239Oxygen Transport – Protein Functions

239Oxygen Transport – Protein Functions


Now, another important thing to be transport 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 as hemoglobin to carry the oxygen to those tissues Now, there’s some important consideration
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 a oxygen carrying capacity of a protein The protein Myoglobin is showing in blue and Hemoglobin is showing in green Now, Myoglobin is also an oxygen carrying
protein It’s 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 on to 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 what we described is 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 for a Myoglobin we see the curve rising very sharply to begin with And at about two millimeters of mercury 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 not gonna let go 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 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 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 – the 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 a hundred percent Meaning that it’s almost the same as Myoglobins and this makes very good sense as well because when hemoglobin is in an environment when 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 sub-unit and hemoglobin has four protein sub-units and because of those four protein sub-units The sub-units 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 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 is passed through the tissues and giving the oxygen away It’s drawn in 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’re starting to become rounded The binding of one oxygen has favored changes in the adjacent proteins such as – 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 the third and the binding the third favors the fourth So because of this hemoglobin gets loaded up with oxygen as its passing though the lungs and then the process reverses as it goes back out through the tissues It’s a remarkable flexibility and 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

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