In this section, we will look at the reactions that convert our two 3-carbon
molecules of glyceraldehyde-3-phosphate (GAP) into pyruvate, the product of
glycolysis. This conversion occurs in five steps that we will review below.
At this point, we will also see where oxygen comes into play in glycolysis so
that in the next section, we can look at the differences between aerobic and
anaerobic glycolysis. Keep in mind in this section that since we have split
our 6-carbon molecule into two 3-carbon molecules, each of these reactions is
occurring in both of the 3-carbon molecules.
Step 5: Glyceraldehyde-3-phosphate Dehydrogenase
In this step, two main events take place: 1) glyceraldehyde-3-phosphate is
oxidized by the coenzyme nicotinamide adenine dinucleotide
(NAD); 2)
the molecule is phosphorylated by the addition of a free phosphate group.
The enzyme that catalyzes this reaction is glyceraldehyde-3-phosphate
dehydrogenase (GAPDH).
Figure %: Step 5.
The chemistry that takes place in this reaction is more complex than that of the
previous reactions we've discussed. Knowledge of organic
chemistry is needed to
understand the specific mechanisms
of the conversion. Generally, the enzyme GAPDH contains appropriate structures
and holds the molecule in a conformation such that it allows the NAD molecule to
pull a hydrogen off the GAP, converting the NAD to NADH. The phosphate group
then attacks the GAP molecule and releases it from the enzyme to yield 1,3
bisphoglycerate, NADH, and a hydrogen atom. We will come back to the role of
this NAD/NADH molecule in the next section.
Step 6: Phosphoglycerate Kinase
In this step, 1,3 bisphoglycerate is converted to 3-phosphoglycerate by the
enzyme phosphoglycerate kinase (PGK). This reaction involves the loss of a
phosphate group from the starting material. The phosphate is transferred to a
molecule of ADP that yields our first molecule of ATP. Since we actually have
two molecules of 1,3 bisphoglycerate (because there were two 3-carbon products
from stage 1 of glycolysis), we actually synthesize two molecules of ATP
at this step. With this synthesis of ATP, we have cancelled the first two
molecules of ATP that we used, leaving us with a net of 0 ATP molecules up to
this stage of glycolysis.
Figure %: Step 6.
Again, we see that an atom of magnesium is involved to shield the negative
charges on the phosphate groups of the ATP molecule.
Step 7: Phosphoglycerate Mutase
This step involves a simple rearrangement of the position of the phosphate group
on the 3 phosphoglycerate molecule, making it 2 phosphoglycerate. The molecule
responsible for catalyzing this reaction is called phosphoglycerate mutase
(PGM). A mutase is an enzyme that catalyzes the transfer of a functional
group from one position on a molecule to another.
