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  • JOANNE STUBBE: So we've been talking

  • about iron metabolism in general in the first lecture.

  • And in the second lecture we started

  • to focus on iron metabolism in humans,

  • and the third set of lectures is going

  • to be iron metabolism and bacteria with a focus on hemes.

  • And the two things you want to talk about in the lecture today

  • are, how does iron get taken up into cells in humans,

  • with a focus on receptor mediated endocytosis,

  • and then we're going to start talking about hopefully iron

  • regulation--

  • how you sense iron, ion regulation

  • at the translational level.

  • By sort of a unique mechanism, at least

  • at the time of its discovery.

  • So in the last lecture, we introduced you

  • to some key features about iron chemistry

  • in general that we're going to use throughout this lecture

  • and next lecture.

  • So you need to go back and review your notes

  • if you don't remember that.

  • Or hopefully you've had it somewhere before,

  • and it's a review from you from freshman chemistry

  • or inorganic chemistry.

  • And so iron metabolism-- what do we know?

  • We know the average human being has 3 to 4 grams of iron.

  • We talked about this at the end of the last class,

  • of how is the iron distributed.

  • We all went through that most of our iron

  • is in our red blood cells in the form of hemoglobin.

  • But it's also-- so in the form of hemoglobin,

  • it also can be stored in proteins called ferritins,

  • which we're not going to spend much time on,

  • but I will introduce you to today.

  • And then many of you may know that red blood

  • cells die every 120 days.

  • And we'll see that the iron is really continually recycled,

  • and we'll talk a little bit about the mechanism

  • of how that's regulated.

  • So instead of excreting it, what happens is you recycle.

  • The iron unit's recycled by macrophages in the spleen.

  • And so the other place you see a fair amount of iron

  • is in the macrophages.

  • And the third place you see a fair amount of iron

  • is in the tissues, because myoglobin, again,

  • has to deliver oxygen to the respiratory chain.

  • So what I want to do now, and I'm

  • going to go back and forth between the PowerPoint

  • and notes.

  • And so some things I'm going to write down some things not.

  • Hopefully you have these cartoons in front of you

  • so you can write down some of the things

  • that I will say here, and say it again and say it again.

  • So this is sort of the big picture

  • that I took from some review.

  • And most of these big pictures have some issues with them.

  • But I think it still gives you the big picture.

  • So here's a duodenum, where we can take up iron from the diet.

  • And we'll talk about this in more detail,

  • but a key player in allowing the iron from the diet

  • to go into our system is going to be FPN--

  • that's going to be ferroportin, I'm

  • going to describe this again.

  • But you're going to see FPN over and over again.

  • It allows iron to be transferred in the plus 2 state,

  • and that's going to be important.

  • And so what we see, that if you look at iron from the diet,

  • there's not that much.

  • [AUDIO OUT] somebody's guess as to how much there is.

  • A few milligrams.

  • And the question is, where does it go in the bloodstream?

  • And it goes to a protein that we're

  • going to talk about that's a carrier for iron in the plus 3

  • state.

  • So we're going to see plus 2, plus 3 into conversions

  • over and over again.

  • And sort of what the strategy that

  • has evolved to be able to deal with these different oxidation

  • states is.

  • We'll see that this little protein, TF, is transferrin,

  • and we're going to look at transferrin

  • for a-- very briefly, but it binds iron 3 and bicarbonate,

  • and then delivers this to tissues,

  • and also delivers it to marrow.

  • And marrow, which is-- accounts for approximately,

  • by mass, 4% of the body weight, makes all of our red and white

  • blood cells.

  • So that's going to be important.

  • And so the marrow makes the erythrocyte,

  • the heme for the erythrocytes makes the erythrocytes,

  • and the erythrocytes are the red blood cells

  • that have all the hemoglobin.

  • So out of the 4 grams, you have 2 and 1/2 grams of hemoglobin.

  • And then these red blood cells die every 120 days,

  • and instead of just discarding everything, they're recycled.

  • And they're recycled by the macrophages in the spleen.

  • And somehow you want to take the iron from these red blood cells

  • and reuse it.

  • And so there's a series of reactions that happen.

  • Ultimately you get iron 2, and the iron 2-- here's

  • again our iron 2 transporter, ferroportin, is

  • going to take the iron that's recovered and put it

  • back into transferrin, where, again, it can be distributed,

  • depending on the sensing of iron.

