#15 Biochemistry Enzyme Regulation II Lecture for Kevin Ahern's BB 450/550


Uploaded by oharow on 31.10.2011

Transcript:
Kevin Ahern: Everybody had a good weekend?
Student: You ruined it.
Kevin Ahern: I ruined it?
Student: Yes.
Kevin Ahern: How did I ruin your weekend?
Student: You gave us our tests back on Friday!
Kevin Ahern: That's supposed to be good news!
Well, okay, I'm sorry.
You didn't have to pick it up, okay?
Nobody said, "Go pick up your exam."
Student: Is the curve shown now or at the end of the course?
Kevin Ahern: The curve is right here on the web page.
If you look right there, there he be.
I showed everybody that.
So it's on the web page.
You can see it.
I have a fair amount to get through today.
I probably won't make it all the way through,
but that's fine, too.
I want to continue our discussion that I started
last time about mechanisms of controlling enzymes.
If you recall, I finished just about the place
where I was talking about a model of control.
When I talked about hemoglobin,
I talked about how binding the first oxygen
affected the binding of the second,
affected the binding of the third,
affected binding of the fourth.
I said that this was a sequential model,
that is, that each one influenced the one after it.
That's one way of explaining how it is that
cooperativity that we see in hemoglobin exists.
When we talk about enzymes,
of course, we don't talk about cooperativity
so much as we talk about control,
and there are other models that are used to explain that,
and I want to explain a couple of those models to you.
The first of these models is called the "concerted model."
The concerted model is a little odd,
at least to understand, well, let me back up.
It's a little odd to understand,
to begin with, because it has a different setup
than the sequential model.
The sequential model looks like what you see on the screen.
The sequential model said that we started out with a protein,
maybe it was hemoglobin or maybe it was an enzyme,
and that protein was in one state.
We can think of this as being in the T state.
And binding of the molecule to one subunit
caused it to affect the other subunit.
So you see how this guy has bound a substrate
and you can see how these other subunits next
to it have changed from being squared off
to being rounded off.
That is, their structure has been affected
by the binding of the first one to the very first subunit.
We can think of this as a cause and effect.
This causes these to change.
This causes these to change.
This causes this to change, et cetera, et cetera.
So we see a cause and effect.
Binding of one causes the next subunit to change,
causes the next subunit to change, et cetera, et cetera.
So that sequential model is sort of an intuitive
one that we think about,
but there are other ways of explaining
cooperativity in hemoglobin, and also control in enzymes.
The second one that I want to talk about
is called the "concerted model."
The concerted model,
I don't have a good one to show you here, visually,
unfortunately, but I'll have to describe it to you.
The concerted model says...
let's go back and look here.
The concerted model says there's not a cause and effect.
There's not a cause and effect.
The enzyme can exist in two states.
It can exist in a T state and it can exist in an R state,
and the flipping from T to R has nothing
to do with the binding of the substrate.
You think, "Well, how does it do it, then?"
It has nothing to do with the binding of the substrate.
It has nothing to do with the binding
of the allosteric effector.
What the concerted model says is the flipping
is an inherent activity of the enzyme.
The enzyme is capable of flipping from R to T,
completely independent of another molecule.
The sequential model says it was a cause and effect.
The concerted model says it's flipping independent of that.
Well then how do we see the phenomenon exhibit itself?
What the concerted model says is that
after the flipping occurs,
then something combined,
and that something that binds locks it
into one state versus the other state.
So if have an enzyme that flips into the R state,
and if this enzyme is ATCase,
and this enzyme binds to ATP,
it locks it in the R state.
Therefore, that enzyme will remain
in the R state and be very active.
If the enzyme lets go of ATP
and it flips back to the T state,
all on its own, and it binds to CTP,
it locks it in the T state,
and it stays locked in that state until it lets go of CTP.
So these are fundamentally different ways
of explaining R state versus T state.
In the sequential model, the R-to-T flip occurs
as a result of binding.
In the concerted model,
the flipping is independent of binding,
but binding locks it into one or the other, okay?
Now, I know that's always a little conceptually
difficult to get your heads around,
so I'll stop and take a brief question for that
Student: Is one or the other more accurate?
Or is it a hybrid of the two?
Kevin Ahern: Is one or the other more accurate,
or is it a hybrid of the two?
