Articles

#25 Biochemistry Glycogen Metabolism I Lecture for Kevin Ahern’s BB 450/550

September 25, 2019


Captioning provided by
Disability Access Services at Oregon State University. Kevin Ahern: Okay,
folks, let’s get started! How was your Thanksgiving break? Student: Good. Kevin Ahern: I bet
everybody was really dying to get back and get
into biochemistry? Student: No. Kevin Ahern: No? Student: I was, a little bit. Kevin Ahern: Look at this way, you go from one
turkey to another. Student: Oh. Ha, ha, ha. Kevin Ahern: The
old lead balloon, that’s as good of a joke as
I have for you today, folks. Believe it or not, we’re
in the home stretch. We have, counting
today, three lectures. Whoo! The end is near. I did not bring note
cards with me today. I said I was going to. I did not bring note cards, so I apologize for
that, number one. I’ve got number two
and number three, I think, here, also. I will bring them on
Wednesday, so on Wednesday you will have note
cards, for sure. If you’re really
desperate and you really have to
have your note card, you can come by my
office and pick one up, but realistically it’s
not going to change things an awful lot if you
go 48 hours different without your note card. But it’s a 5-by-8,
I can tell you. If you want to get a
5-by-8 and practice on it, you can see it’s a pretty
good-sized note card. Student: Wow! Kevin Ahern: But you have to get the note card from
me, remember that. So the note cards have
to be gotten from me, and you don’t have to
use it, but you have to turn in a note card,
with your name on it, that you got from me,
with your final exam. If you don’t, you
will lose points. So make sure that you
get a note card from me and you turn it in
with your final exam. Are we clear? Okay, that’s number one. Number two, I had a big
presentation this morning and I was working my
presentation last night and forgot to send you
guys an email saying the exams are graded! So the exams are graded,
and let me just say a few words about the exam. I was totally
delighted with the exam. The performance on
the exam was one of the highest averages
I’ve ever had. The average was 76.5. I have a curve. I will post it as soon
as class is over today. I will post the curve on
the website so you can see it for the overall sum of
your grades for the first two exams, so you can see
exactly where you stand. The low score on the exam was 15. Student: Ugh. Kevin Ahern: The
high score was 103. I had, I think, two
or three people who had a perfect 103, including
the extra credit questions. So I was very impressed
and I was looking, as I looked at the
grades, themselves, I saw that that average went
up because a lot of people in the low-to-mid
jumped quite a bit, and so it was very,
very satisfying to me. I just felt very,
very good about that and I really respect
what people did with that, so that’s kind of cool. So it was worth
the wait, hopefully, for you to get your exam back. As always, if you have questions, let me know and
we’ll go with that. Number three is we have
a final exam coming up. That final exam is in
here, on Monday, at 9:30. I will do a review
session for it. In fact, I have put in for,
I believe it’s Friday evening at 6:30, I have put in
a request for a room. I will announce that when
I get the room for sure, but the review session
will almost certainly be Friday at 6:30. That gives you a
chance to get dinner and then get review session. As before, I will videotape that. Now, let’s see. What else do I want to say here? I want to say we finished almost everything about gluconeogenesis. The last thing I did not
talk about on Wednesday of last week, which seems like
a long time ago, by the way… Student: I know, it does. Kevin Ahern: The last
thing I did not talk about there I wanted to
save for today because it’s kind of involved and
so I wanted to make sure everyone had the same
opportunity to see it and ask questions
and so forth about it, and it’s the combined regulation of glycolysis and
gluconeogenesis. I’m going to start
out by showing you a complicated figure. Actually, no, I’m not
going to start with that. I’m going to start by telling
you the sort of philosophy of glycolysis and
gluconeogenesis regulation. The philosophy is that glycolysis is a catabolic pathway. Gluconeogenesis is
an anabolic pathway. These pathways,
for the most part, occur in the same place,
which is the cytoplasm. Gluconeogenesis only
has two reactions that aren’t in the cytoplasm. One’s in the
endoplasmic reticulum and one is in the mitochondrion. All the other enzymes of both
pathways are in the cytoplasm. Moreover, many of the
enzymes are the same enzymes in both pathways, which
