Stage 1: Glucose Breakdown
Glycolysis involves nine distinct reactions that convert
glucose into
pyruvate. In this section, we will cover the first four of these reactions,
which convert glucose into glyceraldehyde-3-phosphate. Glucose is a six-
memebered ring molecule found in the blood and is usually a result of the
breakdown of carbohydrates into sugars. It enters cells through specific
transporter proteins that move it from outside the cell into the cell's
cytosol. All of the
glycolytic enzymes are found in the cytosol.
Step 1: Hexokinase
In the first step of glycolysis, the glucose ring is phosphorylated.
Phosphorylation is the process of adding a phosphate
group to a
molecule derived from ATP. As a
result, at this point in glycolysis, 1 molecule of ATP has been consumed.
Figure 1.1: Step 1.
The reaction occurs with the help of the enzyme hexokinase, an enzyme that
catalyzes the phosphorylation of many six-membered glucose-like ring structures.
A kinase is the name given to an enzyme that phosphorylates other
molecules. Atomic magnesium (Mg) is also involved to help shield the negative
charges from the phosphate groups on the ATP molecule. The result of this
phosphorylation is a molecule called glucose-6-phosphate (G6P), thusly called
because the 6' carbon of the glucose acquires the phosphate group.
Step 2: Phosphoglucose Isomerase
The second step of glycolysis involves the conversion of glucose-6-phosphate to
fructose-6-phosphate (F6P). This reaction occurs with the help of the enzyme
phosphoglucose isomerase (PI). As the name of the enzyme suggests, this
reaction involves an isomerization reaction.
Figure 1.2: Step 2.
The reaction involves the rearrangement of the carbon-oxygen bond to transform
the six-membered ring into a five-membered ring. To rearrangement takes place
when the six-membered ring opens and then closes in such a way that the first
carbon becomes now external to the ring.
Step 3: Phosphofructokinase
In the third step of glycolysis, fructose-6-phosphate is converted to fructose-
1,6-bisphosphate (FBP). Similar to the reaction that occurs in step
1 of glycolysis, a second molecule
of ATP provides the phosphate group that is added on to the F6P molecule.
Figure 1.3: Step 3.
The enzyme that catalyzes this reaction is
phosphofructokinase (PFK). As in
step 1, a magnesium atom is involved to help shield negative charges.
Step 4: Aldolase
The final step of the first stage of glycolysis utilizes the enzyme
aldolase, which catalyzes the cleavage of FBP to yield two 3-carbon
molecules. One of these molecules is called glyceraldehyde-3-phosphate (GAP)
and the other is called dihydroxyacetone phosphate (DHAP).
Figure 1.4: Step 4.
GAP is the only molecule that continues in the glycolytic pathway. As a result,
all of the DHAP molecules produced are further acted on by the enzyme
triphoshpate isomerase (TIM), which reorganizes the DHAP into GAP so it
can continue in glycolysis. At this point in the glycolytic pathway, we have
two 3-carbon molecules, but have not yet fully converted glucose into pyruvate.