  • Now, the major player in the sensing and storage of iron

  • is the liver.

  • So the liver, we're going to see there's

  • a protein there not indicated on the slide called ferritin,

  • and ferritin binds 4,500 molecules of iron.

  • And this is also-- the liver is the organ that generates,

  • biosynthesizes the key regulator of iron homeostasis,

  • which is a peptide hormone that we're not

  • going to spend a lot of time on, but I'm

  • going to show you what it does.

  • So that's called hepcidin.

  • And what we'll see is hepcidin in some way controls

  • the levels of ferroportin.

  • So we also see that we lose some iron daily,

  • but the iron losses are small.

  • So we have a lot of iron units, but the iron

  • is continually recycled, and the question

  • is, how does that happen?

  • So I just want to look at one place where,

  • in the duodenum, where we're going to take up iron.

  • So what I'm going to do is--

  • this is a cartoon of what I just showed you in more detail.

  • But I'm going to focus on iron absorption from the diet.

  • And I want to make a couple points

  • about this, which are general.

  • And so what we'll see is we have enterocytes,

  • so this is an enterocyte.

  • And you have an apical brush border membrane.

  • And then you have a second membrane

  • which is going to get us into the bloodstream.

  • So this is called a basolateral membrane.

  • So we get iron from our diets mostly in the plus 3 state.

  • But to do anything with iron, probably

  • because of the ligand exchange issues

  • we talked about last time, the rate constants for exchange

  • are much slower with iron 3 than iron 2.

  • So from the diet, we have iron 3.

  • And iron 3 needs to be reduced to iron 2.

  • And that can be done-- we'll see this is going

  • to happen over and over again.

  • And this can be done by a ferric reductase.

  • And what we will see is in this membrane,

  • we're going to have an iron 2 transporter.

  • So in addition to the ferroportin

  • I just briefly introduced you to,

  • and will introduce you to again, we

  • have an iron 2 transporter, that's called DMT 1.

  • Again, the acronyms are horrible.

  • But it's a divalent predominantly iron

  • 2 metal transporter.

  • And we're going to see, when we think

  • about regulation of iron homeostasis,

  • this is going to be a key player.

  • Because it takes iron from the diet into our cells.

  • And in this membrane of the enterocyte, what

  • we will see is that we have--

  • and this is what you saw in the previous slide--

  • you have ferroportin-- so I'm only

  • going to write this down once.

  • But this is going to take the iron 2

  • and then transfer it into, ultimately,

  • the carrier in the bloodstream, which

  • is going to be transferrin.

  • So here we have iron 2, but for it

  • to get picked up by transferrin, it gets oxidized to iron 3.

  • So what you're going to see over and over again

  • is going back and forth between iron 2 and iron 3.

  • And so this gets oxidized to iron 3.

  • And these proteins-- there's a copper iron oxidase.

  • And if you look at the handouts, you'll

  • see that this is also called--

  • again, I don't expect you remember the names.

  • What I think is key here is that you need to transfer this

  • to the plus 3 oxidation state.

  • So now what happens in the plus 3 oxidation state--

  • so let's go over to the next board here--

  • we have a protein called transferrin,

  • and we'll look at this a little bit.

  • And transferrin is going to bind iron in the plus 3 state,

  • but it also requires bicarbonate.

  • So in the blood, is that unusual that you

  • would require bicarbonate?

  • Or why might you require bicarbonate?

  • What do you know about blood cells and hemoglobin?

  • So we have iron 3 that's regenerated enzymatically,

  • through some kind of oxidation reduction equipment.

  • And we're going to see this, again, over and over again.

  • And they each have different names,

  • so that's confusing as well.

  • But you're cycling between 2 and 3.

  • And then transferrin, we have a structure

  • of this picks up the iron in the plus 3 state,

  • and also picks up bicarbonate.

  • So where do you think that bicarbonate

  • comes from in blood cells?

  • AUDIENCE: CO2.

  • JOANNE STUBBE: Yeah, so it comes from CO2.

  • Why?

  • Because a major function of red blood

  • cells is to transfer CO2 from the tissues back to the lungs.

