How Glycolysis Utilizes Glucose To Make Pyruvate
Glycolysis occurs in the cytosol of individual cells and breaks down glucose to pyruvate
for storage in the body. The overall process can be separated into two phases, the
Preparatory Phase and the Pay-Off Phase. During the Preparatory Phase, energy is
invested and consumed in the form of adenosine triphosphate (ATP). Likewise, glucose is
phosphorylated and converted to Glyceraldehyde 3-phosphate (G3P). During the PayOff Phase, G3P is converted into an energy carrier called pyruvate. High-energy carriers
like nicotamide adenine dinucleotide (NADH) and ATP are also produced.
Phase 1 of Glycolysis - The Preparatory Phase
Phosphorylation of Glucose
The first step of glycolysis is adding a phosphate group to glucose in a process known as
phosphorylation. A family of enzymes called hexokinases is responsible to catalyzing this
reaction to form glucose 6-phosphate (G6P) from glucose. The first priming event takes
place during this reaction. During this priming reaction, low glucose concentrations are
maintained and a high-energy molecule known as ATP is consumed. Likewise, the
reaction uses charged ions to block glucose from leaking or diffusing freely from the cell.
The change in free energy (ΔG) of this step is -16.7 kj/mol. The negative value indicates
that the reaction is spontaneous in the forward direction.
Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate
Glucose 6-phosphate (G6P) is then converted into Fructose 6-phosphate (F6P) by a group
of enzymes called glucose phosphate isomerases. Specifically, an enzyme called
phosphohexose isomerase is required for this reaction. This enzyme catalyzes an
isomerization reaction, which changes the molecular structure of G6P. A simple sugar
such as Fructose can insert itself into the glycolytic pathway at this step of the overall
glycolysis process. This step of glycolysis is reversible under normal cellular conditions.
At low concentrations of F6P, the reaction moves in the forward direction to produce
F6P. At higher concentrations of F6P, the reaction runs in reverse to produce G6P. The
change in free energy (ΔG) of this step is 1.7 kj/mol. The positive value indicates that the
reaction is nonspontaneous in the forward direction.
Phosphorylation of Fructose 6-Phosphate to Fructose 1,6-Biphosphate
Phosphofructokinase 1 (PFK-1) is an enzyme that catalyzes the phosphorylation of
Fructose 6-phosphate (F6P) to Fructose 1,6-Biphosphate. The second priming event takes
place during this reaction. During this step of glycolysis, energy is consumed once again
in the form of ATP and the pathway becomes irreversible. This essential reaction acts as
a regulatory point and a rate-limiting step in the breakdown of glucose. The
phosphorylation event that occurs during this reaction allows formation of two negatively
charged phosphate groups on Fructose 1,6-Biphosphate. Creating dual charges on
Fructose 1,6-Biphosphate prevents diffusion of intermediates out of the cell. The change
in free energy (ΔG) of this step is -14.2 kj/mol so the forward reaction is spontaneous.
Cleavage of Fructose 1,6-Biphosphate
Upon cleavage at carbon 3 of Fructose 1,6-Biphosphate, a split occurs in the hexose ring.
An enzyme called aldolase catalyzes this cleavage reaction to produce two 3-carbon
molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate (G3P). The
change in free energy (ΔG) of this step is 23.8 kj/mol so the forward reaction is
nonspontaneous.
Interconversion of the Triose Phosphates
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Triose phosphate isomerase is an enzyme that catalyzes rapid interconversion of
dihydroxyacetone phosphate to glyceraldehyde 3-phosphate (G3P). In doing so,
regulation is simplified and two molecules of G3P are now present. These two molecules
of G3P are ready to be inserted into the Pay-off Phase of glycolysis. The change in free
energy (ΔG) of this step is 7.5 kj/mol so the forward reaction is nonspontaneous.
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Phase 2 of Glycolysis - The Pay-off Phase
Oxidation of Glyceraldehyde 3-Phosphate to 1,3-Bisphosphoglycerate
Oxidation and Phosphorylation both occur during this step of glycolysis. An enzyme
called Glyceraldehyde 3-Phosphate (G3P) dehydrogenase removes two molecules of
hydrogen from each of the two G3P to form two 1,3-Biphosphoglycerates. Two
phosphate groups (Pi) and two NAD+ groups are consumed during this reaction. Two
molecules of NADH are produced during this reaction. The change in free energy (ΔG)
of this step is 6.3 kj/mol so the forward reaction is nonspontaneous.
Phosphoryl Transfer from 1,3-Biphophoglycerate to ADP
During this reaction, an enzyme named phosphoglycerate kinase catalyzes the production
3-phosphoglycerate. Enzymatic transfer of a phosphate group occurs during this step of
glycolysis. Each of the two 1-3-biphosphoglycerate loses a phosphate group. This
phosphate group then interacts with an energy-poor molecule known as adenosine
diphosphate (ADP). The interaction produces adenosine triphosphate (ATP), which is an
energy-rich molecule. The first substrate level phosphorylation occurs during this
reaction. Substrate level phosphorylation is the activity that produces the energy-rich
ATP. When ATP is readily available to cells, this reaction does not occur. Therefore, key
parts of regulation occur during this step of glycolysis. The change in free energy (ΔG) of
this step is -18.9 kj/mol so the forward reaction is spontaneous.
Conversion of 3-Phophoglycerate to 2-Phosphoglycerate
An enzyme called phosphoglycerate mutase catalyzes the formation of 2Phosphoglycerate from 3-Phophoglycerate. During this mutation reaction, a phosphate
group is shifted one carbon to the 2nd position from the 3rd position. The change in free
energy (ΔG) of this step is 4.4 kj/mol so the forward reaction is nonspontaneous.
Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate
An enol condensation takes place during this step of glycolysis. An enol condensation is
a dehydration reaction where a water molecule is removed from the compound. An
enzyme called enolase catalyzes the formation of two phosphoenolpyruvate from two 2Phosphoglycerate. As a result, a carbon-carbon double bond is formed between the 2nd
and 3rd carbon of phosphoenolpyruvate. The change in free energy (ΔG) of this step is 1.8
kj/mol so the forward reaction is nonspontaneous.
Transfer of Phosphoryl Group from Phosphoenolpyruvate to ADP
During the final step of glycolysis, the second substrate level phosphorylation occurs.
Substrate level phosphorylation uses ADP and a phosphate group to produce energy-rich
ATP once again. An enzyme called pyruvate kinase catalyzes the formation of 2 pyruvate
molecules from 2 phosphoenolpyruvate. Regulation occurs at this step, and the final
pyruvate product can be metabolized to produce oxygen. The change in free energy (ΔG)
of this step is -31.7 kj/mol so the forward reaction is spontaneous.
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Conclusion
Glycolysis is a 10-step process that utilizes the breakdown of glucose to produce
pyruvate. Pyruvate can then be broken down to produce oxygen for cells. During this
process, two high-energy ATP are invested and four ATP are produced. Likewise, a
single glucose is broken down into two pyruvate molecules. While steps 1,3,7, and 10
have negative changes in free energy (ΔG), the other steps have positive changes in free
energy. Therefore, only 4 of the reactions occur spontaneously.
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