>>Welcome to our next segment of the chemistry of beer. The topic for our discussion today
is glycolysis. Glycolysis is a very interesting pathway, and it should be relevant for us
to help us understand how yeast process glucose, but it also applies to us. It's an ancient
pathway, and it is common to both yeast and humans. To begin our discussion of glycolysis,
we'll first of all think about the big picture, and we'll answer the question: what does glycolysis
do for an organism? Next we'll consider sources of glucose, and in particular we'll be thinking
about yeast. Then we'll move on to a discussion of the overall pathway, and then finally we'll
tie up some loose ends and summarize what we've discussed.
Before we dig into the details of glycolysis, it really helps to take a step back and think
about the big picture. To begin let's define what glycolysis means. It literally means
"sweet loosening" or "splitting." As I noted, it's a very ancient pathway, and it's present
in nearly all organisms. The net reaction, which you should know, is as follows. We start
with one glucose, two NAD+ (that's our old friend again), two ADPs (two adenosine diphosphates),
and two Pis. Pi stands for inorganic phosphate. At the end of the pathway we form two molecules
of pyruvate, plus two molecules of NADH, plus two molecules of ATP. As I've mentioned before,
ATP is sometimes called the energy currency of the cell. So this pathway produces ATP,
which the cell can use to power its many functions.
What are the sources of glucose? As you learned in an earlier part of this course, some of
the initial steps of the brewing process yield glucose and maltose. If we look at our diagram,
we see that the glucose can enter the cell through a glucose transporter. And we see
that the maltose can enter the cell along with a proton. That is called symport. So
there is a maltose-hydrogen ion or maltose-proton symporter that brings in maltose and the hydrogen
ion. The maltose can then undergo a reaction catalyzed by the enzyme maltase and form two
glucose molecules.
So now we are ready to discuss glycolysis. Shown in the diagram is the entire glycolytic
pathway, as it is sometimes referred to. If we look at the upper left-hand corner of our
diagram, we see that we start with glucose. Now the first reaction converts glucose to
glucose-6-phosphate. This reaction is catalyzed by the enzyme hexokinase. Kinase is a general
term for an enzyme that catalyzes what we call ATP-dependent phosphorylations. So you
see above the reaction arrow ATP is being converted to ADP plus a proton, and a phosphoryl
group has been transferred from ATP to glucose to make glucose-6-phosphate. One of the things
that this reaction does is it helps keep the glucose inside the cell because in its phosphorylated
form, it cannot pass through the glucose transporter.
The next reaction is an isomerization reaction, and it converts the glucose-6-phosphate to
fructose-6-phosphate. The third reaction is another kinase reaction. The name of the enzyme
is phosphofructokinase, and it catalyzes the ATP-dependent phosphorylation of fructose-6-phosphate
to fructose 1,6-bisphosphate. So now we have phosphoryl groups attached to the number 1
carbon and the number 6 carbon. What we've essentially done is made a more symmetrical
molecule, and that will make sense when we consider the next reaction.
The next reaction is the aldolase reaction. Aldolase catalyzes the cleavage of fructose
1,6-bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. These two
molecules themselves are interconverted by another enzyme called triose phosphate isomerase,
and it is the glyceraldehyde 3-phosphate that goes on through the rest of glycolysis. So
let me back up and repeat because it's important that you are clear on this. Fructose 1,6 bisphosphate
is cleaved, and that happens by the enzymatic action of aldolase, and we make two 3-carbon
molecules. One of them, glyceraldehyde 3-phosphate, can continue on in glycolysis. The other molecule,
dihydroxyacetone phosphate, can be converted to glyceraldehyde 3-phosphate by the enzyme
triose phosphate isomerase. So effectively we have two glyceraldehyde 3-phosphates that
continue on in the rest of the pathway, and so that's why I'm showing the 2X is to indicate
that everything I'm showing from this point on happens twice.
The next enzyme in this pathway is glyceraldehyde 3-phosphate dehydrogenase. It catalyzes the
conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate. As this occurs, we
form NADH. So please keep that in mind. You've been seeing the participation of NAD+ and
NADH several times, and this will really make sense when we finish our discussion of the
metabolism of of carbohydrates. So please note that we form an NADH at this point.
Now the 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, so it's giving up one
of its phosphoryl groups. The enzyme that catalyzes that reaction is phosphoglycerate
kinase, and in this reaction we have formed ATP. Because this happens twice, we actually
form two ATPs. Now the product of this reaction, 3-phosphoglycerate, can undergo a mutase reaction
catalyzed by the enzyme phospoglycerate mutase, and we form 2-phosphoglycerate. 2-phosphoglycerate
can undergo a dehydration reaction, which is catalyzed by the enzyme enolase, to form
the product phosphoenolpyruvate. Phosphoenolpyruvate can then undergo a reaction with the enzyme
pyruvate kinase, and form the molecule pyruvate. In the process, we have formed another ATP
and because this happens twice, we have formed two ATPs.
So let's take a step back and think about this pathway. We started off by spending an
ATP in the first reaction, the hexokinase reaction. Then we had to spend another ATP
during the third reaction, the phosphofructokinase reaction. So we had to prime the pump. We
had had to invest some cellular energy currency, ATP, to get the pathway going. But because
everything happens twice from the formation of glyceraldehyde 3-phosphate on, we form
one, two, three, four ATPs. So we have formed a net of two ATPs. We can consider what we've
been talking about with the aid of another figure, and that figure is shown here. Now
you have access to this figure, and so we will let you work through this figure on your
own.
So now we're ready to tie together some loose ends and to summarize our discussion. We've
seen that the net reaction of glycolysis is: We start with one glucose, two NAD+, plus
two ADP, and we form two pyruvate, plus two NADH, plus two ATP, plus two protons. We also
saw that the first step of glycolysis converts glucose into glucose-6-phosphate, a negatively
charged molecule that is "trapped" in the cell. We learned that two ATPs are required
to "prime the pump" and start glycolysis. These steps occur during the first reaction,
the hexokinase reaction, and the third reaction, the phosphofructokinase reaction. Finally,
we saw that four ATPs are formed, and thus a net of two ATPs, along with two NADHs and
two pyruvates. We will consider the fates of these molecules shortly.