  • So CO2 is not there, at pH 7, it gets rapidly hydrated to form

  • bicarbonate and protons.

  • And so this is unusual.

  • I think this is one of the few systems where you have-- we'll

  • see bicarbonate as a ligand.

  • So in addition to these enterocytes, which again

  • are involved in iron uptake, we also

  • have macrophages in the spleen.

  • And so this, again, is due to the diet.

  • And this is due to basically recycling--

  • iron recycling.

  • And so what you have is macrophages in the spleen,

  • and you have in the macrophages dead red blood cells,

  • which I'll abbreviate RBC.

  • And so the idea is we want to get the iron out

  • of the red blood cells somehow to reuse it.

  • So that's the goal.

  • And so somehow in a complicated process, we get iron 2 out.

  • And then iron 2--

  • here we have our friend ferroportin,

  • that I just showed you in the previous slide,

  • is going to take and put into the extracellular mirror

  • in the plasma the iron 2.

  • So what happens to the iron 2?

  • We just saw over here, the iron 2 gets oxidized to iron 3.

  • The same thing is going to happen over here.

  • So we have iron 2 that needs to get oxidized to iron 3.

  • And again, let's just call it a copper iron oxidase.

  • I'm not going to go through the details.

  • And then what happens to the iron 3?

  • So the iron 3 then gets picked up by the transferrin.

  • And then depending on what the needs are the cell,

  • the transferrin can deliver.

  • If you have a lot of iron, it could deliver it back

  • to the liver.

  • We'll see that's the storage place for the iron.

  • So the iron 3 transferrin needs to get taken up,

  • just like we saw with cholesterol.

  • Or if we need iron in some other tissues,

  • we'll see that there are receptors for iron 3

  • transferrin that can, again, take iron into the cells

  • to meet the needs of the cell for iron requirement.

  • Now, the one thing I wanted to tell you in the first slide,

  • which I had forgot, was that in addition

  • to all of these requirements for iron,

  • and the predominant form being hemoglobin and myoglobin, what

  • we see is that iron is found in only 4%

  • of the metabolic enzymes.

  • So iron is found in many proteins that

  • catalyze all kinds of reactions, like we talked about last time.

  • But that's a small percentage of the total amount of iron.

  • So this sort of diagram is pointing out

  • a few things that sort of is indicative of iron

  • mediated metabolism in many cases.

  • And so what I briefly want to do is

  • sort of summarize the functions of these different proteins.

  • So this is phenomenological.

  • And if you're going to have--

  • if you were given an exam on this,

  • I'll give you the names of all of these things.

  • Because I think the names are actually confusing.

  • So number one, we have DMT1.

  • And again, when ion is trans--

  • it's a transporter of iron 2.

  • And so that's an important thing to remember.

  • But even though it's transferred into the cell

  • and it moves around inside the cell as iron 2, likely because,

  • again, the ligand exchange, though iron starts here

  • and it needs to move here, and it needs to move here,

  • and this is the way nature--

  • because of the exchangeability of the ligands--

  • has decided to move iron, and also

  • oftentimes copper 1 around, instead

  • of in the oxidized state that this transporter deals

  • with iron 2.

  • The second key thing is ferroportin, and this goes--

  • brings, again, iron 2 to the extracellular milieu.

  • And so this is bringing it inside the cell.

  • This is bringing it extracellularly

  • the outside the cell.

  • And this leads to the next thing that we

  • see over and over again--

  • while iron 2 is brought outside the cell,

  • it then gets oxidized to do anything with it.

  • So then we have general ways of iron 2

  • being oxidized to iron 3.

  • And this could be a copper iron oxidase.

  • But again, there are multiple--

  • there are multiple names for these [INAUDIBLE]----

  • we'll see in a few minutes, steep is one.

  • I mean, they have five different iron oxidases.

  • And iron 3 is going to be the key for transferring this

  • to ferritin, which is the way that iron is transferred,

  • just like the LDL particles are the way cholesterol is

  • transferred around the cell.

  • Ferritin-- transferrin is the way

  • the iron is transferred around the cell.

  • So so this iron 3--

  • so iron 3 is picked up by transferrin.

  • And again, this is iron 3.

  • I'll show you-- we have structures

  • of all these proteins.

  • This is, again, iron 3 bicarbonate.

