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  • [Ahern laughing]

  • Student: Do it.

  • Ahern: Do it.

  • [laughing]

  • I'm guessing if I gave everybody who came to class

  • an A today then they'd never come to class again.

  • That's just my hunch.

  • Which would kind of be a self-defeating thing, right?

  • So...

  • Student: Not necessarily.

  • Ahern: Maybe we would, right?

  • What she should say is, "Ahern, you're a scientist,

  • "let's do the experiment and fight out," right?

  • Student: Exactly.

  • Ahern: Well, we can't do that.

  • Student: You can give us extra credit.

  • Ahern: I could give you extra credit.

  • There's a lot of things I could do.

  • I could give you money.

  • [class laughing]

  • We could go have beer.

  • We could have pizza.

  • Student: How many of us would not get in

  • trouble for you buying beer.

  • Some of us are still under age, so...

  • Student: Yeah, you could get in a lot of trouble.

  • Ahern: No, actually the way I do that is I go to a place

  • where they can serve people underage

  • and you have to show an ID so it's not my responsibility.

  • A lot of energy.

  • I hope everybody's got a big Thanksgiving planned.

  • Wild plans?

  • Student: Family.

  • Ahern: Oh.

  • Family, huh?

  • Like I said, if any of you are in town

  • and would like to come over, give us a holler

  • and I'll let you know where we're going to live and everything,

  • but if you'd like to come over,

  • we've got plenty of turkey and other things.

  • And no, I won't get you drunk.

  • But we'll have a good time.

  • Today we're going to have a good time

  • because we're going to be thinking of the making of glucose.

  • I know that for many of you,

  • that's been something you've dreamed of doing

  • and you're going to get that dream today.

  • Happy days are here again.

  • Glucose synthesis is an interesting process.

  • The phenomena of course is known as gluconeogenesis

  • and it is a pathway that is very similar,

  • very similar to glycolysis.

  • Very similar.

  • It's very similar to the reverse of glycolysis.

  • However, there are important differences and specifically,

  • there are two reactions.

  • I'm sorry, specifically that there are 3 reactions.

  • There are 3 reactions in gluconeogenesis,

  • in glycolysis that are replaced

  • by 4 reactions in gluconeogenesis.

  • So gluconeogenesis has 11 steps, glycolysis had 10.

  • One of the steps takes two steps to get around.

  • So it's 2 step.

  • If you learned glycolysis, gluconeogenesis for 8 of the steps,

  • Let's get that right for 7 of the steps.

  • I can't get my head right today.

  • for 7 of the steps is identical to glycolysis

  • except for in the reverse.

  • Same enzymes, same intermediates going to the opposite direction.

  • Three of the steps that are in glycolysis

  • as I said are replaced by 4 steps.

  • So let's take a quick look at that.

  • Before I take a look at that, I'll show you something

  • your book is distracted by and that is this process here,

  • which is the making of glucose from glycerol.

  • Why do we care about the making of glucose from glycerol.

  • One of the reasons we care about the making of glucose

  • from glycerol is glycerol is a byproduct

  • of fat metabolism and so it turns out

  • that the only portion of the fat molecule

  • that can be converted directly into glucose is the glycerol.

  • We don't convert fat into glucose for the most part,

  • with the exception of the glycerol.

  • I just show you this, I'm not going to go through

  • and expect you to memorize these are anything,

  • but just show you that glycerol is a 3-carbon molecule.

  • It gets made in a couple of reactions

  • into an intermediate in glycolysis,

  • dihydroxyacetone phosphate.

  • And of course, once it's dihydroxyacetone phosphate,

  • we can now do the upwards pathway,

  • going into making glucose via gluconeogenesis.

  • And we see this, this is a phenomena you've seen before.

  • We saw how other sugars entered glycolysis

  • and gotten broken down by being converted

  • into glycolysis intermediates.

  • We saw, for example, fructose got converted

  • into fructose-6-phosphate and then got metabolized

  • as an intermediate in glycolysis.

  • In this case, we see glycerol being converted

  • into an intermediate in glycolysis or gluconeogenesis,

  • it can actually go either way,

  • and be made into either glucose or ultimately into pyruvate.

  • Let's focus on gluconeogenesis

  • since that's our main topic of the day.

  • You'll notice in looking at the screen

  • that we oriented very much like we oriented glycolysis

  • except that we're going upwards in gluconeogenesis

  • where as we were going downwards in glycolysis.

  • So we start at the bottom and the place where we will

  • start gluconeogenesis is actually pyruvate.

  • But again, we remember that all these designations

  • about where something starts and stops is really arbitrary.

  • We could just as easily start it at lactate

  • for some types of metabolism.

  • We can start at amino acids as well,

  • but we're going to start right here at pyruvate.

  • So starting at pyruvate, and that's a good place to start

  • because that's where we finished glycolysis,

  • starting with pyruvate, cells can convert pyruvate into glucose.

  • Well, not surprisingly, if pyruvate is a 3-carbon molecule

  • and we want to make a 6-carbon glucose,

  • we need to have two pyruvates to start everything.

  • We're going to start with 2 of everything

  • and eventually they're going to combine

  • into 1 as we get higher up in the pathway.

  • What we discover in gluconeogenesis is the first

  • instance that we see of a phenomenon I call

  • sequestration meaning we're sequestering something.

  • All of glycolysis occurs in the cytoplasm of the cell.