It varies from one protein to another,
in terms of which one it tends to be more like.
So there are some that will be very much more
on the concerted side,
others will be very much more on the sequential side,
and all I'm expecting you to do
is not memorize which one is which,
but to just understand what
Student: Is there something that causes
it to unlock then, later?
Or is it time driven, or...?
Kevin Ahern: When you say "unlocked,"
do you mean to let go of the molecule?
So is there something that causes it to let go
of CTP or something that causes it
to let go of ATP or aspartate?
The answer is, like every other binding
we've talked about that's not covalent,
these are associative things.
They come on, they go off.
They come on, they go off.
So we always see reversible binding of these intermediates.
If they're covalently bound then,
of course, we don't have any way of getting them off again.
Yes, sir?
Student: Would the state change be spontaneous of itself,
or would it result from a reaction
of something other than the intended substrate,
like the [unintelligible]?
Kevin Ahern: Good question.
So his question is, is the change R to T completely
independent of everything else?
And the answer is, yes, it is.
Yes, it is.
And we will see preferences for one versus the other.
I told you that when we started studying ATCase,
we didn't even really know the R state existed
until we locked it with PALA.
That's because the preference was something
like a couple hundred to one of T over R
when there's nothing there,
a couple hundred to one,
when there's nothing else there.
But if we lock one of them in the R state,
then that equilibrium starts moving
in the direction of the R state.
More and more start flipping into the R.
Yes, sir?
Student: Does this rate of T-to-R transition switch,
is there any trend to where something
that switches states spontaneously, very quickly,
favors the concerted model?
Kevin Ahern: I'm not sure I understand the question.
Student: If an enzyme spontaneously changes
states very rapidly...
relative to other enzymes,
would that be better described by the concerted model
since there would be more opportunity
for [unintelligible]?
Kevin Ahern: So his question has to do with how rapidly
the T-to-R state occurs within an enzyme,
and does this tend to favor one model versus the other.
The answer is, if it's independently changing,
it already is in the concerted model.
So if the changing is independent
of a substrate binding or an allosteric effector binding,
then it automatically is a concerted model, okay?
Okay, so I think that you should, as I say,
understand the generalities of those models
and be able to explain them to me in words.
That would be useful.
I want to turn our attention now from talking
about models to talking about some interesting enzymes.
This first enzyme I'm going to talk about
is one you're going to hear a lot about.
Yeah? Question?
Student: So that graph was not so important?
That you kind of,
for that concerted model graph?
Kevin Ahern: You're talking about here?
Student: Yeah.
Kevin Ahern: Yeah, so all this graph is showing is, okay,
so here is the ratio of T to R of 70.
This is when ATP's present.
Here's the ratio of T to R that's 200
when there's nothing present.
And here's the ratio of 1250 to 1 when there's CTP present.
This is consistent with what I told you before.
That is, ATP favors the R state more,
so we see less of a ratio.
"L" is the ratio of T to R, okay?
Nothing is 200, and when we have the inhibitor,
in this case, CTPóit's 1250 to 1, okay?
Now, as I said, I'm going to talk a little bit
about an enzyme you're going to hear
a lot more about this term.
We're going to see that this enzyme
plays a very, very important role
in a phenomenon we refer to as "signaling."
Signaling is a way for cells to talk to each other.
For one cell to talk to another,
it must release something like a hormone.
That hormone goes to that other cell
and binds to that other cell,
and causes changes to happen inside the target cell.
One of the enzymes that plays a role in those changes
in the target cell is protein kinase A.
You're going to hear a lot about protein kinase A,
so it's important that we understand,
first of all, how it works.
Protein kinase A is allosterically affected
by a molecule called cyclic AMP,
or, as we will refer to it in this class, cAMP.
You see it right here.
The structure of cAMP is shown here.
No, you don't need to know the structure.
It's related to AMP, AMP being adenosine monophosphate.
The cyclic part is this little ring out here.
So AMP doesn't have a ring.
It just has a phosphate hanging out there,
but when we make it cyclic,
we actually rejoin it to this and make a circular structure.
Cyclic AMP turns out to be very,
very important for this communication process
that cells go through, very important.
The reason it's very important is because cyclic AMP
affects protein kinase A.