  • And then the question is, how does this transferrin

  • get into cells?

  • So this is the major carrier.

  • Iron.

  • And it's carried in the plus 3 oxidation state.

  • Maybe, and we'll see that the KD for binding--

  • what do you think the KD for binding to a transferrin

  • might be?

  • Do you think it's weak?

  • Do you think it's strong?

  • And what would you, if you were designing something

  • that was carrying around iron to all the tissues,

  • what would you design?

  • Something weak or strong?

  • Say it was weak, what would happen?

  • Yeah, it comes unbound.

  • And then if it gets reduced, into the realm where

  • you have iron 2 and then you have reactive oxygen species.

  • And so nature has developed, I would say,

  • you've seen with siderophores you

  • can get things 10 to the minus 35 for dissociation constants

  • to 10 to the minus 50.

  • The KD for this is 10 to the minus 23 molar

  • for iron binding to transferrin.

  • And so the next thing that happens

  • is that the iron binding to transference

  • goes to the transferrin receptor.

  • And so transferrin then binds to the transferrin receptor,

  • just like the LDL particle binds to the LDL receptor.

  • So this is the transferrin receptor.

  • And so what you're going to see is that, in contrast

  • with iron transported across--

  • in the case of the enterocyte, or in the case of ferroportin,

  • where it's iron 2, this is all transferred in the iron 3

  • state.

  • So this-- again, this is important to see

  • the differences in the oxidation states that

  • are used to control uptake into the cell.

  • And this occurs by--

  • we'll briefly look at this, but it's

  • very similar to what you saw with the LDL receptor.

  • The receptor mediated endocytosis.

  • So we're going to look at a cartoon of this.

  • So there's one other player that I want to introduce you to.

  • And this player becomes really critical

  • because we don't have ways-- we don't

  • produce a lot of excess iron and then export it.

  • All the iron is recycled.

  • So what controls that iron recycling?

  • So the key regulator is a peptide hormone

  • which I introduced you to in the previous slide, called

  • hepcidin.

  • And we know quite a bit, actually,

  • about the structure of this peptide hormone.

  • And I'll tell you what its proposed function is.

  • We're not going to spend a lot of time discussing this.

  • But it is made in the liver.

  • So it's bio synthesized in the liver.

  • And it's basically-- its function

  • is, it's a major site of regulation,

  • and it controls iron from the diet, and iron cycling

  • through extracellular factors, like the transferrin--

  • like transferrin.

  • So how does it do this?

  • So here we have a little peptide hormone.

  • It's made in the liver.

  • And how can a control iron recycling?

  • And so the one guy that we see now

  • is ferroportin, ferroportin.

  • And so its major function-- it has a lot of functions,

  • and it's complicated, and people are still studying this.

  • But one of the major functions is

  • to control the amount of ferroportin.

  • So if you look at the way it's described,

  • the hepcidin combined extracellularly

  • to the ferroportin.

  • So I'll draw a little cartoon of that.

  • And then targets it for degradation

  • by the proteosome inside the cell.

  • So that's the key feature of hepcidin

  • that you need to remember.

  • So we're going to see, if you look

  • at-- if you look at a lot of the cartoons I've given you,

  • you have your ferroportin, watch transfers

  • iron from the inside extracellularly.

  • I forgot my colored chalk today.

  • I was on drugs or something.

  • But people were bothering me up until five minutes.

  • I didn't have time to think before this lecture.

  • So I'm sorry I'm a little discombobbled here.

  • But this is hepcidin--

  • hep-cid-in.

  • And so it binds to the extracellular side.

  • And what does that does when it binds?

  • It causes-- somehow things change,

  • and it causes it to be degraded inside the cell

  • by the proteosome.

  • So.

  • This interaction, extracellular, causes

  • ferroportin to be degraded inside the cell

  • by our friend the proteosome.

  • So does everybody sort of understand what the model is?

  • So this is the key regulator.

  • And you've seen ferroportin--

  • we only looked at two cell types.

  • We looked at the enterocyte, and we looked at the macrophages

  • in the spleen, both of which have ferroportins,

  • but ferroportins that are in a number of additional cell

  • types.

  • And when we look at regulation, one of the key regulators

  • of everything is going to be that we need to control

  • are the levels of ferroportin.

  • Because that allows all the iron to somehow be recycled.