means that many reactions are driven by concentration, which side of the equation
has necessarily large enough amounts to drive a
reaction one way or the other. That means that we
have to be careful to regulate these pathways. If we don’t control
these pathways, we’re going to have that
futile cycle that I talked about before, where,
imagine, let’s imagine the following scenario. Let’s imagine I had glycolysis and gluconeogenesis
going at the same time. What would happen? I would start with pyruvate. I would put in six
triphosphates to get to glucose. I would burn glucose
and get two triphosphates and be right back at pyruvate. I wouldn’t have gained anything, but I would have lost
four triphosphates. And then I start it again
and I go up, I go down, and each time I turn that
cycle I lose four triphosphates. That’s futile because
it doesn’t give the cell anything but heat. So it’s important the cell
not waste its energies, and the cell doesn’t waste
its energies by controlling pathways like that
in what we call a “reciprocal” fashion. Reciprocal regulation
is something you’re going to hear a lot about today
and you’re going to hear a lot about it on
Wednesday, also. Reciprocal regulation. Well, we start to
see it right here. Here’s a schematic,
going down for glycolysis on the left, going up for
gluconeogenesis on the right. Some of these things
we’ve talked about already. Let’s look at the
regulators of this pathway. What you see on the screen
are the allosteric regulators, the allosteric effectors
of the important enzymes. In glycolysis, we know
there are hexokinase, which is not shown,
PFK, and pyruvate kinase. In gluconeogenesis, there
are these two enzymes. They are this guy and also
glucose-1,6-phosphatase, which is also not shown. So we just sort of throw
out the first one up here. We just throw it out. These guys and these guys,
we’re very interested in. As I said when I talked
about glycolysis earlier, the most important pair
are these two, right here. PFK and FBPase-1, or
fructose 1,6-bisphosphatase, if you want to call it that, PFK and FBPase-1 are
regulated reciprocally. F2,6BP we talked about before. Notice that in very
tiny amounts it turns this enzyme on. In the same tiny amounts,
it turns this enzyme off. It has opposite effects
on the two enzymes. Look at AMP. AMP turns this guy on. AMP indicates low energy. With low energy, we
want glycolysis to go. PFK is activated. We look over here. PFKóI’m sorry, AMP
turns off FBPase-1. Citrate turns off this guy. Citrate turns on this guy. It’s reciprocal. It’s not perfectly reciprocal. There are things
that affect this one that don’t affect this one. But when we look at
the thing as a whole, F2,6BP is a reciprocal regulator. AMP is a reciprocal regulator. It has opposite
effects on catabolic and anabolic enzymes. To a lesser extent, we
see some of that down here. ATP turns this guy off. ADP turns this guy off. But we don’t see the same kind of reciprocal regulation that
we saw with PFK and FBPase-1. Now, reciprocal regulation
turns out to be very, very important when we
have pathways occurring in the same place,
at the same time, or that can occur at the
same place, at the same time. Cells generally
regulate them so that they don’t occur that
way, for the most part. Well, it gets even a bit
more complicated than that, because the question
arises, I told you earlier when I talked about PFK and
I said the most important regulatory effector
for PFK was fructose 2, 6-bisphosphate, right? And I just showed you that
it was a very important regulator for FBPase-1, as well. So, unfortunatelyóand
you’re going to say this as well as I doóunfortunately, it means we need to
understand how do cells make and break down fructose
2,6-bisphosphate. That, you’re going to see,
it’s going to look much more complicated than it is, so
I’m kind of conditioning you for what I’m going to show
you, and I’m also going to tell you that I can throw
a million words at it. It’s kind of like
the mechanisms of serine protease action. We can throw a
million words at it, but until you sit down with
it and look at it yourself, it’s going to seem
like a million words. So let’s take a look at
the overall pathway by which fructose 2,6-bisphosphate
is made and regulated. Remember, this is the
reciprocal regulator of PFK and FBPase-1. This looks complóoh, Jesus,
yeahóthis looks complicated. That’s always the first reaction. It’s not as bad as it seems. There’s a lot of
information on here, and the guts of its right here. All this is showing
us is, on the side we’re breaking it down. On this side over
here, we’re making it. There’s an enzyme that makes it, and there’s an enzyme