  • All of the enzymes of glycolysis are found

  • in the cytoplasm of the cell.

  • In the case of gluconeogenesis, we see 2 enzymes

  • that are not found in the cytoplasm of the cell.

  • These are sequestered into other organelles

  • in the cell and I'll show you those.

  • They actually end up being the first

  • and the last enzyme in the pathway.

  • All the other enzymes between the first and the last

  • are all found in the cytoplasm.

  • Let's look at what's happening in making glucose from pyruvate.

  • If we recall in glycolysis, in going from PEP to pyruvate,

  • I said that was the big bang.

  • I said that was a reaction that was extraordinarily energetic.

  • It had a large delta-G-zero prime.

  • And as a consequence, that,

  • you might imagine going in the reverse direction,

  • would be an enormous energy barrier to overcome.

  • And in fact, that's exactly what it is.

  • It's because of this enormous energy barrier

  • that cells can't go directly back from making

  • pyruvate to PEP in one step.

  • They have to do a two step around it.

  • And the two steps around it are these two enzymes here.

  • Pyruvate carboxylase and phosphoenolpyruvate carboxykinase,

  • which you are more than welcome to abbreviate as PEPCK.

  • Let's talk about the first one first,

  • pyruvate carboxylase is an enzyme that is found

  • in the mitochondrion of cells.

  • It's not found in the cytoplasm.

  • The very first reaction of gluconeogenesis

  • occurs in the mitochondrion, not the cytoplasm.

  • In this reaction, as you can see on the screen,

  • carbon dioxide in the form of bicarbonate and ATP

  • are used to convert pyruvate into oxaloacetate.

  • You can see the structure here.

  • Here's a 3-carbon, here's a 4-carbon over here.

  • We've put an additional carboxyl group onto the end of pyruvate.

  • The carboxyl group going right here.

  • We can see the new carboxyl group on the right side.

  • Here it was what pyruvate looked like over here.

  • So what we did is we took this methyl group

  • and we added a carboxyl group to it.

  • That takes energy to put that on there.

  • It makes a 4-carbon intermediate

  • and that 4-carbon intermediate you're going to hear

  • a lot about next term because oxaloacetate is one

  • of those molecules that appears in so many metabolic pathways.

  • It's a very, very important molecule.

  • It's important in amino acid metabolism.

  • It's important in the citric acid cycle.

  • And it's also important as you can see here

  • in the synthesis of glucose.

  • This is an energy requiring reaction

  • so if we started with 2 molecules of pyruvate,

  • it's going to take 2 molecules of ATP

  • and 2 molecules of bicarbonate to make

  • 2 molecules of oxaloacetate.

  • We haven't gotten to PEP yet because even with all

  • that energy that we've put in, we've made a 4-carbon

  • intermediate and we have to convert

  • that 4-carbon intermediate into phosphoenolpyruvate or PEP.

  • To do that, the oxaloacetate which was made

  • in the mitochondrion has to be moved

  • out of the mitochondrion and into the cytoplasm.

  • Next term we'll talk about how molecules

  • move across an organelle.

  • But it turns out there are specific proteins

  • that will shuttle a molecule across a membrane.

  • There are specific proteins that will transport

  • oxaloacetate out into the cytoplasm.

  • When it's out in the cytoplasm,

  • oxaloacetate can be acted upon by this

  • second enzyme that's unique.

  • By the way, all the unique enzymes are shown in red on here.

  • By the unique enzyme phosphoenolpyruvate carboxykinase, or PEPCK.

  • Notice that it also takes energy

  • and that energy in this case comes from GTP.

  • So GTP is just like ATP, a high energy source.

  • GTP is used in some places in the cell for energy.

  • The most common place we actually see GTP used

  • is in the synthesis of proteins because all proteins

  • are made using GTP, not ATP.

  • We'll talk about that next term.

  • If we have 2 molecules of oxaloacetate

  • and we want to make 2 molecules of PEP,

  • it takes us 2 molecules of GTP and

  • this enzyme to accomplish that.

  • In the process, a CO2 is released.

  • Look what we did.

  • We put a CO2 on the form of bicarbonate

  • and we've released the CO2 up here,

  • so no net gain of carbons have occurred.

  • We've done a molecular rearrangement and in the process

  • of doing that molecular rearrangement,

  • we've also put a phosphate onto the molecule,

  • creating that very high energy PEP molecule.

  • As I said, this reaction occurs out in the cytoplasm.

  • To go from pyruvate to PEP in terms of synthesizing glucose,

  • we had to use 2 high energy phosphates for each

  • molecule for a total of 4.

  • So in order to go from here 2 pyruvates to 2 PEPs,

  • we need 4 triphosphates.

  • 2 ATPs and 2 GTPs.

  • From an energy perspective, GTP is exactly equivalent to ATP.

  • There's no difference.

  • Going down, if you recall, when we went from PEP to pyruvate,

  • we only got a total of 1 ATP for each one,

  • or a total of 2 ATPs.

  • So we can see that building molecules in anabolic pathways

  • takes more energy than we get out in catabolic pathways.

  • That's not surprising.

  • We're thinking okay, well we're going.

  • Yes?

  • Student: So how does this prevent PEP from immediately

  • switching back to pyruvate?

  • Professor: She's reading my mind.

  • My next point is, her question was

  • how does the cell keep PEP from just going back to pyruvate?