As I said, it is an allosteric effector of the enzyme,
and the way it allosterically affects the enzyme
is different than we've talked about
other allosteric effectors.
So far, with allosteric effectors,
we've talked about the T to R
or the R to T conversion.
We've seen shape changes that occurred
as a result of binding of allosteric effectors.
Cyclic AMP is an allosteric effector,
but it's not doing T to R changes.
It's doing something like shown here.
It turns out protein kinase A has multiple subunits,
like you've seen before in other enzymes.
However, this guy has catalytic and regulatory subunits,
like you saw with ATCase,
but, in this case, the binding of cyclic AMP
causes something different to happen.
The something different that happens
is binding of cyclic AMP causes the catalytic subunit
to let go of the regulatory subunits.
This is, literally, an on/off switch.
Remember I said that with T and R
we don't really have on/off switches?
I said we turn the volume down or we turn the volume up.
This is an on/off switch.
When the regulatory subunits are bound
to the catalytic subunits,
as we see on the left over here,
this guy is dead in the water.
It's not catalyzing anything.
I'll tell you what it catalyzes in a second.
When cyclic AMP is present,
it binds to the regulatory subunits,
releases them from the catalytic,
and these catalytic subunits are active as all get out.
So cyclic AMP is literally turning the enzyme
from the off position to the on position.
How do we turn it back off?
Well, if we start degrading cyclic AMP in the cell,
when this guy comes off, it'll get degraded.
Remember, this is a reversible binding again.
These are coming on, coming off,
coming on, coming off.
As we start degrading cyclic AMP in the cell,
these guys will move back to this state
and they will grab a hold of the catalytic subunits
and turn them off.
Now, these guys work by literally blocking
the active site of protein kinase A.
They block the active site.
They physically provide a barrier.
The active site can't get at what it normally catalyzes
and, as a result, is dead in the water.
It doesn't do anything.
What does protein kinase A catalyze?
Well, first of all,
I need to tell you a term that you should absolutely know,
and that's the term "kinase."
First of all, whenever you hear the name
of a molecule or the name of a protein
in this class ending in "-ase"
it is always an enzyme, always.
So the fact that "kinase" ends in "-ase"
tells us that it's an enzyme.
Kinase is an enzyme that puts phosphates onto molecules.
It puts phosphates onto molecules.
So when you hear the term "phosphate,"
it's putting a phosphate onto something.
The name "protein kinase" tells us that this enzyme
puts phosphates onto other proteins.
So protein kinase A,
that just happens to be the very first one,
protein kinase A catalyzes the addition of phosphates
to target proteins.
Now, on these target proteins,
the effects can be enormous, as we shall see.
But for the moment,
all you need to know is that protein kinase A,
when it's active, is putting phosphates
onto target proteins, okay?
Everybody got that?
So kinase puts a phosphate onto target proteins.
There have to be enzymes that take phosphates off,
and I'll talk about those in just a few minutes.
But a kinase puts it on.
A protein kinase puts it onto other proteins,
and, as we will see in this signaling process
that I have described to you,
addition of a phosphate to a protein
can make a protein be much more active or much less active.
So now we start to see another level of control of enzymes.
Allosteric control was one,
and we've seen a couple of examples
in the form of ATCase and also protein kinase A.
Protein kinase A covalently modifies targets,
those targets being other proteins,
and that covalent modification
is a regulatory mechanism, as well.
Covalent modification in the cell occurs in many ways.
Putting a phosphate on or taking a phosphate off
is one covalent modification.
There are other covalent modifications,
and you've already seen one example of those
when I talked about the proteases.
Chymotrypsin is covalently modifying its target
because it's cutting it in half.
It's breaking a covalent bond.
These are different modifications
that can occur inside of a cell...
phosphorylation, acetylation, meaning putting
on an acetyl group, myristoylation, putting on myristic acid,
blah, blah, blah.
I'm not giving you this table
to give you something to memorize.
But I do want you to be aware
that there's many different kinds of modification
to proteins that can occur.
Those modifications affect the proteins in significant ways,
because frequently they will change
the charge of the protein.
If we put a phosphate onto one section of a protein
where there wasn't a charge before,
now, all of a sudden, it might be attracted to another part
of the protein that's positively charged,
it might be repelling a part that's negatively charged.
