 ATP + 2 NADH + 2 H+
At this time, concentrate on the fact that glucose with six
carbons is converted into two pyruvic acid molecules with three
carbons each. Only a net "visible" 2 ATP are produced
from glycolysis. The 2 NADH will be considered separately later.
The major steps of glycolysis are outlined in the graphhic
on the left. There are a variety of starting points for glycolysis;
although, the most usual ones start with glucose or glycogen
to produce glucose-6-phosphate. The starting points for other
monosaccharides, galactose and fructose, are also shown.
Glycolysis - with
white background for printing
Overview
of Metabolism
Link to: Great Animation of entire Glycolysis
- John Kyrk
Link to: Interactive
Glycolysis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.
Link
to Glycolysis Aninmation 1
Link
to Glycolysis Aninmation 2
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Reaction 1: Phosphate Ester Synthesis
Phosphate is added to the glucose at the C-6 position. The
reaction is a phosphate ester synthesis using the alcohol on
the glucose and a phosphate from ATP.
This first reaction is endothermic and thus requires energy
from a coupled reaction with ATP. ATP is used by being
hydrolyzed to ADP and phosphate giving off energy and the phosphate
for reaction with the glucose for a net loss of ATP in the overall
glycolysis pathway.
Hydrolysis: ATP + H2O --> ADP + P + energy
P = PO4-3; ATP = adenine triphosphate;ADP
= adenine diphosphate
This reaction is catalyzed by hexokinase.
Off-site chime link: Boyer Tutorial - Hexokinase
Reaction 1 - Chime
in new window
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Reaction 2: Isomerization
The glucose-6-phosphate is changed into an isomer, fructose-6-phosphate.
This means that the number of atoms is unchanged, but their positions
have changed.
This works because the ring forms may open to the chain form,
and then the aldehyde group on glucose is transformed to the
keone group on fructose. The ring then closes to form the fructose-6-phosphate.
This reaction is catalyzed by phosphoglucoisomerase.
Off-site chime link: Phosphoglucoisomerase
Reaction 2 - Chime
in new window
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Reaction 3: Phosphate ester synthesis
This reaction is virtually identical to reaction 1 The fructosee-6-phosphate
has an alcohol group on C-1 that is reacted with phosphate from
ATP to make the phosphate ester on C-1.
Again this reaction is endothermic and thus requires energy
from a coupled reaction with ATP. ATP is used by being
hydrolyzed to ADP and phosphate giving off energy and the phosphate
for reaction with the glucose for a net loss of ATP in the overall
glycolysis pathway.
Hydrolysis: ATP + H2O --> ADP + P + energy
This reaction is catalyzed by phosphofructokinase.
Off-site chime link: Phosphofructokinase
Link to: Rodney Boyer Animation of Phosphofructokinase
Reaction 3 - Chime
in new window
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Reaction 4: Split Molecule in half
The six carbon fructose diphophate is spit into two three-carbon
compounds, an aldehyde and a ketone.
The slit is made between the C-3 and C-4 of the fructose.
The ring also opens at the anomeric carbon. The product on the
right is the glyceraldehyde.
Technically this is called a reverse aldol condensation.
This reaction is catalyzed by aldolase.
Off-site chime link: Aldolase
Reaction 4 - Dihydroxyacetonephosphate Chime
in new window
Reaction 4 - Glyceraldehyde-3-phosphate Chime
in new window
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Reaction 4A: Isomerization
The dihydroxyacetone phosphate must be converted to glyceraldehyde-3-phosphate
to continue the glycolysis reactions. This reaction is an isomerization
between the keone group and an aldehyde group.
As a result of this reaction, all of the remaining glycolysis
reactions are carried out a second time. The first series of
reactions occurs with the first glyceraldehyde molecule from
the orginal split. Then the second series of reactions occurs
after the isomerization of the dihydroxyacetone into the glyceraldehyde.
This reaction is catalyzed by triose phosphate isomerase.
Off-site chime link: Triose Phosphate Isomerase (TIM)
Reaction 4A - Isomerization Chime
in new window
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Reaction 5: Oxidation/Phosphate Ester Synthesis
This reaction is first an oxidation involving the coenzyme
NAD+. The aldehyde is oxidized to an acid as an intermediate
through the conversion of NAD+ to NADH + H+.
Then an inorganic phosphate is added in a phosphate esteer synthesis.
This and all remaining reactions occur twice for each glucose-6-phosphate
(six carbons), since there are now two molecules of 3-carbons
each.
This reaction is catalyzed by glyceraldehyde-3-phosphate.
Off-site chime link: G3P Dehydrogenase
Reaction 5 - 1,3-diphosphoglycerate Chime
in new window
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Reaction 6: Hydrolysis of Phosphate;
Synthesis of ATP
One of the phosphate groups undergoes hydrolysis to form the
acid and a phosphate ion, giving off energy. This first energy
producing reaction is coupled with the next endothermic reaction
making ATP. The phosphate is transferred directly to an ADP to
make ATP.
This reaction is catalyzed by phosphoglycerokinase.
Off-site chime link: Phosphoglycerate Kinase
Reaction 6 - 3-phosphoglycerate Chime
in new window
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Reaction 7: Isomerization
In this reaction the phosphate group moves from the 3 position
to the 2 position in an isomerization reaction.
This reaction is catalyzed by phosphoglycerate mutase.
Off-site chime link: Phosphoglycerate Mutase
Reaction 7 - 2-phosphoglycerate Chime
in new window
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Reaction 8: Alcohol Dehydration
In this reaction, which is the dehydration of an alcohol,
the -OH on C-3 and the -H on C-2 are removed to make a water
molecule. At the same time a double bond forms between C-2 and
C-3. This change makes the compound somewhat unstable, but energy
for the final step of glycolysis.
This reaction is catalyzed by enolase.
Off-site chime link: Enolase
Reaction 8 - phosphoenol pyruvic acid Chime
in new window
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Reaction 9:Phosphate Ester Hydrolysis;
Synthesis of ATP
This is the final reaction in glycolysis. Again one of the
phosphate groups undergoes hydrolysis to form the acid and a
phosphate ion, giving off energy. This first energy producing
reaction is coupled with the next endothermic reaction making
ATP. The phosphate is transferred directly to an ADP to make
ATP.
This reaction is catalyzed by pyruvic kinase.
Off-site chime link: Pyruvate Kinase
Reaction 9 - pyruvic acid Chime
in new window
Conclusion:
Starting with glucose-6-phosphate with 6 carbons, the final
result of the glycolysis reactions is two molecules of pyruvic
acid, since reaction 5-9 are each carried out twice.
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