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01.01 Hemoglobin and Myoglobin

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Overview

  1. Overview
    1. Abbreviations
      1. Hemoglobin is abbreviated as Hb
      2. Myoglobin is abbreviated as Mb
    2. Why we need Hb and Mb
      1. We use oxygen that we breathe to carry out a combustion reaction consumes that produces carbon dioxide along with the energy (i.e. ATP) we need to live
      2. Oxygen can’t penetrate into our bodies so we need transport proteins (Hb & Mb) to deliver oxygen from the air we breathe into places deep inside our bodies
      3. Hb & Mb are proteins that bind oxygen in places where oxygen is more plentiful and then travel to a place where the oxygen levels are lower, and thus let go of the oxygen they are carrying. This is what is meant by “transport”.
  2. Structures
    1. Protein structure
      1. Hb is a tetramer composed of 4 protein subunits whereas Mb is a monomer, composed of 1 protein unit
      2. The Hb and Mb monomers have very similar secondary structures
      3. For our purposes I am going to represent each monomeric group as a little blob
    2. Oxygen binding
      1. Each Hb and Mb monomer contains an iron-heme group which is where the oxygen binds
      2. Note that carbon monoxide can bind the iron-heme; however, carbon dioxide does NOT
  3. Hb & Mb Functions
    1. Starts with Hb in the lungs
      1. Hb is located in red blood cells within our blood vessels (arteries, capillaries and veins). This means Hb can go from the lungs where oxygen levels are high to tissues throughout the body where oxygen levels are lower.
      2. Mb is located inside cells, particularly muscle cells which use a lot of energy. It is more like a storage protein than an actual transporter.
    2. Oxygen dissociation curves–getting oriented on the graph
      1. The fraction of binding sites bound to oxygen (Y) is on the y-axis
      2. The concentration of oxygen in the atmosphere (pO2) is on the x-axis
    3. Hb has a sigmoidal oxygen dissociation curve
      1. The Hb in blood vessels in our lungs binds oxygen we breathe; Hb then travels through the body, eventually entering capillaries where the concentration of oxygen (pO2) is low; this causes Hb to let go of the oxygen which then diffuses into cells
      2. Hb delivers about 38% of its cargo (i.e. it doesn’t let go of all 4 oxygens)
      3. The S-shaped curve is a result of Hb’s tetrameric structure
    4. Mb has a hyperbolic oxygen dissociation curve
      1. Mb binds oxygen in cells and “delivers” oxygen to the mitochondria
      2. Mb’s curve is said to be hyperbolic.
  4. Diseases
    1. Thalassemias are caused by mutations in Hb and can result in severe anemia (low oxygen delivery)
    2. Sickle-cell disease is caused by mutations in Hb that cause Hb subunits to stick to each other in a way that creates long fibers; these fibers cause red blood cells to adopt a sickle shape
  5. Summary
    1. We breathe oxygen so that it can be used in a combustion reaction that creates energy
    2. Hb & Mb are transport proteins that bind and release oxygen depending on how much oxygen is in the environment
    3. Hb is a tetramer that resides in red blood cells; it binds up to 4 oxygen molecules: and it has a sigmoidal oxygen dissociation curve
    4. Mb is a monomer that resides in cells; it binds just 1 oxygen molecule: and it has a hyperbolic oxygen dissociation curve
    5. Mutations in Hb can cause thalassemias and sickle cell anemia

Reference Links

Video Transcript

Hey there. What we’re going to talk about today are hemoglobin and myoglobin. We’re going to look at their structures, their functions, and we’re going to look at a couple of diseases associated with hemoglobin. Before we get talking about hemoglobin and myoglobin, and why we need them, let’s give them a couple of abbreviations: HB for hemoglobin and MB for myoglobin. The basic idea is that we need energy in order to live. That’s why we breathe. So we’re breathing in oxygen and we’re creating cellular energy (ATP) from the oxygen we breathe in. And we do that through a big combustion reaction: hydrocarbons plus oxygen are going to make CO2 and water. And this is really not all that different from what happens in your car, right? You can have gas as a hydrocarbon, whereas in us, it’s the foods that we eat, but this whole process is going to make lots of energy.

But the key thing here is the oxygen that we need in order to do this in our cells…it can only penetrate a few millimeters into our bodies. And so that’s why we have to have a big system of blood vessels in order to get the oxygen all around our bodies. Hemoglobin and myoglobin are proteins that help transport oxygen to the places that we need it. And the way they do this…the idea of them being transporters…is really just that when there’s a higher concentration of oxygen, that’s when these proteins are going to bind the oxygen. And then they, each in their own way, in their own environments, are going to travel to places that are lower in oxygen. And then they just naturally are going to let go of the oxygen in those places.

Let’s start by looking at the structures of hemoglobin and myoglobin. Over here, we’ve got hemoglobin and it’s got four subunits. Two of them are in red and two of them are in blue and each of those subunits has a heme. You can see this really beautiful structure here…this is a heme and it has an iron sitting in the middle of it, and this is where the oxygen binds. So there’s four of these heme groups in the hemoglobin. Now myoglobin is similar in structure: If you were to line up these alpha helices, you would see that myoglobin has almost the same secondary structure as hemoglobin, but it’s just one single protein. So, whereas a hemoglobin is going to be four separate proteins and myoglobin is just one single protein.