  • It's a key player controlled by hepcidin

  • that allows the iron to be recycled

  • to the different tissues.

  • So we have a number of proteins that I'm

  • going to very briefly introduce you to,

  • in addition to these guys.

  • And so we're getting into more acronyms cities.

  • But the additional proteins that we need to think about--

  • so involved in iron homeostasis.

  • Our number one, the ferritin, which

  • in the introductory slide--

  • and let me just show you.

  • So what I'm going to do, these are the list

  • of proteins that I'm going to go through one by one

  • and tell you a little bit.

  • This is sort of an amazing protein.

  • It has 24 protein subunits.

  • It has two kinds of protein subunits.

  • You don't need to remember this.

  • But what is this function?

  • It's a key-- and this is found in all organisms--

  • it's involved in iron storage.

  • And why is this important?

  • It's important because it keeps iron soluble so that it's not

  • precipitating sort of as rust.

  • There are, in yeast, if you look at some of yeast homeostasis,

  • when things start going awry you can

  • you can look at it in an electron microscope,

  • you see iron all over the inside of the mitochondria,

  • just these big black blobs where the iron has precipitated

  • and mineralized.

  • So we need to keep iron soluble, and we

  • need to keep iron non-toxic.

  • So what do I mean by non-toxic?

  • In the last lecture, I told you that iron 2 can easily

  • be oxidized to iron 3 by oxygen. We're going to talk about that

  • in module 7 a little bit.

  • And that can result in all kinds of damage inside the cell

  • if it's not controlled.

  • So this protein is sort of amazing.

  • You can bind 4,500 irons, most of them

  • are in the iron 3 state.

  • But when you start out, it binds iron 2.

  • So iron 2, again, inside the cell

  • is what gets transferred around in general.

  • So iron 2 binds, and then each ferritin

  • has an oxidase activity that I'm not

  • going to go into in detail that can oxidize it to iron 3, which

  • puts it into this mineral structure

  • that you see in these 4,500 atoms of iron.

  • OK, you don't see it there, all you see is the protein there.

  • So this gets oxidized to iron 3, and this is how

  • it's stored in mineral form.

  • So now the question is, say you needed iron.

  • So we have a lot of iron, we want

  • to keep it sequestered so we don't

  • have to worry about reactive--

  • it doing chemistry that's aberrant.

  • We want to keep it soluble.

  • So we have iron stored in the plus 3 state

  • in some kind of mineral form.

  • How would you, if you wanted to use iron,

  • now what would you do?

  • Do you think you can get it out of the iron 3 mineral?

  • No.

  • What do you have to do to it to make the ligands more labile?

  • All you need to reduce it.

  • So to use it, you now-- and people are still

  • arguing about what the reductants are--

  • so you need to reduce iron 2 plus 2 so you can use it.

  • So that's ferritin.

  • Does anybody have any questions about ferritin?

  • It's got a complex structure, we have lots of structures of it.

  • You can have-- every ferritin is sort of different,

  • it has different ways of dealing with these issues

  • of how you mineralize, and how you remove it.

  • But this is a major storage protein

  • in all organisms of ferritins.

  • It's sort of an amazing structure.

  • So what we were talking about before

  • is that we get iron 3 transferrin.

  • What does iron 3 transferrin look like?

  • So we take iron from the diet, or we're recycling iron

  • from red blood cells.

  • We need to get it to the plus 3 state, where it gets picked up

  • by transferrin.

  • That's what we need to do.

  • And so if you look at this--

  • So we've picked up iron 3 in transferrin.

  • And the structures of transferrin are known.

  • So now we need to look at transferrin--

  • whoops.

  • And if you look at the structure,

  • it is composed-- the protein is composed

  • of two domains, each of which can bind iron 3 bicarbonate.

  • So it has two little lobes over here.

  • You can see this lobe and this lobe, the N terminal and the C

  • terminal lobe.

  • And they each bind--

  • if you look at this carefully, there is the iron,

  • there is the bicarbonate.

  • It has two tyrosines, a histidine,

  • and an aspartate as ligands.

  • And it's in an octahedral environment.

  • So again, why bicarbonate?

  • And people thought for a long time the bicarbonate was

  • related potentially to how do you deliver this iron 3

  • out of the transferrin into something that's useful,

  • namely the enzymes that are going to use

  • it to catalyze transformations.

