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  • I've talked a lot about the importance of hemoglobin in

  • our red blood cells so I thought I would dedicate an

  • entire video to hemoglobin.

  • One-- because it's important, but also it explains a lot

  • about how the hemoglobin-- or the red blood cells, depending

  • on what level you want to operate-- know, and I have to

  • use know in quotes.

  • These aren't sentient beings, but how do they know when to

  • pick up the oxygen and when to drop off the oxygen?

  • So this right here, this is actually a picture of a

  • hemoglobin protein.

  • It's made up of four amino acid chains.

  • That's one of them.

  • Those are the other two.

  • We're not going to go into the detail of that, but these look

  • like little curly ribbons.

  • If you imagine them, they're a bunch of molecules and amino

  • acids and then they're curled around like that.

  • So this on some level describes its shape.

  • And in each of those groups or in each of those chains, you

  • have a heme group here in green.

  • That's where you get the hem in hemoglobin from.

  • You have four heme groups and the globins are essentially

  • describing the rest of it-- the protein structures, the

  • four peptide chains

  • Now, this heme group-- this is pretty interesting.

  • It actually is a porphyrin structure.

  • And if you watch the video on chlorophyll, you'd remember a

  • porphyrin structure, but at the very center of it, in

  • chlorophyll, we had a magnesium ion, but at the very

  • center of hemoglobin, we have an iron ion and this is where

  • the oxygen binds.

  • So on this hemoglobin, you have four major binding sites

  • for oxygen.

  • You have right there, maybe right there, a little bit

  • behind, right there, and right there.

  • Now why is hemoglobin-- oxygen will bind very well here, but

  • hemoglobin has a several properties that one, make it

  • really good at binding oxygen and then also really good at

  • dumping oxygen when it needs to dump oxygen.

  • So it exhibits something called cooperative binding.

  • And this is just the principle that once it binds to one

  • oxygen molecule-- let's say one oxygen molecule binds

  • right there-- it changes the shape in such a way that the

  • other sites are more likely to bind oxygen.

  • So it just makes it-- one binding makes the other

  • bindings more likely.

  • Now you say, OK, that's fine.

  • That makes it a very good oxygen acceptor, when it's

  • traveling through the pulmonary capillaries and

  • oxygen is diffusing from the alveoli.

  • That makes it really good at picking up the oxygen, but how

  • does it know when to dump the oxygen?

  • This is an interesting question.

  • It doesn't have eyes or some type of GPS system that says,

  • this guy's running right now and so he's generating a lot

  • of carbon dioxide right now in these capillaries and he needs

  • a lot of oxygen in these capillaries surrounding his

  • quadriceps.

  • I need to deliver oxygen.

  • It doesn't know it's in the quadraceps.

  • How does the hemoglobin know to let go of the oxygen there?

  • And that's a byproduct of what we call allosteric inhibition,

  • which is a very fancy word, but the concept's actually

  • pretty straightforward.

  • When you talk about allosteric anything-- it's often using

  • the context of enzymes-- you're talking about the idea

  • that things bind to other parts.

  • Allo means other.

  • So you're binding to other parts of the protein or the

  • enzyme-- and enzymes are just proteins-- and it affects the

  • ability of the protein or the enzyme to do

  • what it normally does.

  • So hemoglobin is allosterically inhibited by

  • carbon dioxide and by protons.

  • So carbon dioxide can bond to other parts of the

  • hemoglobin-- I don't know the exact

  • spots-- and so can protons.

  • So remember, acidity just means a high

  • concentration of protons.

  • So if you're in an acidic environment, protons can bond.

  • Maybe I'll do the protons in this pink color.

  • Protons-- which are just hydrogen without electrons,

  • right-- protons can bond to certain parts of our protein

  • and it makes it harder for them to hold onto the oxygen.

  • So when you're in the presence of a lot of carbon dioxide or

  • an acidic environment, this thing is going to let go of

  • its oxygen.

  • And it just happens to be that that's a really good time to

  • let go of your oxygen.

  • Let's go back to this guy running.

  • There's a lot of activity in these cells right here in his

  • quadriceps.

  • They're releasing a lot of carbon dioxide into the

  • capillaries.

  • At that point, they're going from arteries into veins and

  • they need a lot of oxygen, which is a great time for the

  • hemoglobin to dump their oxygen.