So we can see then, it's a mechanism of changing protein shape,
and changing protein shape changes protein function.
Now, let's talk about protein kinase.
As I said, protein kinase or protein kinases,
specifically, protein kinase A,
takes a phosphate and it puts it onto a target protein.
Where does it put it on?
Primarily it puts it onto the amino acids
that contain hydroxyl groups.
These include serine, threonine and tyrosine.
When we look at the enzymes that catalyze these,
we see that they can be grouped
into different categories.
Some protein kinases strongly prefer
or absolutely prefer to put them onto serine or threonine.
Protein kinase A is one of those enzymes.
It strongly prefers to put a phosphate
onto either serine or threonine side chains.
Other enzymes, other protein kinases,
prefer to put it onto tyrosine.
We'll see when we talk about signaling
that the tyrosine kinases are really interesting.
They're intimately involved
in controlling cells' decision to divide or not divide.
This plays a role in cancer and in many other processes.
Kevin Ahern: I'm sorry?
Student: Protein kinase A prefers serine and threonine?
Kevin Ahern: Protein kinase A works on serineand threonine.
So that's the reaction being catalyzed.
We see that ATP is the source of the phosphate.
Here's our target hydroxyl group.
Here's the phosphate that's put onto it.
That leaves us with ADP as a byproduct.
Putting phosphates on is one thing.
Taking phosphates off is another.
Student: Is ATP always the source of that phosphate?
Kevin Ahern: Is ATP always the source of the phosphate?
Never say "always," but mostly I would say "yes."
Removing phosphates is a phenomenon
known as dephosphorylation.
It's not the reversal of a phosphorylation.
Instead, separate enzymes are used,
and the enzymes that remove phosphates off
of molecules or proteins are known as phosphatases.
We'll talk a lot about this enzyme
called protein phosphatase because it reverses
the action of protein kinases.
It catalyzes the hydrolysis,
that is, the use of water,
to split off a phosphate off of a target.
So here is the phosphorylated protein
that we had from the protein kinase.
If we treat that protein with protein phosphatase,
we see that we get phosphate
and the protein back in its original state.
This is important, because, as I've said before,
cells are control freaks.
If they have a way of turning something on,
they darned sure better have a way of turning it off.
Just like you better have a way to turn off your lights,
or your electric bill is going to go crazy,
so, too, cells want to turn off a process
or they may burn up all their resources.
So if they have the ability to put the phosphate on,
they darned sure better have the ability
to take the phosphate off, and that's what we see in the form
of protein phosphatase.
Understanding signals that cells are getting
requires us to understand which of these enzymes
is active at any given time, and that's a fascinating process.
Okay, I'll stop, slow down and take questions.
Clear as mud?
Am I going slow enough for you?
Student: [unintelligible]
Kevin Ahern: What's that?
Student: [unintelligible]
Kevin Ahern: Clear as mud.
Oh, you like that saying.
Well, that's good, alright.
So, phosphorylation is important.
Dephosphorylation is important.
Let's now think of some other covalent modifications,
some other covalent modifications.
I've already alluded to the fact
that breaking peptide bonds is a covalent modification,
and this covalent modification turns out
to have some really cool implications for human health,
very, very cool implications.
They're involved in digestive processes
and controlling the enzymes of digestive processes.
They're involved in blood clotting.
I'll be talking about both of these.
The first one I'll talk about is digestive processes.
If we think about it for a moment,
we sort of take digestion for granted
until we get an upset stomach.
We get an upset stomach
and then we become very aware of digestion.
But in the absence of that, we eat our turkey,
and we go to bed, and we fall asleep and that's that.
We wake up in the morning and we haven't given a thought
to digesting something, right?
But what's been happening after we eat
is that we have digestive enzymes that are breaking down
those things that we've been stuffing
down our gullet the whole day.
With Thanksgiving coming up,
I have turkey on the mind, you can see.
Now, it's important that the body control
those digestive enzymes.
I don't want those digestive enzymes digesting me.
And those digestive enzymes are really,
really good at digesting things.
When we look at the digestive enzymes,
they're made in a variety of places,
but one of the more common places
they're made is by our pancreas.
Our pancreas makes digestive enzymes.