that breaks it down. An enzyme that makes it, and an enzyme that
breaks it down. Now, let’s take a
look at this enzyme. This enzyme is one of the
most fascinating enzymes in biochemistry,
because this enzyme is actually two enzymes. The same protein molecule
catalyzes the synthesis and the degradation of
fructose 2,6-bisphosphate. It’s the same protein. This protein has two activities. One activity makes it. It’s called PFK2. PFK2 catalyzes the synthesis of fructose 2,6-bisphosphate. FBPase-2 is the other half
of it, and it breaks it down. Make it, break it down. Here’s the enzyme. Here’s the two activities. Well, as we can see,
at any given time, only one portion of
the enzyme is active. Only one portion of
this enzyme is active. Oo-ooh! Good job! Not my day today. What’s the difference
between these two? The difference is a phosphate. If we put a phosphate
onto this enzyme, we flip the activities. That turns FBPase-2 on. That turns PFK2 off. If we take the phosphate off, we favor the reversal of that. Well, that’s not surprising. You’ve seen before how
covalent modification of enzymes can affect
enzyme activities. We’re simply putting
a phosphate on, we’re taking a phosphate off. It has opposite effects. This causes the PFK2
to become active. Going to the right causes
the FBPase-2 to become active. What catalyzes these things? Well, protein kinase
Aóthere’s our friendóprotein kinase A, when it’s activated,
catalyzes this enzyme getting a phosphate on it
and FBPase-2 being active. Let’s think about what that
means in terms of the cell. If FBPase-2 is active,
not looking at the screen what’s going to happen? We’re going to break
down F2,6BP, right? When we break down F2,6BP, what’s going to be the
effect on FBPase-1 and PFK1? PFK is activated
by this molecule, so if I take the molecule
away, what’s going to happen? Less active, right? If I go to the right, PFK1 is
going to become less active. F2,6BP is an allosteric
inhibitor of FBPase-1. If I take it away, what’s
going to happen to FBPase-1? It’s going to be active, right? Well, since those are the
critical enzymes controlling whether we’re running
glycolysis or gluconeogenesis, now you can look at
this and say, in general, what’s going to
happen to glycolysis and gluconeogenesis
if I phosphorylate this guy, right here. Well, I’m going to go over here. I’m going to break this guy down. I’m going to favor
gluconeogenesis. And look, when glucose is scarce, that’s exactly what
I want to be doing. I want to be making glucose. Remember the flight or fright? Remember the grizzly
bear chasing me and my adrenaline starts
flowing, and I said that we had that kinase cascade, and the kinase cascade
activated protein kinase A? There’s our protein kinase A. And I said that the
result of activation of protein kinase A resulted
in production of glucose. This is one of the ways
in which we make glucose. Not surprisingly, if
we’re making glucose, we don’t want to be
breaking down glucose, so we inhibit glycolysis, because we’re no
longer activating PFK with fructose
2,6-bisphosphate. So in one simple step,
depending on how you look at it, of course, but in
one simple step, we’ve reversed
those two pathways. Well, what happens
now when I’ve got my glucose stores back up? I’ve escaped the grizzly
bear and I’m sitting around and eating pizza. I’ve got plenty
of glucose around, and glucose is a… poison. So I’ve got to deal
with that glucose. I’ve got two things
I can do with glucose. I can break it down. I can turn it into glycogen. We’ll be turning it into
glycogen later in the week. Today, we’re going
to break it down. So when we no longer are
activating protein kinase A, we are no longer phosphorylating. Phosphoprotein
phosphatase becomes active, and, by the way,
phosphoprotein phosphatase is activated by insulin. Insulin is causing this
process to go to the left. Why? Glucose is a poison. We’ve got to do something
with that poison. We’re going to
take phosphates off. We’re going to activate PFK2. We’re going to inhibit fructose
bisphosphatase-2, FBPase-2. What’s going to happen? We’re going to start making
fructose 2,6-bisphosphate, activate PFK1. Glycolysis
is going to run. When fructose
2,6-bisphosphate is present, FBPase-1 is inhibited
and gluconeogenesis stops. Insulin favors going to the left. Epinephrine favors
going to the right. That, in a nutshell
is what’s happening. Now, I want you to
lay this out yourself. I’ll be happy to
answer any questions, but I want you to just
sit down, lay it out, and you’ll discover it’s
really not that complicated. Yes, back there? Student: What about
non-strenuous activity, where you happen to have an