  • So it's really good that hemoglobin is allosterically

  • inhibited by carbon dioxide.

  • Carbon dioxide joins on certain parts of it.

  • It starts letting go of its oxygen, that's exactly where

  • in the body the oxygen is needed.

  • Now you're saying, wait.

  • What about this acidic environment?

  • How does this come into play?

  • Well, it turns out that most of the carbon dioxide is

  • actually disassociated.

  • It actually disassociates.

  • It does go into the plasma, but it actually gets turned

  • into carbonic acid.

  • So I'll just write a little formula right here.

  • So if you have some CO2 and you mix it with the water-- I

  • mean, most of our blood, the plasma-- it's water.

  • So you take some carbon dioxide, you mix it with

  • water, and you have it in the presence of an enzyme-- and

  • this enzyme exists in red blood cells.

  • It's called carbonic anhydrase.

  • A reaction will occur-- essentially you'll end up with

  • carbonic acid.

  • We have H2CO3.

  • It's all balanced.

  • We have three oxygens, two hydrogens, one carbon.

  • It's called carbonic acid because it gives away hydrogen

  • protons very easily.

  • Acids disassociate into their conjugate base and hydrogen

  • protons very easily.

  • So carbonic acid disassociates very easily.

  • It's an acid, although I'll write in some type of an

  • equilibrium right there.

  • If any of this notation really confuses you or you want more

  • detail on it, watch some of the chemistry videos on acid

  • disassociation and equilibrium reactions and all of that, but

  • it essentially can give away one of these hydrogens, but

  • just the proton and it keeps the electron of that hydrogen

  • so you're left with a hydrogen proton plus-- well, you gave

  • away one of the hydrogens so you just have one hydrogen.

  • This is actually a bicarbonate ion.

  • But it only gave away the proton, kept the electron so

  • you have a minus sign.

  • So all of the charge adds up to neutral and that's neutral

  • over there.

  • So if I'm in a capillary of the leg-- let me see

  • if I can draw this.

  • So let's say I'm in the capillary of my leg.

  • Let me do a neutral color.

  • So this is a capillary of my leg.

  • I've zoomed in just one part of the capillary.

  • It's always branching off.

  • And over here, I have a bunch of muscle cells right here

  • that are generating a lot of carbon dioxide

  • and they need oxygen.

  • Well, what's going to happen?

  • Well, I have my red blood cells flowing along.

  • It's actually interesting-- red blood cells-- their

  • diameter's 25% larger than the smallest capillaries.

  • So essentially they get squeezed as they go through

  • the small capillaries, which a lot of people believe helps

  • them release their contents and maybe some of the oxygen

  • that they have in them.

  • So you have a red blood cell that's coming in here.

  • It's being squeezed through this capillary right here.

  • It has a bunch of hemoglobin-- and when I say a bunch, you

  • might as well know right now, each red blood cell has 270

  • million hemoglobin proteins.

  • And if you total up the hemoglobin in the entire body,

  • it's huge because we have 20 to 30

  • trillion red blood cells.

  • And each of those 20 to 30 trillion red blood cells have

  • 270 million hemoglobin proteins in them.

  • So we have a lot of hemoglobin.

  • So anyway, that was a little bit of a-- so actually, red

  • blood cells make up roughly 25% of all of the

  • cells in our body.

  • We have about 100 trillion or a little bit

  • more, give or take.

  • I've never sat down and counted them.

  • But anyway, we have 270 million hemoglobin particles

  • or proteins in each red blood cell-- explains why the red

  • blood cells had to shed their nucleuses to make space for

  • all those hemoglobins.

  • They're carrying oxygen.

  • So right here we're dealing with-- this

  • is an artery, right?

  • It's coming from the heart.

  • The red blood cell is going in that direction and then it's

  • going to shed its oxygen and then it's

  • going to become a vein.

  • Now what's going to happen is you have this carbon dioxide.

  • You have a high concentration of carbon dioxide in the

  • muscle cell.

  • It eventually, just by diffusion gradient, ends up--

  • let me do that same color-- ends up in the blood plasma

  • just like that and some of it can make its way across the

  • membrane into the actual red blood cell.

  • In the red blood cell, you have this carbonic anhydrase

  • which makes the carbon dioxide disassociate into-- or

  • essentially become carbonic acid, which

  • then can release protons.