These digestive enzymes come in a variety of categories,
but we're going to focus for the moment
on ones that break down proteins.
These are proteases.
We eat meat.
We break the protein in meat down to amino acids
so we can use those amino acids to make our own proteins.
I don't want my digestive enzymes to digest my pancreas.
I'm fond of my pancreas.
So my pancreas is not stupid.
It doesn't make active enzymes.
It makes inactive enzymes called "zymogens."
In other words, it starts out making these enzymes
that will ultimately become active,
but it starts out making them in an inactive form
so it can get it away from itself
without digesting itself in the process of being made.
So these inactive enzymes are known as zymogens.
We can see them being made by the pancreas,
and they're actually secreted outside of the cell
so they can go into our digestive tract,
where they get activated and start breaking things down.
You might say, "Well, don't they sort of digest
"the digestive tract itself?"
And the answer is, to a limited extent, they do.
Some of the cells that have the most rapid turnover
in our body are those found in our digestive tract.
When you hear of ulcers and people getting ulcers,
what's happening is they're not replacing
those cells as rapidly as they're being degraded.
You have damage that's happening as a result of that.
Student: Aren't those primarily attributed
now to Helicobacter pylori?
Kevin Ahern: His question has to do with ulcers
relating to Helicobacter pylori,
and the answer is, yes,
but that plays a role, also, again, in this turnover of cells.
The turnover of cells is a very important
consideration in the digestive tract.
We used to say it was stress that caused that phenomenon.
Now we now there are other factors that play a role in there.
What happens if we release our digestive enzymes
from our pancreas and that activation process,
which I'll show you about in a second,
starts backing up the tubes and getting up to the pancreas?
Will that cause a problem?
Oh, yeah, oh, yeah.
You create a condition that's very painful,
in some cases fatal, called pancreatitis.
What's happening is those zymogens are getting
activated in the wrong place,
at the wrong time,
and the pancreas is getting chewed up.
It's treatable, but it's a very painful condition.
Has anyone in here ever had pancreatitis?
We usually have one or two that have.
Nobody that will raise their hand.
It can be life threatening, in some cases.
It's a very painful condition.
It is treatable, but it has to be treated fairly early.
Well, how do these things get activated?
Here's one example.
It shows how our body activates the enzyme
you learned about last week known as chymotrypsin.
Chymotrypsin is made in an inactive form,
and the inactive form is known as chymotrypsinogen.
Whenever we see the suffix "-ogen" on the end of it,
we know we have a zymogen,
and a zymogen is inactive.
Unfortunately, that "-ogen" is not consistent.
We see that that was used
after they named some other enzymes,
and I'll show you those in a second.
But for the moment, we have chymotrypsinogen.
Here's how it's made.
It's made as a 245-amino acid piece and it's inactive.
It doesn't do anything.
It goes off into the digestive system
where it encounters proteases
that start to chew it up,
and that starting to chew it up actually activates it.
The first thing that happens is an enzyme
known as trypsin makes a single cut
between amino acids 15 and 16,
and no, you don't need to memorize these numbers
or anything like that.
This creates something called "pi-chymotrypsin"
that is partly active,
meaning that pi-chymotrypsin will cut
other pi-chymotrypsins.
They're sort of a self-digesting set of enzymes.
Pi-chymotrypsin cutting pi-chymotrypsins
causes other bonds to be broken.
We see that the bond between position 13 and 14 got broken.
So now we lose a couple of amino acids there.
We see that the bond between 146 and 147,
and also between 148 and 149 got broken,
and now chymotrypsin's in three pieces.
Why don't these three pieces go flying away?
Any thoughts?
Student: Are there disulfide bonds present?
Kevin Ahern: There are disulfide bonds.
Absolutely, so this enzyme has to fold properly.
The disulfide bonds have to form, and then the cleavage occurs,
because if that doesn't happen,
then these pieces all go flying away
and we have no active enzyme, at all.
It's prime evidence for the process of folding.
It's prime evidence for the importance of disulfide bonds.
Now, after we've got this pi-chymotrypsin
making these various cleavages, this guy down here
is what we call chymotrypsin.
It's completely active, and now we can go out
and start breaking down those things
that you've been eating over Thanksgiving.
What's happening in this process is
we're looking at the inactive form in red.
We're looking at the active form in blue.