abundance or scarcity of glucose? Kevin Ahern: So what if you
have the in-between situation, basically, is what you’re saying. We have an in-between response. The body will generally
modulate glucose levels to provide glucose, as
needed, as much as possible. So maybe we’ll
phosphorylate, in this case, we’ll burn some of our glucose. We’ll phosphorylate some of
this, but not all of this. Does that make sense? Student: Yeah. Kevin Ahern: Thanksgiving took
all the questions out of you guys. Yes? Student: So the glucose
production, that’s happening in the liver, only, right? Kevin Ahern: Glucose
production, gluconeogenesis, is happening primarily in the
liver and a portion of the kidney. That’s correct. Okay. So look it over. If you have questions, see me, but that’s basically
what’s up with that. That is the last of
what I want to say. Oh, here’s the
enzyme, by the way. There’s the enzyme that’s there. There’s the part that
puts the phosphate on. There’s the part that
puts the phosphate off, and there’s that
tiny little ribbon that connects the two of them. It’s an amazing enzyme,
absolutely amazing enzyme. We turn our attention
now to something that is an easy
metabolic pathway. It’s going to concern us
for the rest of this week. So you say, “Well, it’s
not an easy pathway!” Well, I’m going to
convince you, I hope, that glycogen metabolism
is actually one of the easiest metabolic
pathways to learn. Its regulation is complicated, but the pathway itself
is extraordinarily simple. Let’s talk about glycogen. We talked about it earlier
in the term, and glycogen is a storage form of
glucose that animals use. It’s a storage form of
glucose that animals use. We talked about how plants
use amylose and amylopectin. We combine those and
we get starch, right? But plants don’t have glycogen. What’s the difference between
glycogen and amylopectin? Anybody remember? Student: The linkages between [unintelligible] Student: There’s more branches? Kevin Ahern: There’s more
branches in the glycogen than there is in the amylopectin. So they’re all
polymers of glucose. Amylose has only
alpha-1,4 bonds, so it’s just a long linear chain. Glycogen has alpha-1,4 linkages, but every now and then
it has 1,6 branches. There’s a 1,6 branch. About every ten
residues or so, glycogen has a 1, 6 branch, which
means that glycogen, even though it’s full of
glucose just like amylose is, is structurally very different. It has a lot of ends. The more branching you have, the more free ends we have
at the non-reducing end. You remember what the
non-reducing end is. Is this a reducing sugar
or not a reducing sugar? How many say it’s
a reducing sugar? How many say it’s not? I’m sorry but the
person who said it was a reducing sugar was right. The very first one
has a free aldehyde. The very first one has a free… it’s alpha-1,4 linkages. There’s 1,4. That means if this is
the end of the molecule that would actually
be an OH there and that could become an aldehyde. Student: So the
last one on the right is the reducing sugar? Kevin Ahern: In this
case, it would be, yeah. Now, that’s not important. I’m just throwing
that out at you, just to see what you remembered
after all that turkey. The difference between glycogen and amylopectin,
they’re both branched. Amylopectin also
has 1,6 branches, but it only has them about
every 50 residues or so. I’m going to tell you in a
second why that’s the case, but that’s the structural
difference between amylopectin and glycogen. Did you have a question? Student: Yeah. Isn’t that initial
glucose subunit before all the branching takes place,
on the very internal chain, wouldn’t it be non-reducing
because it’s covalently attached to that
little seed molecule that starts the whole thing off? Kevin Ahern: That is the
seed molecule, right there. So if this is the end, then that’s going to
be an OH, right there. That OH makes it, a free
anomeric carbon on an aldehyde on an aldose will always
make it a reducing sugar. I’ll show you the
structure of that, if you’d like to see it. Come see me. Now, amylopectin’s
chemically different from glycogen in just the
extent of the branching. Why is that important? Well, the reason it’s important, and this is why you’re
able to be an animal, and I’m not talking
about in any sense except for walking around, you people… [laughter] I know where your minds are! How are you able to be an animal? One of the most important
ways in which you can be an animal is
thanks to glycogen. Glycogen is stored
in our muscles. It’s also stored in our liver. It’s in muscles for
very quick energy. It’s in our liver for
providing that buffer to keep our glucose levels balanced,