  • Well, those protons, we just learned, can allosterically

  • inhibit the uptake of oxygen by hemoglobin.

  • So those protons start bonding to different parts and even

  • the carbon dioxide that hasn't been reacted with-- that can

  • also allosterically inhibit the hemoglobin.

  • So it also bonds to other parts.

  • And that changes the shape of the hemoglobin protein just

  • enough that it can't hold onto its oxygens that well and it

  • starts letting go.

  • And just as we said we had cooperative binding, the more

  • oxygens you have on, the better it is at accepting

  • more-- the opposite happens.

  • When you start letting go of oxygen, it becomes harder to

  • retain the other ones.

  • So then all of the oxygens let go.

  • So this, at least in my mind, it's a brilliant, brilliant

  • mechanism because the oxygen gets let go just where it

  • needs to let go.

  • It doesn't just say, I've left an artery and

  • I'm now in a vein.

  • Maybe I've gone through some capillaries right here and I'm

  • going to go back to a vein.

  • Let me release my oxygen-- because then it would just

  • release the oxygen willy-nilly throughout the body.

  • This system, by being allosterically inhibited by

  • carbon dioxide and an acidic environment, it allows it to

  • release it where it is most needed, where there's the most

  • carbon dioxide, where respiration is occurring most

  • vigorously.

  • So it's a fascinating, fascinating scheme.

  • And just to get a better understanding of it, right

  • here I have this little chart right here that shows the

  • oxygen uptake by hemoglobin or how saturated it can be.

  • And you might see this in maybe your biology class so

  • it's a good thing to understand.

  • So right here, we have on the x-axis or the horizontal axis,

  • we have the partial pressure of oxygen.

  • And if you watched the chemistry lectures on partial

  • pressure, you know that partial pressure just means,

  • how frequently are you being bumped into by oxygen?

  • Pressure is generated by gases or molecules bumping into you.

  • It doesn't have to be gas, but just molecules

  • bumping into you.

  • And then the partial pressure of oxygen is the amount of

  • that that's generated by oxygen molecules

  • bumping into you.

  • So you can imagine as you go to the right, there's just

  • more and more oxygen around so you're going to get more and

  • more bumped into by oxygen.

  • So this is just essentially saying, how much oxygen is

  • around as you go to the right axis?

  • And then the vertical axis tells you, how saturated are

  • your hemoglobin molecules?

  • This 100% would mean all of the heme groups on all of the

  • hemoglobin molecules or proteins have bound to oxygen.

  • Zero means that none have. So when you have an environment

  • with very little oxygen-- and this actually shows the

  • cooperative binding-- so let's say we're just dealing with an

  • environment with very little oxygen.

  • So once a little bit of oxygen binds, then it makes it even

  • more likely that more and more oxygen will bind.

  • As soon as a little-- that's why the slope is increasing.

  • I don't want to go into algebra and calculus here, but

  • as you see, we're kind of flattish, and

  • then the slope increases.

  • So as we bind to some oxygen, it makes it more likely that

  • we'll bind to more.

  • And at some point, it's hard for oxygens to bump just right

  • into the right hemoglobin molecules, but you can see

  • that it kind of accelerates right around here.

  • Now, if we have an acidic environment that has a lot of

  • carbon dioxide so that the hemoglobin is allosterically

  • inhibited, it's not going to be as good at this.

  • So in an acidic environment, this curve for any level of

  • oxygen partial pressure or any amount of oxygen, we're going

  • to have less bound hemoglobin.

  • Let me do that in a different color.

  • So then the curve would look like this.

  • The saturation curve will look like this.

  • So this is an acidic environment.

  • Maybe there's some carbon dioxide right here.

  • So the hemoglobin is being allosterically inhibited so

  • it's more likely to dump the oxygen at this point.

  • So I don't know.

  • I don't know how exciting you found that, but I find it

  • brilliant because it really is the simplest way for these

  • things to dump their oxygen where needed.

  • No GPS needed, no robots needed to say, I'm now in the

  • quadriceps and the guy is running.

  • Let me dump my oxygen.

  • It just does it naturally because it's a more acidic

  • environment with more carbon dioxide.

  • It gets inhibited and then the oxygen gets dumped and ready

  • to use for respiration.

I've talked a lot about the importance of hemoglobin in

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B1 中級

血紅蛋白 (Hemoglobin)

  • 48 4
    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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