Oxygen Transport – Protein Functions

by Kevin Ahern, PhD

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    00:01 chylomicrons.

    00:01 Now another important thing to be transported in the body is that of oxygen. Oxygen of course is necessary for respiration, and as will be discussed in another module, oxygen is necessary for efficient production of ATP. Muscles are very, very dependent upon oxygen, when you're exercising for example. Well if we think about what happens in the body, the lungs are very well prepared to handle oxygen there, but leaving the lungs and going to tissues that are far away, it's important to have a system, the system that we use is hemoglobin, to carry the oxygen to those tissues. Now there are some important considerations about what hemoglobin has to do in that process, and so part of that is illustrated in this slide here. What you see on the screen is a plot of the oxygen saturation or you can think of it oxygen carrying capacity, of a protein. The protein myoglobin is shown in blue and hemoglobin is shown in green. Now myoglobin is also an oxygen-carrying protein.

    01:07 It is not really used much for carrying though, it's used mostly for storing oxygen in muscle cells. So in muscle cells what myoglobin does, is it grabs oxygen when it's available and it holds onto it. And when the oxygen concentration gets really, really low, myoglobin lets go of it and gives it to the muscle. So myoglobin acts a bit like that we describe as an oxygen battery, a battery of course provides electricity when the electricity isn't there. The myoglobin provides oxygen when the oxygen isn't there. If we look at the oxygen binding tendencies from myoglobin, we see the curve rising very sharply to begin with, and at about 2 mmHg, we see that myoglobin is 50% saturated with oxygen, meaning it takes very little to get saturated but that also means the flip side is it's not going let go of very much until the oxygen is almost completely gone. Well that's good if the cell is desperate, but cells don't like to live in a desperate state all the time. Hemoglobin by contrast, has a different profile for binding oxygen than myoglobin does. We can see that hemoglobin's curve is shifted considerably to the right and instead of being a hyperbola, as we see from myoglobin; the hemoglobin curve is what we call sigmoidal. It looks like an S-shape.

    02:34 The S-shape curve for hemoglobin indicates that it actually has two different binding affinities for oxygen. At low oxygen concentrations, such as we find in tissues, the affinity that hemoglobin has for oxygen is low, that means that when hemoglobin is traveling through tissues, it's letting go of oxygen much more easily than myoglobin was doing. That’s the gap between the two curves near the bottom. As we move to the upper right, we see that hemoglobin's affinity for oxygen is almost up to 100%, meaning that it's almost the same as myoglobin's. And this makes very good sense as well, because when hemoglobin is in an environment where there's a lot of oxygen, you want it to bind to oxygen tightly, and that's what happens as hemoglobin is traveling through the lungs. So hemoglobin changes depending upon the environment in which it finds itself. How does this change actually happen? Well it turns out that hemoglobin can do something that myoglobin can't do. Myoglobin has one protein subunit, and hemoglobin has four protein subunits. And because of those four protein subunits, the subunits can affect each other, after one oxygen has been bound. Now this is a phenomenon known as cooperativity and I'll show this in the next slide.

    03:52 In the next slide we can see hemoglobin starting on the left, where it's already dumped off its oxygen. This might be a hemoglobin for example, that has passed through the tissues and given the oxygen away. It's drawn as squares because as squares, hemoglobin has very little affinity for oxygen. Moving to the right, we see first of all that one of the squares has converted to a blue circle that now contains oxygen. This could happen for example as the hemoglobin is approaching the lungs. The square has converted to a circle, and the circle you can see is interacting with two other portions of the hemoglobin that were previously squares, they are starting to become rounded. The binding of one oxygen has favored changes in the adjacent proteins such that they are much more likely to bind oxygen than they were before, so the binding of one oxygen favors the binding of a second oxygen, and the binding of a second, favors a third, and the binding of a third favors the fourth.

    04:55 So because of this hemoglobin gets loaded up with oxygen as it is passing through the lungs and then the process reverses as it goes back out to the tissues. It's a remarkable flexibility and a remarkable ability of hemoglobin to make this process happen. Myoglobin can't do it because it has a single unit, and there's no communicating that it can possibly do.

    About the Lecture

    The lecture Oxygen Transport – Protein Functions by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. Sigmoidal curve
    2. Hyperbolic curve
    3. Parabolic curve
    4. Elliptic curve
    5. J-shaped curve
    1. The myoglobin displays two different oxygen binding affinity patterns in the body; near the lungs it has the high affinity of oxygen; but in muscle cells, it shows the lowest affinity for oxygen.
    2. The hemoglobin displays the positive cooperativity phenomenon in the presence of high concentrations of oxygen
    3. The myoglobin oxygen binding follows the rectangular hyperbolic curve, whereas hemoglobin oxygen binding follows a sigmoidal curve pattern
    4. The hemoglobin participates in the transportation of oxygen from lungs to the tissues, whereas myoglobin stores the oxygen in the muscle cells
    5. The hemoglobin is composed of four subunits, whereas myoglobin is comprised of only one subunit

    Author of lecture Oxygen Transport – Protein Functions

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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