For the most part, the structure doesn't change much,
but we look at what happens as a result of this cleavage.
Look where this amino acid number 16 is, up here,
and look how far it gets moved after that cleavage has occurred.
Why is that important?
Well, what this is doing is it's opening up access
to the active site.
The movement of this amino acid this distance
allows substrate to now come
into the active site and be cleaved.
Shape change is essential for protein function.
This shape change happens as a result of proteolytic cleavage.
Student: So those three pieces do stay linked
because of their disulfide bonds?
Kevin Ahern: The three pieces stay linked
because of their disulfide bonds.
That's correct, so that's pretty cool.
How am I doing on time?
We're doing very well.
Now, I've got to tell you a cool story.
Actually, before I tell you the story,
let me show you also the bigger process.
I was showing you the activation of one of these guys.
It turns out that a bunch of the proteolytic enzymes
in our body, as well as other enzymes in our body
in the digestive system,
are all synthesized in the inactive form.
Trypsin itself is synthesized as trypsinogen.
Here's chymotrypsinogen that you saw, here.
Here's procarboxypeptidase.
That's another enzyme that breaks down proteins.
Here's proelastase.
Generally, when you see a "pro -" on the front,
that's the equivalent of an "-ogen" on the end.
It means it got named before they started using
"-ogen" to name them.
Proelastase gets converted to elastase,
and I'm going to talk more about that one in just a minute.
There's a master switch on this scheme,
and that master switch comes from an enzyme known as
enteropeptidase, and, no, I'm not going to tell you
how enteropeptidase gets activated.
We could imagine we could follow this scheme
back quite a ways, and we can.
The control of this system is important.
If we lose control of this system,
we end up making things like pancreatitis.
We have enzymes active where we don't want them to be active
and that's not a good thing.
Notice that we have, over here, prolipase going to lipase.
A lipase is something that breaks down fat.
We're also controlling enzymes that break down fat.
So it's important, in this big picture
that we've been talking about in terms of regulation,
that cells be able to control enzyme activity.
If they don't control enzyme activity,
then they've got real problems, whether it's an enzyme
that breaks down proteins, or it's an enzyme that breaks down
fat, or, as we'll see later in the term,
enzymes that break down glycogen and glucose.
Being able to control these is a very important thing,
and the different ways in which cells do
that are important for us to understand.
Yes, sir?
Student: [unintelligible]
Kevin Ahern: Oh, yeah.
That crazy, damn thing.
There we go.
Okay, questions about the scheme?
Yes, back here.
Student: [unintelligible]
Kevin Ahern: So, as you can see,
trypsin is a master controller of all of these down here.
That's correct.
Over here, you.
Student: So, in this, would...
is the trypsin more important [unintelligible]
that we want to worry about, or the enteropeptidase?
Kevin Ahern: Well, I don't want to say "more."
I mean, this obviously controls four of these guys,
and we'll see trypsin plays roles with other enzymes,
as well, and enteropeptidase controls that.
So there's just a hierarchy of these
is really all that there is.
Did you have a question?
Student: Yeah, do these other molecules
follow a similar scheme,
where they start off as a single long [unintelligible]
chain which is disulfide cross-linked,
and then selectively cleave to be activated?
Kevin Ahern: His question is, do the other enzymes
have this phenomenon of getting chopped into pieces,
hanging onto pieces, like with disulfide bonds,
and then becoming activated?
The answer is, there's a wide variety of schemes that are used.
In some cases, loss of the piece is important.
In some cases, simply structural change is important.
So there's no one answer to that question.
You had a question back here?
Student: With the arrow going back from trypsin
back to itself...
Student: ...right there.
How does that work?
Does trypsin also regulate itself?
Kevin Ahern: Good eyes.
You have good eyes.
As the figure implies, trypsin actually does play a role
in helping to activate itself, and it's a complicated process.
So trypsin can, once you've made some active trypsin,
trypsin can then act on trypsinogen, kind of like we saw
with chymotrypsinogen becoming partly activated.
So, yes, that can happen.
So that's cool.
Now I said I was going to tell you something else about elastase,
and this is a really cool story.
So let me tell you this story.
Student: Someone had a question.
Kevin Ahern: Question?
Student: So the trypsin [unintelligible]
it seems like if trypsin can activate itself
[unintelligible].
