Another consideration for the
control of metabolic pathways
is covalent modification of enzymes and I am
going to give a couple of examples here;
because, they each illustrate important principles.
The first is this reaction that
you can see on the screen.
Trypsinogen is an inactive form
of a proteolytic enzyme that we have in
our digestive system called trypsin.
Now our digestive system has some
enzymes that are designed to do some
really nasty things to
the food that we eat.
Nasty in the sense that
they just rip them apart.
These proteases take proteins, that are in
our diet, and break them into smaller pieces.
And they work very very efficiently and they
are very very powerful in doing what they do.
The problem is if they are not
controlled properly, they can attack
the very tissues that make them.
And that's a problem. So for that reason when we
have very powerful enzymes that could act chaos
within a cell. Many times those proteins are made in an
inactive form and that's the zymogen form that I referred to.
Well, how is it that these proteins become active?
One of the ways it happens for
the proteases that we can see here
is by action of another protease
that has to break a bond
to convert the zymogen, which is the
inactive form, into the active form.
Now as you might imagine that
enteropeptidase that's seen here
is located some place away from the
place where the trypsinogen is made.
The trypsinogen is made in our pancreas
as are many proteolytic enzymes.
Now if trypsin were active at the pancreas
where the trypsinogen is synthesized,
then trypsin being a protease could
attack the proteins in the pancreas.
Now, that actually does happen sometimes.
There is a sickness called pancreatitis
that is very painful and happens
when the zymogens that are made in
the pancreas get activated too soon.
When that happens they attack the pancreas
and some severe problems can result
as a result of that including fertility. So,
managing this zymogen activity is very important.
Well, this is only one of
several different zymogens
that are made in the pancreas and we can see some
others and we can also see some hierarchy here.
Trypsin becomes activated by enteropeptidase
and when it's active, it starts
activating other proteases.
One of those proteases is chymotrypsinogen
which become chymotrypsin upon activation.
Trypsin can also activate another protease called
proelastase into the active form called elastase.
Now these activations are typically happening
inside of the digestive system where
those proteases are really
aim to work in the first place.
A third zymogen that trypsin can
activate is procarboxypeptidase
and again this is involved in breaking
down proteins in the digestive system.
And last trypsin can activate an
enzyme that breaks down fat as well
and that enzyme that breaks
down fat is called lipase.
So managing where that lipase
is active is important
just like managing where the
proteases are active as well.
Now I show you this scheme; because, there is a very
important principle and that's known as cascading affects.
Cascading affects happen in a
circumstance like what you see here
where an enzyme on the left activates an
enzyme in the middle, in this case, trypsin.
Well, enzymes are very rapid in their function.
So the enzyme in the left isn't activating just
one trypsin. Let's imagine it's activating 10,000.
Well, each of those 10,000 trypsins was able to go and activate
another 10,000 of the individual molecules on the other side.
The result of cascading affects like this is the enormous
amplification of the signal very very quickly.
So, by this doing cascading
scheme that you see here
what happens is, a few enteropeptidases can activate
a lot of proteases and lipases ultimately.
Now, I mentioned chymotrypsinogen and I
wanna bring up its activation as well;
because, it puts an additional ring to
the process that I haven't talk about.
Chymotrypsinogen is synthesized obviously
in an inactive form. That's what the
"ogen" on the end of the molecule's name says.
Trypsin activates it by cutting a bond between
amino acids 15 and 16. So you can see going
on from the top line to the second line
that there is a gap between 15 and 16.
You also notice that the piece
between 1 and 15 doesn't go
flying away but it's held
in place by that S-S bond.
Now in a different presentation, I talked about the importance
of disulphide bonds in terms of maintaining protein structure
and here we can see a very graphic example
of the value of a disulphide bond.
Now chymotrypsin, in fact, the form of it's
shown on the second line is called π-chymotrypsin.
π-chymotrypsin is only partly active.
It only works on a very select substrate and
the select substrate it works on, is itself.
The catalytic action of π-chymotrypsin
is to make additional cuts
in the chymotrypsin,
as you see here, such that a fully active
chymotrypsin is synthesized
known as the α-chymotrypsin.
The additional cuts include cutting
off a two amino acid piece
at amino acid number
13 and cutting out off
three amino acid segment between
amino acids 146 and 149.
Now the result of those actions is to actually
open up the active site of the enzyme.
Prior to that final cleavage, the active
site of the enzyme is not fully open
but after that happens it is fully open and substrates
can then get in and the enzyme can work on them.
So in the very top form the enzyme is completely
closed, or as I described earlier, sealed
for your protection in
a previous presentation
and in the bottom line the
enzyme is open for business.