  • And what is the bi-- is there a role for bicarbonate

  • in that process?

  • So what's unusual about the transferrin,

  • again, I get-- the KD is tight.

  • What's most unusual is it's got bicarbonate,

  • it's got two tyrosines, and it's got a histidine,

  • and it's got an aspartate, and it's an octahedral environment.

  • And how do you think-- what do you think the proteination

  • state of the tyrosines are?

  • Everybody know what tyrosine is?

  • Do you think it's proteinated?

  • Non-proteinated?

  • This brings up another sort of general principle

  • we talked about last time.

  • If you have water attached to a metal,

  • what can it do to the pKa of the water?

  • It decreases it so that you lose the proton

  • under physiological conditions.

  • What's the pKa of tyronsine?

  • It's on the order of 10, 10 1/2.

  • And in fact, this is bound-- it's the phenylate.

  • So both of these are in the phenylate form.

  • So both of these are phenylate.

  • And again, if you want to think more about this,

  • both Liz and Lippert have taught a course,

  • are teaching a course now, in bio inorganic chemistry, where

  • you really sort of talk about the details of these kinds

  • of interactions, which are key to the way everything

  • functions.

  • So we have transferrin, and the unusual part

  • is the binding of bicarbonate, and then, again,

  • let me just re-emphasize it's in the plus 3 state,

  • and you have fairly tight binding.

  • And what we're going to see is, it's going to bind just

  • like the LDL particles bind to the LDL receptors,

  • it's going to bind to the transferrin receptor.

  • So we now have a transferrin receptor.

  • So this is the receptor.

  • And we know we have structures, actually, of the receptors.

  • It's a 90 kilodalton dimer.

  • So and its transmembrane.

  • So you have-- so this is the transferrin receptor.

  • I'm going to show you a cartoon of this in a minute.

  • 90 killodalton dimer, and so this is extracellular.

  • This is intracellular.

  • And this is the membrane.

  • So let me just show you that cartoon over here.

  • So extracellular, intracellular.

  • And if you remember back to the LDL receptor,

  • how did we trigger receptor mediated endocytosis?

  • We had a zip code.

  • Here we also have a zip code.

  • And the zip code is YTRF.

  • So there's also, on the intracellular side,

  • a zip code for triggering transferrin uptake.

  • So those are the players that we need to think about.

  • So the transferrin, in the transferrin receptor,

  • have parallels with LDL.

  • LDL receptor-- of course every one of these things

  • is different.

  • But this was one of the other systems that

  • had been characterized quite extensively,

  • the first one being the LDL receptor.

  • And so the model is shown here.

  • This model hasn't really been--

  • this model's not completely correct.

  • I'll tell you where things need to be changed a little bit.

  • But really people haven't studied this model

  • in a long time, even though there's

  • a lot we don't understand.

  • So here's the surface.

  • Here's transferrrin, these little things here.

  • Here's the transferrin receptor purple.

  • So the transferrin binds to the transferrin receptor.

  • To get uptake into the cell, you need to have clustering.

  • So that's not shown here, because this cartoon

  • was drawn before we realized that you had a cluster--

  • the transferrin receptors.

  • When you transfer, when you cluster,

  • and you bind transferrin, again, just like we saw with the LDL

  • receptor, in some way, you have machinery

  • that attracts the clathrin, and then it's

  • going to pinch off the clathrin coated vesicle.

  • And they skip here the clathrin coated vesicle.

  • So that should be in between-- this is clathrin.

  • And then what happens, just like in the LDL receptor,

  • you remove the clathrin from the external part

  • of your little vesicle.

  • So that's what's indicated here.

  • So what do we have?

  • We have the transferrin receptor, and transferrin,

  • and this is-- the internal pH of this system is about 5-5.

  • So if you think about this, how would you--

  • how would you remove the iron from the transferrin?

  • Why might bicarbonate be there?

  • So I just told you that bicarbonate in iron

  • are bound to the transferrin.

  • Can you think of a mechanism by which that could happen?

  • X inside the cell, at lower pH?

  • We don't know the answer to this.

  • It's still open to debate.

  • So-- but what happens to the bicarbonate at low pH?

  • Think about hemoglobin.