Kevin Ahern: Yeah, so, as I said, it's a complicated process.
Trypsin can activate itself, to some extent.
A lot of this is going to be controlled by when
and where that activation occurs.
In the digestive system, you want to be able to activate
as much as you can very quickly.
So if you don't activate any of it until it gets to the
digestive systemóthat is, you don't have any of the master one,
the, I can't remember the name of it,
but the master enzyme,
if you don't have any of that present where
trypsin is being made, you're okay.
But, as you could imagine,
you could see things kind of go haywire.
Maybe things do back up a little bit.
Maybe you're activating trypsin
where you don't want it to be activated,
and that's what's going to be a mechanism
for making pancreatitis.
Connie?
Student: On the [unintelligible] of the protein
that you just showed, do they all just digest proteins except
for lipase which digests [unintelligible]?
Kevin Ahern: Let me make sure I understand
and answer your question properly.
So, if I talk about this, all the enzymes here are proteins.
All the enzymes from here over to here
digest protein down into other proteins,
and this one simply breaks down lipids, in this case, fats.
But all this, including this, is a protein.
Yeah?
Student: Could these be used to regulate themselves?
Say you have excess lipase or something,
but you don't want to shut down all [unintelligible]
inhibition further up on their own chain that would maybe, say,
get rid of the prolipase or something so that you didn't
[unintelligible] the other ones?
Kevin Ahern: I'm not sure I understand your question.
Are you saying if you'd want to be able to activate
only this one and not these guys?
Is that what you?
Student: Or something like that.
Or maybe you just want only [unintelligible].
Kevin Ahern: Yeah.
It's, as you could imagine, a fairly complex system,
and in the context of our digestive system,
we basically want all these guys to be active.
We want them there breaking stuff down.
So it would be unusual that we would have a situation
where we'd want to make this one be active and not these,
and as you can see, it would be hard to do that
because we've got one master enzyme that's catalyzing it.
Does that make sense?
Student: Yeah.
Kevin Ahern: Okay.
Glad to see you guys are thinking about this stuff.
I want to tell you my story.
This story is a very cool one.
This story has to do with what you see on the screen here,
and it's something called "trypsin inhibitor."
More specifically, it's called "alpha 1-antitrypsin."
What is alpha 1-antitrypsin?
Alpha 1-antitrypsin is a protein that acts kind of like that
regulatory protein of protein kinase A.
It will sit on trypsin, in the active site,
and prevent trypsin from breaking other proteins down.
Alpha 1-antitrypsin is another way of controlling trypsin.
So your question back here about if I have trypsin active
and I don't want it to be active, do I have a problem?
Yes, you could have a problem.
But if you have alpha 1-antitrypsin present,
the body has another way of controlling trypsin
once it's been activated.
Once it's been activated, you have alpha 1-antitrypsin
that can bind to it and you're set.
Well, it turns out that alpha 1-antitrypsin is misnamed.
It was named because the first enzyme
that people discovered that it worked on was trypsin.
But it turns out it works on some of the other proteases,
as well, and the one that it works on really well is elastase.
Some people say, in fact, that it's better named
as anti-elastase instead of alpha 1-antitrypsin,
because it's really good at knocking down the activity
of elastase, and this is where the story gets interesting.
Elastase is an enzyme that breaks down proteins,
and it's present not only in our digestive systems,
but it's also present in our lungs.
Why do we have a proteolytic enzyme present in our lungs?
Well, think about what our lungs are exposed to all the time.
We're continually breathing in crap from that air.
That crap from the air includes bacteria and viruses
and all kinds of things.
Our lungs are an interface
between the rest of the world and our bodies.
Having some sort of a protection
against those things coming in is very good.
Our lungs have elastase in them to break those guys down.
They also have in the lungs alpha 1-antitrypsin.
Now why do they have both?
Well, it turns out that,
just like we worried about
having too much trypsin be active,
so, too, do we want to control
how much elastase we have active.
Because if we have too much elastase active,
the elastase starts doing something we don't want it to do,
which is attack our lung tissue, and that creates emphysema.
So having the proper balance of alpha 1-antitrypsin
in our lungs that's functional is very, very important.
If we don't have enough alpha 1-antitrypsin in our lungs,
we will develop emphysema.
