This video, we're going to talk about the structure of hemoglobin.
And so we already know from our last lesson video that hemoglobin is a molecule found inside of red blood cells or inside of erythrocytes, and it's found in very large numbers. Functionally, we already know hemoglobin is important for the transport of oxygen and carbon dioxide gases. Although recall, hemoglobin plays a larger role in the transport of oxygen gas since most carbon dioxide gas is transported by the blood's plasma, not by hemoglobin. Now, structurally, hemoglobin, which again can be abbreviated as HB, is a 4 subunit protein, which means that it has 4 separate protein chains that come together to form the complete hemoglobin molecule, and this 4 subunit protein is called globin. Now this 4 subunit protein globin specifically transports oxygen gas using what are known as heme groups. And so we'll be able to see this down below in our image. But, really, this is where hemoglobin gets its name from, from the presence of the heme groups and the presence of this 4 subunit protein globin.
Now what's really important to note is that each of the 4 subunits in the hemoglobin molecule is going to have its own heme group, And so there are 4 heme groups, one for each of the 4 subunits of the hemoglobin molecule. And each heme group has its own central iron atom, or its own central Fe2+ atom, and each iron atom is capable of reversibly binding just one oxygen gas molecule. And so when hemoglobin binds to oxygen gas in this way, we refer to it as oxygen-rich hemoglobin. And so once again, because there are 4 subunits in one hemoglobin molecule, each with their own heme group and each capable of binding 1 oxygen gas molecule, what this means is that each hemoglobin molecule can actually carry up to a maximum of 4 oxygen gas molecules at once. Again, since each oxygen each subunit can carry 1 oxygen gas molecule. And so we can actually indicate this with this chemical formula that you see right here, where again the HB is the abbreviation for hemoglobin, the O2 is the chemical formula for oxygen gas, and then the 4 subscript here is indicating that there are 4 oxygen gas molecules bound to this one hemoglobin, which has 4 subunits. And so this detail here is very important to keep in mind that each hemoglobin can bind up to a maximum of 4 oxygen gas molecules.
Now again, we know hemoglobin can also bind and transport carbon dioxide gas, And so it turns out that hemoglobin can also bind up to 4 carbon dioxide gas molecules at once. And when hemoglobin is bound to carbon dioxide gas, we specifically refer to it as deoxygenated hemoglobin or carbaminohemoglobin, But what's really important to note is that hemoglobin will bind oxygen and carbon dioxide via different mechanisms. Once again, hemoglobin will bind to oxygen gas using heme groups. But the heme groups do not bind to carbon dioxide gas. Instead, the carbon dioxide gas is going to be bound via amino groups, not the heme group. So let's take a look at our image down below to get a better understanding. And notice on the left hand side, we're showing you the same exact buses that we showed you in our last lesson video, which represent hemoglobin. And notice that this red bus at the top is bound to 4 oxygen gas molecules, and so the red bus represents oxygenated hemoglobin. And the blue bus down below, notice, is bound to 4 carbon dioxide gas molecules, and so the blue bus represents deoxygenated hemoglobin or carbaminohemoglobin.
Now one thing to notice is that the deoxygenated hemoglobin is binding to the carbon dioxide gas at a different position than the oxygenated hemoglobin binds to the oxygen gas. And so this is supposed to represent that hemoglobin will bind to oxygen and carbon dioxide via different mechanisms.
Now on the right, we're focusing in on hemoglobin structure, and we know once again that hemoglobin is a 4 subunit protein. In fact, hemoglobin has 2 subunits, shown here in blue, which are alpha subunits, and the alpha can actually be written out as alpha or symbolized with the Greek letter alpha, and then it has these 2 beta subunits, and again, it can be written out as beta or symbolized with the Greek letter beta. And so the 2 alpha subunits are identical to each other, and the 2 beta subunits are identical to each other. But, of course, the alpha and beta subunits are different from one another. But regardless, all 4 of these subunits, which you'll notice, has a heme group that is present. And so notice on the right, we are zooming in to the structure of the heme group. And so it does have a pretty complex structure. And, in most cases, you wouldn't be expected to have to memorize the structure. But this structure here is the heme group, so we can go ahead and label it as so. And what you'll notice is that right in the center of the heme group, is going to be that iron atom, the Fe2+ atom. And so this iron atom is what is capable of reversibly binding oxygen gas. And so although it's not being shown, again, it's important to know that the iron interacts with the oxygen gas for binding. And so again, because each of these subunits has a heme group, that allows each one of these subunits to reversibly bind 1 oxygen, and that means that this entire hemoglobin molecule can bind a maximum of 4 oxygen gas molecules as we discussed.
And so this here concludes our brief lesson on the structure of hemoglobin, and as we move forward, we'll be able to apply these concepts and learn more. So I'll see you in our next video.