  • Think about 5.07 and hemoglobin.

  • We spend so much time talking about bicarbonate

  • as a key player inside red blood cells.

  • What happens to bicarbonate in the presence of acid?

  • Yeah, so it forms carbonic acid.

  • What happened to the carbonic acid?

  • To CO2 in the water.

  • Yeah.

  • So this is something we banged into you

  • over and over again in 5.07.

  • There's an equilibrium that happens over and over again

  • inside cells.

  • So maybe that's a way to deliver the iron.

  • I don't know.

  • So we somehow lose iron.

  • But the iron is in the plus 3 state.

  • To get it into the cytosol, which

  • is where we're going to use it, to deliver it

  • to all of the proteins, what do we need to do?

  • Hopefully you now remember this.

  • We need to reduce it.

  • So steep is a reductase, a ferric reductase,

  • that converts this into iron 2.

  • Where did we see this guy before?

  • DMT1.

  • We've see that before as a key player in uptake

  • into enterocytes.

  • So you see these same players over and over again.

  • You see this shift from iron 2 to iron 3 over and over again,

  • actually, in yeast, where I know a lot about iron metabolism

  • as well as in human systems.

  • Now-- so we've got iron 2 out of the transferrin, transferrin

  • receptor.

  • And then the iron 2 goes into the cytosol.

  • And then we've got to figure out how to use it in a way

  • so that we don't have oxidative stress

  • and deliver it to the proteins to biosynthesize

  • all our co-factors.

  • So then the question is, remember in the LDL receptor,

  • it got recycled.

  • So what happens here is distinct from what

  • happens in the LDL receptor.

  • In that now the transferrin and the transferrin receptor

  • are both recycled.

  • So that's distinct from what we briefly

  • talked about in the case of cholesterol.

  • So we have two ways of taking iron into the cell one--

  • is through these di--

  • iron 2 transporters, the DMT molecules, and the second way

  • is through iron transfer-- iron transferrin

  • which circulates in the blood and delivers it

  • to all the tissues.

  • So these are the major mechanisms of iron delivery,

  • and recycling within the cell controlled by hepcidin,

  • this peptide hormone.

  • So what I want to do now is look at how this iron is sensed.

  • How do we control everything?

  • And iron sensing--

  • So iron sensing, there are going to be two players.

  • And so we're going to look at iron sensing.

  • And I'm going to introduce you to the two players,

  • and then I'm going to show you the general logic of how you

  • control all these proteins we've talked about-- ferritin, DMT1,

  • transferrin receptor--

  • all of these things are going to be controlled

  • by the mechanism we're going to talk

  • about now, which is regulation at the translational level.

  • So this is iron sensing by translational control.

  • So who are the two players?

  • They're written up there.

  • But we have iron responsive element,

  • and we're going to see that's a little piece of RNA.

  • So-- and I'll show you what it looks like.

  • So this is RNA, a little piece of RNA,

  • stem loop piece of RNA, that has defined characteristics.

  • I'm going to show you what it is.

  • And then we have iron responsive protein 1,

  • or iron responsive binding protein 1.

  • They're called both of these things,

  • I don't remember what was in the articles you had to read.

  • They're sort of used interchangeably.

  • And there are two of these, so there's a 1 and there's a 2.

  • And they're structurally homologous to each other,

  • and I'll tell you a little bit about each one of these.

  • So we also have a one and a two.

  • So those are the two guys.

  • These are proteins.

  • So these are proteins, that's why the name binding protein.

  • So it turns out that iron responsive binding proteins

  • are homologous to aconitase--

  • where you seen aconitase before?

  • Yeah, so in the TCA cycle.

  • It catalyzes the conversion of citrate to isocitrate.

  • So-- and where is the TCA--

  • TCA cycle located?

  • In the mitochondria.

  • So this is a TCA cycle enzyme found in the mitochondria.

  • But what we'll see is, we're working on RNA,

  • we're going to regulate somehow.

  • We're going to use interaction between this protein

  • and a piece of RNA to control the translational process,

  • where is that located in the cytosol?

  • So these proteins are located in the cytosol.

  • So if you think about what happens with aconitase,

  • let me just write that down for you.

  • So we have citrate.

  • And I asked the question, do you think

  • it's interesting that citrate is involved

  • in this overall process that I'm going to be describing?