If we have too much,
we may be more susceptible to infections.
Balance is good.
Balance is important.
What disturbs that balance?
Well, it turns out smoking disturbs that balance.
Smoking oxidizes a very critical methionine side chain
in alpha 1-antitrypsin.
When it oxidizes that side chain,
we create the side chain that looks like this.
This is what we started with, over here.
Remember how I said alpha 1-antitrypsin fits
into the active site and blocks it?
This guy doesn't fit in the active site anymore.
The more you smoke,
the more you damage your alpha 1-antitrypsin,
the more elastase starts doing the thing
that it's supposed to,
which is attack proteins, but those proteins are unfortunately
in your lung tissue, and you're much more likely to develop
emphysema as a result of smoking... very nasty thing.
Many of you who know me know how anti-smoking I am,
and with good reason.
This stuff is designed to kill you.
Yes, sir?
Student: So is protein that elastase attacks,
I'm assuming elastin or something along those lines,
is that present in large capacity in, like, the alveoli?
And that's what leads to
the alveoli collapse [unintelligible]?
Kevin Ahern: Yeah.
His question is, does elastase preferentially attack
a protein in the alveoli,
and I don't know the answer to that question.
I don't think it's preferential.
No, I think it's a byproduct of that.
But it is a concern and consideration.
It is the alveoli that are affected by emphysema,
but I don't know if it's really targeted that one.
Are you guys ready for a song?
Student: Oh, yeah.
Kevin Ahern: I just happen to have one.
It's actually relevant to this, and it's a song...
I'm not going to sing it.
I've got somebody who's recorded it for me.
It's on YouTube and it's called "I Lost a Lung."
I've got to set it up here... and here we go.
If you want to sing along, you can sing.
[David Simmons performing "I Lost a Lung"
(to the tune of "I Lost My Heart in San Francisco")]
Lyrics: You've gone and left me breathless,
so especially today an absence from my body
makes it hard to say,
I'm so horribly upset and reminded evermore.
Kevin Ahern: I'm not done yet, in case you guys are interested.
Lyrics: I'll not forget, how you took my breath away.
I lost a lung from smoking Camels.
Emphysema kills, it seems to me.
You see those nicotines and tars, leave alveolar scars.
My raspy throat will often choke from the smoke.
I've no respect for RJ Reynolds and its cor-por-a-toc-rac-y.
Now when I hear the name
RJ Reynolds,
I only think malignancy.
Kevin Ahern: So that's my little sermon of the day, I suppose.
I've got about five minutes and so I want to finish up
talking or at least introducing one last topic
that I'll finish up next time,
and that's another important process
that's controlled with zymogens.
Now this process turns out to be absolutely critical for us
to function, because it controls our blood clotting.
Again, our blood clotting is kind of like our digestion.
We don't think about it, but by golly,
where are we without it?
Let's think of the challenge that our body has.
Our body wants to deliver blood to our tissues,
and it wants to repair itself if there's damage.
So if I poke a hole in one of my arteries or one of my veins,
I want to be able to stop that flow from happening.
Not only do I want that flow to stop from happening,
I want the repair to be done in minutes,
because if I don't I will lose all of my blood.
So I need to have a system that is extraordinarily rapid.
The way that I make it be rapid is I load my bloodstream
with zymogens that are capable of forming that clot.
We can think of our bloodstream as kind of like a loaded gun.
A loaded gun, because it's ready to clot
as soon as the sense of damage occurs.
Can we see problems happening with that?
You betcha.
If we clot when we don't want it to clot,
we may be blocking blood flow to a critical tissue,
like our heart or our brain, and we have a stroke.
If we don't make that clot,
let's say that we are a hemophiliac,
we will bleed ourselves to death.
So, again, balance is important.
We have to be able to stop that blood flow
and stop it in minutes, and,
this is the most amazing thing to me, it's got to be water tight!
We have to assemble this block from molecular pieces.
They have to be self-assembling
and it has to be tight enough to hold water.
I'll stop with that and that's where we'll pick it up next time,
but I want you to think about that.
Yes?
Student: [unintelligible]
Kevin Ahern: Can [unintelligible] activate zymogens?
Generally, no.
Captioning provided by Disability Access Services
at Oregon State University.
[END]