hopefully, over time. The reason that the
structure of glycogen is so important to being an
animal is because glycogen has so darned many ends. All those branches, all
those ends, are important, because, as you will soon
see, the way that glycogen is broken down is from the ends. More ends, more breakdown, more quick release of glucose. Animals have to run. They have to escape. They have to catch prey. They have to take
notes in biochemistry. All those things
require quick energy. Having a system that
has a lot of ends allows for a lot of glucose to be released very quickly,
when necessary. Plants don’t have those needs. Plants don’t go running
away from their prey. If they could, they might
evolve into something different. But they never made
anything of themselves. They just kind of sit
around like plants, right? “If only we had thought
of making glycogen,” plants say to themselves,
“where would we be now?” But, no. You guys are really quiet today. Student: It’s a Monday. Kevin Ahern: It’s a Monday. Student: Thanksgiving,
we had a four day break. Student: Yeah. Kevin Ahern: So do you see
the fundamental difference? That chemical difference
really plays out as a very important thing. Well, let’s look at the
metabolism of glycogen. Actually, this is
whatóthere you go. Is that the figure
you were referring to? Student: Wasn’t the very,
very internal molecule not a made-of-glucose
molecule, though… Kevin Ahern: It is. It’s a glucose, yeah. Everything in it is a glucose. What’s that? You want to draw it
on the exam, you said? Student: No! Student: What?! Student: You could just
draw a bunch of lines. Kevin Ahern: No,
you’ve got to draw it, we’ll line it up and we’ll
put it on top and see. Nope. No partial credit. Sorry. [laughter] Student: Oh, god. Kevin Ahern: Let’s look at
the breakdown of glycolysis. There’s what glycogen looks like. That’s these little
black guys here. Fates of glycogen. Glycogen turns
out to be important as a source of glucose. But, of course, we know
glucose is not the end of the story because
glucose, by itself, doesn’t do anything
except poison us. We want to have the
energy from glucose, which is why we have glucose
around in the first place, and what this is showing
you is what happens when we break down glycogen and how
it’s converted into energy, the glucose in it. I’m going to show you in a
second an unusual reaction. It’s a really cool reaction. The glycogen isn’t broken
down directly into glucose, for the most part. Ninety-nine percent of it, or, ninety percent of it is
broken down into this guy, right here, glucose 1-phosphate. Where did we see glucose
1-phosphate before? Anybody remember? Student: Glycolysis? Kevin Ahern: Not glycolysis, no. Galactose metabolism. Do you remember when
we had the UDP glucose and it got released,
and it was released as you don’t remember
thatóglucose 1-phosphate. I told you, at the
time, glucose 1-phosphate would be important in
glycogen metabolism because it can readily be converted
into glucose 6-phosphate. This enzyme phosphoglucomutase
allows this interconversion. It can go up, It can go down. It’s pretty much
equal in terms of which direction it goes. Student: Is it “phophoglucomutase”? Kevin Ahern:
Ha-ha-ha-ha! What are they doing
this in this textbook? “Phophoglucomutase.” [laughter] That is now an acceptable
name for this enzyme. If you want to call it
“phosphoglucomutase,” you can. If you want to call
it “phophoglucomutase” [laughing] or “phuphuglucomutase,”
I don’t care. [laughter] Now, glucose 6-phosphate
can go to glycolysis. That’s important. Glucose 6-phosphate can
get released as glucose and go into the bloodstream, if this happens in the liver. Glucose 6-phosphate
can be converted by the pentose phosphate
pathwayówe’ll briefly talk about that next termóinto ribose, and ribose is very important
for making nucleotides. So this molecule
is central to a lot of different pathways. How do we get
glucose 1-phosphate? Let’s take a look at that. Here’s the end of
a glycogen molecule. One of those ends
that we talked about, one of those millions
or thousands of ends that are on the end of a
glycogen, we’re sitting at it right now with an
enzyme that breaks it down. The enzyme that breaks
this down, that catalyzes this reaction, is known as
“glycogen phosphorylase,” P-H-O-S-P-H-O-R-Y-L-A-S-E, unless you’re a
textbook publisher, in which case it’s
called “phophorylase.” [scattered laughter] Now, this is a reaction
like you haven’t seen before. It looks very straightforward. Here, we’ve got a
glycogen molecule. Here, we’ve clipped off
a glucose 1-phosphate, and here’s the glycogen that’s
