In a previous lecture I described
the way in which glucose is broken down.
But glucose is not the only sugar
that we ourselves need to deal with.
In this lecture I will talk about
the metabolism of other sugars.
I will discuss the process of gluconeogenesis,
that's the way cells make glucose that they can use.
And then last I will talk about the phenomenon of reciprocal
regulation, the way cells control sugar metabolism.
Now cells have a lot of different sugars
that they get besides glucose. So it's
important that we understand
something about what they do.
In this pathway we see that metabolism
of a sugar known as galactose.
Now we get galactose in our diet fairly
regularly if we eat dairy products because
galactose comes from lactose
which is known as milk sugar.
So our cells have to be
able to metabolize galactose.
To metabolize galactose, the metabolism
is actually shown on the screen.
I am gonna step you through it and it looks a little
complicated but it is actually fairly simple.
The reactions of galactose
basically involve its conversion
into an intermediate in glycolysis.
Now this starts with an enzyme that has a mouthful
of a name: galactose-1-phosphate uridyl transferase.
Hopefully, the reaction is simpler
then the name of the enzyme is.
The reaction catalyzed by this enzyme takes
galactose-1-phosphate on the upper left
and combines it with
UDP glucose on the lower left.
And what it’s doing is essentially
swapping the galactose for the glucose.
So we see when this process happens that we start
with galactose-1-phosphate and we end up with UDP galactose.
UDP glucose releases a glucose-1-phosphate.
So because of that we now
have glucose-1-phosphate that is free.
A glucose-1-phosphate is relatively easy to
get into glycolysis because there is an easy
enzymatic conversion of glucose-1-phosphate to
glucose-6-phosphate and we get into glycolysis.
This means we have converted part of the molecule
shown on the screen into glycolytic intermediates.
The galactose in the UDP, however, has
to also be converted into intermediate.
And that happens in the next reaction which
is catalyzed by UDP-galactose-4-epimerase.
Again a mouthful of a name, but
the reaction is quite simple.
The galactose at the end of the
UDP is converted into a glucose.
That reaction means that we have now got into a
point where we have made our starting material.
UDP-glucose on the right.
UDP-glucose on the left.
The UDP-glucose on the left now continues
the process metabolizing more lactose.
So in this way all galactose can ultimately
be converted into glycolytic intermediates.
Now other sugars get metabolized, I have
mentioned lactose. Lactose is milk sugar
and it's metabolized in a very simple way.
There is an enzyme called
lactase and lactase breaks
lactose into its constituent
sugar which are galactose and glucose.
You have seen how galactose is metabolized.
Now lactase is an important
enzyme; because, in some adults
the reduced quantities of this enzyme leads to
a phenomenon known as lactose intolerance.
And lactose intolerance, if you ever had to deal
with, that is pretty discomforting; because,
it causes the bacteria in
the stomach to be able to produce
a lot of gas because they are working
on lactose not galactose and glucose.