Hemoglobin can bind up to four oxygen molecules. Myoglobin is only gonna bind one oxygen molecule. And before we leave this slide, I want to point out that the oxygen here–you can see it binding to that heme–carbon monoxide also binds in this place. That’s why carbon monoxide is so serious because can replace the oxygen at the heme. But do note that carbon dioxide does not bind here. That’s not how carbon dioxide leaves our bodies. And also I want to point out that I do like to draw little artistic version of hemoglobin, and that would look like this. So it’s just these four little blobs. Each one of those blobs can bind oxygen. Whereas down here, if we look at the myoglobin, it’s just one of those little blobs.
Let’s go through the process starting from the beginning. So we’ve got five steps here. S
tarting from the beginning, step one, we’re going to take a nice deep breath and bring oxygen into our lungs. We’ve got blood vessels all around our lungs. That oxygen is going to transfer across the lung tissue and get into the blood vessels. And that’s where the hemoglobin is….hemoglobin is in your red blood cells. You’re going to get the oxygen into the red blood cells, and then they are just going to travel all through your body, down through arteries and into these little capillaries and eventually making it down to the myoglobin, which is deep inside cells inside tissues. The word (prefix) ‘myo’ is for ‘muscle’….there’s lots of myoglobin in muscle because muscles do need a lot of energy….that’s why it’s there. Let’s look at step two: hemoglobin is taking the oxygen to the tissues and then eventually, the concentration of oxygen (that’s in these darker areas down in here) is pretty low.

And so the oxygen naturally just leaves the hemoglobin and diffuses into the cells. And that’s where myoglobin picks it up. We can think of myoglobin as kinda like a transport protein, like it’s taking it from the outside of the cell to the mitochondria, but I don’t think it’s quite that ‘intentional’. Myoglobin is more like a storage protein. When the cell is not low in oxygen, the myoglobin is going to be binding to the oxygen, but when the mitochondria really get up and going–like, say you’re exercising and oxygen levels get low–that’s when myoglobin would release its oxygen, so that the mitochondria can burn more oxygen. So there we are, five steps of how the process happens. If you’ve been taking some biochemistry classes, you’ll know that one of the ways that we talk about how myoglobin and hemoglobin function is through what’s called an oxygen dissociation curve.

Let’s first just try and get oriented to the graph here. The Y axis has a thing called Y on it. Capital Y–this is either considered the percent or fraction of oxygen binding sites that have oxygen bound. So one way to look at that is: let’s say we have our little hemoglobin here with its four subunits. Each of these subunits is going to be bound with an oxygen. That’s what a hundred percent means. Whereas if we popped down here at the 0%, that would mean none of those subunits has an oxygen bound, and you can imagine the same thing with the myoglobin. You can have myoglobin bound to oxygen up here, myoglobin not bound to oxygen down there. Alright, so now let’s go and look at the X axis. On the X axis, what we have here is the partial pressure of oxygen, and this is in torrs.

Over here, we’ve got 100 torr…that is the oxygen level in your lungs at sea level. And then over here would be the oxygen level in your tissue. So it’s quite a bit lower. What we’re going to see is that hemoglobin functions from the lungs over to this area, but myoglobin functions from this area down over to the ‘zero area’. Let’s first look at the hemoglobin oxygen dissociation curve. First thing notice that this has a sigmoidal shape–S–so it’s easy to remember, that’s the Sigmoidal shape. This is where you’re going to be binding the oxygen in the lungs. And then hemoglobin is going to go to about maybe right in this area here, and that’s where it just naturally lets go of the oxygen. So this right here is about 38%.

So 38% of the oxygen gets dropped off by the time it comes over here and then the hemoglobin is going to go back out to the lungs and get more. And it just keeps going back and forth like that. Myoglobin on the other hand, it has most of its change happening early on in this curve. This is the shape of it. This is not an S shape. This is hyperbolic. That’s what this is called. This is where myoglobin functions. So kind of in that tissue area, down to the zero, that is where it’s going to be doing its work. Let’s look at both of those together. You can really see the difference: here’s the hyperbolic curve and here’s that sigmoidal curve, and you can see how they each work in their own unique environment in terms of binding oxygen at high oxygen levels and then letting go of the oxygen when they get to low oxygen levels.

The last thing I want to touch on are diseases. We’ve got thalassemias…there could be like 800 different amino acid mutations in hemoglobin that can cause poor oxygen delivery. These are types of anemias and are just mutations that don’t allow the hemoglobin to function like it normally should. And that could cause people to not feel good and have weakness, et cetera. There’s also another disease that’s very important for you to know about: sickle cell disease. This is caused by a particular mutation…one of those subunits of the hemoglobin is called the beta subunit and there’s a mutation in the beta subunit that causes the hemoglobin tetramers to stick together. So you kind of get a structure that looks like this, let’s say here’s a hemoglobin and then you’ll get another one down here.

And then you get another one stuck up here and what this ends up looking like it starts to create a fiber. And then that fiber links up to another fiber and you just get lots of fibers. Normally a red blood cell would look kind of like a pillow, but having all these fibers in there changes its structure and it gets this type of shape. That’s the fibers in there. This is a sickle cell and it does not deliver oxygen properly and is apparently super painful. Sadly it’s quite a common disease. So hopefully we’ll see some kind of a cure for it soon because it’s not a nice thing to have. All right. So let’s do a little bit of summarizing here. We breathe in oxygen so it can be used for energy.

So again, this whole thing is really about energy. We’ve got these transport proteins, they bind and release the oxygen depending on how much oxygen is in the environment. And their two environments are quite different: the blood vessels in lungs for hemoglobin versus what’s inside of a cell for the myoglobin. Hemoglobin is a tetramer and it binds four oxygen molecules. Whereas myoglobin is a monomer and it binds one oxygen molecule. Hemoglobin has that sigmoidal curve. Myoglobin has the hyperbolic curve. And then the last thing to say is that hemoglobin mutations can cause thalassemias and sickle cell disease. We love you guys! Go out and be your best self today. And as always happy nursing!

 

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