  • What do we know about citrate, besides the fact

  • that it's an intermediate in the TCA cycle?

  • So this is citrate.

  • It undergoes a dehydration reaction.

  • So we're going to lose water to form aconitate, cis-aconitate--

  • and then it becomes rehydrated.

  • So that's the reaction you learned about a long time ago

  • in the Krebs cycle or the TCA cycle.

  • Why is it interesting that citrate is involved?

  • I don't know why it's really involved.

  • But do you think it's interesting?

  • What is citrate, if you look at the structure of it?

  • Yeah.

  • AUDIENCE: Combined iron.

  • JOANNE STUBBE: Yeah, combined iron.

  • And in fact, there are iron siderophores that use citrate.

  • I don't think this is an accident.

  • And thinking about, again, how nature uses primary

  • metabolites over and over again in ways other than what

  • you see in primary metabolism.

  • So what's unusual about this protein is the following.

  • And this is the key to the way the sensing is

  • going to work for the iron responsive binding proteins.

  • So if you look at-- if you go back and you look--

  • if you go back and you look at the Krebs cycle, or you go back

  • and you think about this, this is something

  • that probably confused you all.

  • You have an iron 3, a 4 iron 4 sulfur cluster.

  • Remember I talked a little bit about this,

  • trying to show you that this was going

  • to be highlighted later on?

  • and what we have in this 4 iron 4 sulfur cluster--

  • you have a cysteine attached to three of the irons.

  • We have one iron that's unique, OK

  • that doesn't have the cysteine that you see in normal 4 iron 4

  • sulfur clusters.

  • So this is the unique iron.

  • So if you look at that over here-- so here's

  • the cartoon of this.

  • So here you have cysteine, cysteine, cysteine in the 4

  • iron 4 sulfur cluster.

  • Here's citrate.

  • And that iron-- so most of you probably

  • learned in respiration, iron sulfur clusters

  • are involved in electron transfer.

  • They do one electron chemistry.

  • They undergo oxidation reduction, which we briefly

  • discussed in the last lecture.

  • But what's it doing here?

  • What it's doing here is binding the citrate.

  • So here's citrate.

  • This is the hydroxyl that we're going

  • to eliminate to lose water to form cis-aconitate.

  • So this is the first example.

  • But this was discovered by Helmut Beinert at Wisconsin

  • many years ago, where the iron sulfur

  • classes were doing something other than redox chemistry.

  • This is just the tip of the iceberg.

  • Remember, I talked to you about radical SAM proteins,

  • 100,000 proteins doing interesting chemistry.

  • This is the first example of this.

  • And these really are seminal experiments

  • to figure out how this all worked.

  • So the unusual thing is that most iron sulfur clusters look

  • like this, and they all have 16 on each of the iron,

  • and they do redox chemistry, but now we're

  • finding that a lot of iron sulfur clusters

  • have unique iron they can end up doing interesting chemistry as

  • well, namely binding S-adenosyl methionine.

  • So if you go back and you think about what happens,

  • this is helping dehydration.

  • So you're going to dehydrate.

  • But now you have to reorganize the thing.

  • This is one where they talk about the Ferris--

  • spinning around the Ferris wheel if you

  • look at an introductory TCA cycle thing,

  • how this reorganizes.

  • I don't think this is a very good picture.

  • But it needs to reorganize because you're

  • going to rehydrate another carbon, using the same iron.

  • So if you sit here and you stare at this, what you see

  • is this carboxylate.

  • Now, here was the initial carboxylate bound,

  • this one wasn't bound.

  • Now, this one ends up being bound.

  • And now you're adding water back across this double bond.

  • So the purpose of this system is simply

  • to catalyze the dehydration reaction.

  • So what the heck are we doing with an iron

  • responsive binding protein being a cytosolic aconitase

  • equivalent?

  • And so what I'm going to come back and tell

  • you one Friday is, this is going to be the key switch for iron

  • sensing.

  • Whether the iron is in the apostate, with no metal,

  • or whether it moves to the 4 iron 4 sulfur cluster state.

  • And we'll talk a little bit then about how

  • those two states, and the presence of RNA,

  • can control which of all these proteins

  • I've thrown at you today actually get translated.

  • OK.

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