lost one of its residues. Very straightforward, right? Well, not quite. Look what’s happened. We put a phosphate on
there, in the process. How did we put that
phosphate on there? We didn’t use ATP. When we talked about putting
ATP onto glucose before, we said that took energy, right? Where did the energy come to
put this phosphate on here? Any thoughts? Wild ideas? Yes, sir? Student: It’s
energetically favorable? Kevin Ahern: Why is it
energetically favorable? It is energetically
favorable, but why? Student: Negative
Delta G zero prime? Kevin Ahern: Why is the
Delta G zero prime negative? Student: Is there energy
in breaking that bond? Kevin Ahern: There’s energy
in breaking this bond. This bond has some energy in it. The energy in breaking
this bond is transferred to making glucose 1-phosphate. So it tells us that that
alpha-1,4 bond has some energy in it and that we can use
that energy to make something. Well, why do we want to do that? Well, it turns out,
whenever we can save energy, that’s good, just
in general, right? Insulate your glycogen, right? So that you don’t… no. Alright. You don’t waste energy, you see, if you insulate your glycogen. Alright, Anyway. We got a phosphate
onto here and we didn’t have to invest ATP energy. We just saved a triphosphate. Muscle cells, if I am
running and jumping, I don’t want to burn my ATP
breaking down my glycogen. I want to burn my ATP using
the energy from glucose. This allows me to put
a phosphate on there without using any ATP energy. This is really cool
because now I can isomerizes this guy to make glucose
6-phosphate andóbang! I’m in glycolysis without
investing any ATP to start. Very good. So this saves a reaction. The enzyme is called
a phosphorylase. The name, again,
tells us what it does, meaning it uses a
phosphate to break a bond. It uses a phosphate
in breaking a bond. It’s different than a hydrolase, which uses water to break a bond. So instead of using water, we’re using phosphate
to break that bond. We’re almost done, okay? We’re almost done. There’s only one other
thing I have to tell you, and that is the fact that
glycogen phosphorylase is a finicky enzyme. Of course it’s a finicky enzyme. It has to be, right? Glycogen phosphorylase
will only work to within about four residues of a branch. It gets to that point. It starts up here. It keeps chewing,
chewing, chewing. It takes these red guys
off here, and it says, “I ain’t going any further.” It will not work any closer
than about four residues to a branch, the branch
being a 1,6, right there. Then, something
else has to happen. Well, the something else
that has to happen is another interesting enzyme that has
two activities associated with it, but we branch
them into one name. We could memorize that
it’s called a transferase and we could memorize
that it’s called an alpha-1,6-glucosidase,
but we, being biochemists,
are kind of lazy. We like to call both
of these activities “debranching enzyme.” I’m going to tell you what
debranching enzyme does, but these two reactions
are catalyzed by the same enzyme known
as “debranching enzyme.” What happens? Well, let’s look to see
what this enzyme is doing. Follow the blue guys. Here’s the three blue guys here. The three blue
guys get transferred from this branch
down to this branch. That leaves behind one green guy. They’re all glucoses, by the way. So they’re all glucoses. They’re not different. The difference being this
guy is linked by an alpha-1,6. The enzyme, debranching
enzyme, uses water to break that guy off and
we get free glucose. This is the only place
we get free glucose in glycogen metabolism. Student: And then the
glycogen phosphorylase will then be able to… Kevin Ahern: Then glycogen
phosphorylase now has a new template it can work
on and it can go chewing back until it gets
back to another branch. Student: The free
glucose, the green one, is that [unintelligible]? Kevin Ahern: The green one
is the only free glucose that’s released in the process. Student: [unintelligible] Kevin Ahern: What’s that? Student: [unintelligible] Kevin Ahern: Right. So you might wonder, well, why in the other case does
it use glucose 1-phosphate it used glucose to make
glucose 1-phosphateówhy, in this case, is it
releasing free glucose? It’s not being consistent. No, there’s something
that’s different here. What’s different here? Student: water Kevin Ahern: It’s using water, but why doesn’t the
other one use water? Why doesn’t this one use
phosphates, is my question? Student: It’s not a high energy Kevin Ahern: It’s not a
high enough energy bond. An alpha-1,6 does not have as
much energy as an alpha-1,4 does. It doesn’t have the option. Well, fortunately, there’s
only one of these per branch that’s made, so the cell
