Cooperativity

00:02 Now, I'm going to show you schematically here what actually happens with that process I've just described to you.

00:08 Hemoglobin can exist in what are called "two states".

00:11 A T-state, which is described as the tight state and R-state, which is described as the relax state.

00:17 And on this figure, the T-state is shown in squares and the R-state is shown as circles.

00:24 Well, what's the difference between the two? The difference is that the T-state does not tend to bind oxygen very well at all.

00:32 Whereas the R-state binds oxygen really well.

00:37 What happens when hemoglobin gets to the lungs if it's in the T-state? It's in state that we see on the left.

00:44 Four units, none of which like to bind oxygen very well.

00:49 Well, the oxygen concentration in the lungs is relatively high.

00:53 In fact, it's very high compared to what it was in the rest of the body.

00:56 It's so high, in fact, that oxygen forces its way onto one of those units that doesn’t want to bind oxygen.

01:04 Well, that's depicted in the second figure over.

01:07 We see the blue circular form.

01:09 That is that molecule or that protein of globin that got changed in structure by the binding of the first oxygen.

01:17 So that change in structure changed it from a T-state into an R-state.

01:23 And we can see that as a result of that change, that blue circle is interacting with two other subunits.

01:29 And those two other subunits have started to change their shape, meaning that the change in shape of the first one has been communicated to the other two subunits touching it.

01:40 Well, because they have had their shape changed slightly, each of those subunits is more like an R than it was like a T.

01:50 The result being that they will tend to bind oxygen much more readily.

01:54 And we can see that happening in the third, fourth and fifth that this change in shape is being communicated across the overall molecule.

02:05 So this process I have just described to you is a very important process that's happening inside of our lungs.

02:12 We're going from a non-oxygenated state to a fully oxygenated state in very short amount of time.

02:19 And the binding of the first oxygen was difficult.

02:23 The binding of the second was less difficult.

02:25 And the binding of the third was less difficult.

02:27 It got easier the further we went along.

02:30 That process that I just described to you is known as cooperativity.

02:35 And cooperativity is important because hemoglobin has to get that oxygen quickly and it has to fill itself up when it encounters the oxygen in the lungs.

02:45 As hemoglobin is leaving the lungs, it's fully in the R-state and it's full of oxygen.

02:53 Now, this graph, I'm a biochemist so I have to show graphs.

02:57 This graph shows the binding of oxygen for hemoglobin compared to myoglobin.

03:03 Remember that myoglobin is like hemoglobin but it only has one subunit, whereas hemoglobin has four, two alphas and two betas.

03:12 If we look at the graph, we see on the Y axis that the oxygen saturation of the molecule being studied is plotted.

03:20 On the X axis is plotted the concentration of oxygen that each of these units is placed in.

03:26 And we can see that myoglobin has a very different binding curve for oxygen than hemoglobin does.

03:32 We see that very low concentrations of oxygen as shown in the left part of the graph.

03:38 Myoglobin has a pretty high percentage of oxygen bound to it.

03:43 If we compare that to hemoglobin, we see that at the same concentration oxygen where myoglobin has 50%, hemoglobin has much less oxygen bound.

03:55 Maybe 10% or less of its oxygen is bound When we compare that to the high oxygen concentrations seen on the right, we see that both curves have about 100%.

04:07 Now, what this tells us is that myoglobin grabs oxygen really readily and holds on to it, and hemoglobin's affinity for oxygen changes as a function of the concentration of oxygen that's found in the cell.

04:22 Now, that's important because at high oxygen concentrations, you want hemoglobin to load up such as found in the lungs.

04:30 At lower oxygen concentrations such as found in the rest of the cell, we want to oxygen to let go of that oxygen and give that oxygen to cells.

04:39 This graph shows very clearly that hemoglobin is much better suited for delivering oxygen than myoglobin is.

04:48 So what's myoglobin good for? Well, myoglobin is what we call an oxygen storage protein and it's found in our muscles.

04:56 That’s the myo- part of the myoglobin.

04:59 Myoglobin is really good at grabbing oxygen when it's available and holding onto it for muscles when the muscles really need oxygen.

05:08 So, when you're out running that marathon or you're doing that really fast race, your muscles very quickly run out of oxygen and your muscles can -- need oxygen faster than your blood can supply it.

05:22 Well, that's where you want to have a backup supply of oxygen and that's where myoglobin comes in.

05:26 Because as the muscles get to very low oxygen concentration, that's when myoglobin gives up its oxygen.

05:33 It's good for those emergency situations, but it's not good for regular day-to-day activities.

05:39 Hemoglobin is much better.

05:41 Now, myoglobin's curve has a shape that we call hyperbolic as you can see on the label on the graph, whereas hemoglobin's curve is called sigmoidal, and that is sort of S shaped.

05:51 And these distinct shapes really tell us a lot as I said about the differences of the binding of myoglobin and hemoglobin for oxygen.

05:59 Now, I want to show you a principle that shows in the graphs I'm going to show you in a minute that I want you to keep in mind.

06:05 As we see graphs like this one moving from the left, myoglobin, to the right for the same thing for hemoglobin, what this means is that the molecule that corresponds to the graph on the right is exhibiting less affinity for oxygen than the graph for the molecule on the left.

06:23 So, at higher oxygen concentrations, it takes more oxygen to get hemoglobin bound with oxygen then it does myoglobin.

06:33 And at low oxygen concentrations, myoglobin is great at holding on to it.