says, “Okay, I’ll take and use some ATP and put
you into glycolysis.” Bang! You got it. Student: So the debranching
enzyme requires ATP? Kevin Ahern:
Debranching enzyme? No. There’s nothing here
that requires ATP. Getting that into
glycolysis requires ATP. Okay, questions? Now, believe it or not,
with the exception of the phosphoglucomutase
that’s needed oop, turn that guy off the phosphoglucomutase
that’s needed to convert the glucose 1-phosphate
into glucose 6-phosphate, you’ve just seen how
you break down glycogen. Bang! What enzymes did we see? Phosphoglucomutase
interconverts glucose 1-phosphate and glucose 6-phosphate. It’s a mutase, so what
does that tell you? It has a 1,6
intermediate, and, yes, that can get released
as a free molecule. It does get released
as a free molecule. The second enzyme was
glycogen phosphorylase, that broke 1,4 bonds
close to a branch, and the third enzyme
was debranching enzyme, which changed the branch
and released free glucose. Three enzymes in
the entire pathway. Cool! Glycogen breakdown
is very simple. I’m going to talk about
glycogen synthesis in a second and you’re going to see
it’s almost as simple. Here’s the phosphoglucomutase. This is the glucose 1-phosphate. There’s the intermediate. There’s the product,
glucose 6-phosphate. This is a reversible
reaction, either direction. If we have excess
glucose 1-phosphate, it’ll go to the right. If we have excess
glucose 6-phosphate, it’ll go to the left. When would we have excess
glucose 6-phosphate? What conditions
would give us excess glucose 6-phosphate? What metabolic pathway…hint, would give us excess glucose 6-phosphate? Student: Gluconeogenesis. Kevin Ahern:
Gluconeogenesis, right? So if a cell is building glucose, it’s going to be
building glycogen, too. We’ll see in a second
that glucose 1-phosphate is needed to make glycogen. So if we’re making
things in gluconeogenesis, we’re going to the left. If we’re breaking things
down in glycogen breakdown, in glycolysis, we’re
going to the right. Yes, ma’am? Student: Which one did
you say is reversible? Kevin Ahern: The entire
reaction is reversible. Student: Oh. What are the yellow things? Kevin Ahern:
That’s just part of the enzyme. So there’s the active
site of the enzyme. There’s the rest of the enzyme. There’s the serine
residue that’s involved. That’s really all it is. It’s just showing
you that side chain. Alright. DIPF would to do
what to this enzyme? Student: Inactivate it. Kevin Ahern:
Inactivate it, right? Okay. I should have asked
you what the molecule was that’ll do it. Okay. I’m going to jump down
to glycogen synthesis, because I think if we
talk about the metabolism and then we save the regulation
for later we’ll be better off. So let’s talk about the
synthesis of glycogen. It’s just about as simple
as the breakdown is. There’s one extra
enzyme, one extra enzyme. So, the enzyme, again, we think “phosphoglucomutase”
for interconverting. Now we want to make
glucose 1-phosphate, because we want to make glycogen. But it turns out that
glucose 1-phosphate can’t be added to a growing
glycogen chain. Why? Well, remember that alpha-1,4
bond had some energy in it? Right? If it has energy in it, then
we have to put some energy into making that bond, and
there’s not enough energy in water, essentially,
to make that bond. So we have to use
a high-energy intermediate in order to make that
alpha-1,4 linkage. The high-energy intermediate
we use is this guy, right here. You saw it before. You saw it when we talked
about galactose metabolism. This was a molecule I described as an “activated intermediate.” An activated intermediate
is a molecule that has a high-energy bond, and
there is the high-energy bond. It’s a molecule that
has a high-energy bond that uses the energy of that bond to transfer a part of
itself to something else. So an activated intermediate
is a molecule that has a high-energy bond
and it uses the energy of that bond to transfer a part
of itself to something else. Well, the part of itself
it’s transferring is this guy, right here, glucose. What it’s going to do is
attach it to position 4 of a glucose on the end of
a growing glycogen chain. If we’re going to
talk about the enzymes of glycogen synthesis,
we have to talk, first of all, about how
do we make this molecule. Once we know that,
everything else is pretty much like glycogen breakdown. Let’s take a look
at how we make that. Here’s the reaction
that makes UDP-glucose. Glucose 1-phosphate, okay,
you know how that’s made now. Glucose 1-phosphate
we combine with UTP. We make UDP-glucose and we make what’s called pyrophosphate. Those are two phosphates
joined to each other. Let’s count the phosphates. One, two, three, four. One, two, three, four. We haven’t lost any phosphates, but they’ve reorganized. Now we have this guy and
we have this guy, over here. Student: What did
you say the name was? Kevin Ahern: It’s
called “pyrophosphate,” P-Y-R-O-P-H-O-S-P-H-A-T-E. Pyrophosphate means two
phosphates covalently linked to each other. Well, we’ve just made an
activated intermediate. What did it take to do it? It took a triphosphate. UTP has the same
energy as ATP does. It has the same
energy as GTP does. That triphosphate is high energy. The cell is having
to invest some energy into making this bigger molecule. That’s a fundamental
principle of anabolism. Building bigger
things takes energy. It took energy to make glucose. It’s now taking energy
to make glycogen. We’re nearing the
end, believe it or not. UDP-glucose. What’s the next
step in the process? Well, the next step in
the process is adding that glucose to a
growing glycogen chain. This is the reaction
that’s catalyzed, here. There’s the UDP-glucose
that we just made. Here’s carbon number 4 of the end of a glycogen chain, right there. In this reaction,
this glucose gets transferred over there. The energy of this
bond is used to make this high-energy bond. We’ve now made a
glycogen that has one more glucose on it. The enzyme that
catalyzes this reaction has a very simple name. It’s called “glycogen
synthase,” S-Y-N-T-H-A-S-E. Glycogen synthase
catalyzes the addition of glucose to a
growing glycogen chain. The product is UDP, of course, and UDP can be converted into
UTP and then reused again. Now, we’re only
missing one thing. What are we missing? How do we get branches? Well, for branches, we’ve
got a really complicated enzyme name that’s used
to do it, but I prefer to call it “branching enzyme,”
as I’m sure you will, too. There is, believe me,
it’s a mouthful of a name. It’s about that long, okay? But, in essence, branching
enzyme will create alpha-1, 6 branches about
every ten residues. Here’s an alpha-1,4 linkage. Here’s a branching enzyme. Bang! Got it! So branching enzyme is
creating the branches. So what enzymes have we
seen in glycogen synthesis? Well, we saw
phosphoglucomutase, as before. I didn’t give you the
names of the UDP-glucose synthesizing enzyme, did I? Student: No. Kevin Ahern: Do
you really want it? Student: Nope. Kevin Ahern: Should
we give it a name? I’ll tell you what the real
name is and then you can tell me perhaps a more humorous name. The real name is UDP-glucose
pyrophosphorylase. Student: Steve! [laughing] Kevin Ahern: Steve. These are all male names. Do we have any female…there’s
never a female…it’s true, every year when
I ask for names people always give me male names. Student: Helga. Kevin Ahern: Ursula! Student: Tina. Kevin Ahern: Tina? Student: Amaryllis. Kevin Ahern: Amaryllis? Student: Shaniqua. [laughing] Kevin Ahern: I’m sure they’d
like to spell that one. So you may call it either
UDP-glucose pyrophosphorylase, which is the real name, or…
I’m going to vote on this. I don’t know. I think the best names
I’ve heard were Steve… Tina… Student: Lucy. Kevin Ahern:…and Ursula…Lucy! And Lucy. Okay. Steve, Tina, Ursula, Lucy. Steve? Tina? Ursula? Lucy? Lucy is the simplest
one, I think. People wanted Lucy. “Lucy in the Sky
with Diamonds,” right? Student: What was the real name? Kevin Ahern: It’s the
enzyme that catalyzes this reaction right here. Its real name is UDP-glucose
pyrophosphorylase. UDP-glucose pyrophosphorylase. That’s the breakdown. That’s the synthesis of glycogen. I’m going to cut
short early today but I’m not going
to finish quite yet. I just want to say one
last thing, and that is, on Wednesday I’m
going to talk in detail about the regulation. The regulation is reciprocal, but it’s also complicated. It involves both
covalent modification and allosteric regulation. If you want to look over
a lecture material before you come to lecture, next
time might be a good one. See you Wednesday. [indistinct conversation] Kevin Ahern: Yes, sir? Student: [unintelligible], why was that a pyrophosphate
instead of a bisphosphate? Kevin Ahern: What’s that? Student: [unintelligible] Why is that a pyrophosphate
instead of a bisphosphate, when it’s free floating? Kevin Ahern: I think the
term’s interchangeable. Student: Okay. Kevin Ahern: Yeah. Student: So “pyro
-” means “bond”? Kevin Ahern: Just bond, yeah. Yeah. Student: Okay. Thank you. Student: I didn’t
catch where you said we could pick up our exams. Kevin Ahern: Yes, they’re
at the BB office, in ALS-21. Student: Okay. Thank you. Kevin Ahern: Sure. [indistinct conversations] [no audio] [END]

No Comments

Leave a Reply