Did you ever wonder how red blood cells can store enough oxygen to supply the entire human body? Join us in this lesson on hemoglobin to learn why red blood cells are so good at transporting oxygen and carbon dioxide. This lesson will make you see red (as well as blue and purple-maroon)!
What is Hemoglobin?
You may recall that hemoglobin is an iron-containing protein that can bind to either oxygen or carbon dioxide. Hemoglobin belongs to a class of proteins known as respiratory pigments, which are metal-containing proteins that are used to bind and transport oxygen and carbon dioxide through the circulatory system. But why do we have so much of this protein in our red blood cells? Well, it is true, oxygen and carbon dioxide are soluble in blood, but the amount that can be in solution is limited, which would limit the effectiveness of the blood to transfer these gases, especially oxygen, if it weren’t for hemoglobin.
Let’s just look at oxygen for right now. When blood is saturated with oxygen, the oxygen binds to the hemoglobin molecule. When this happens, it essentially becomes part of the hemoglobin and is taken out of solution, which creates a vacancy for more oxygen in solution. Think of it like artwork in a museum. In a museum, there is a limited amount of space to display artwork. When museums acquire more artwork than they can display at one time, they store some of the artwork away for use at a later time. In this way, a museum may be able to acquire and preserve a collection that is many times the size of the collection that can be displayed at once. The same is true for oxygen in the blood. If the blood can store oxygen by using hemoglobin to bind it, then many times more oxygen can be transported by the blood than could be in solution at one time.
But that’s only half the problem, because not only does hemoglobin need to store the oxygen, but it also has to be able to release it into the blood at just the right time so that it is available to the cells that need it. So how does the hemoglobin ‘know’ when to bind oxygen and when to release it? The answer is that hemoglobin is constructed in such a way that it takes its cues from the conditions and concentrations of oxygen and carbon dioxide around it. Hemoglobin is a tetramer composed of four nearly identical subunits, each of which contains an iron atom at a specific site called the heme group, which is the part of the hemoglobin protein that can bind an oxygen molecule. Each hemoglobin subunit is capable of changing its shape, or conformation, between two basic forms, depending on a variety of factors. In one conformation, oxygen has easy access to the heme group and can easily bind. This conformation of hemoglobin is called the relaxed conformation, or R-structure for short. In the second conformation, oxygen cannot easily bind to the heme group. This conformation of hemoglobin is called the tense conformation, or T-structure for short.
When oxygen binds to a heme group, that hemoglobin subunit adopts the R-structure and causes the other three subunits to adopt the R-structure too. The result is that when hemoglobin binds one molecule of oxygen, it will usually bind three more almost instantly. This ability of one subunit to facilitate the binding of oxygen to the other subunits is called cooperativity, the ability of one subunit of a protein to positively influence the activity of another subunit of the same protein.
So, in an oxygen-rich environment, like the capillaries of the lungs, oxygen can be rapidly taken out of solution and stored in hemoglobin because of cooperativity. As blood enters capillaries in tissues that are actively using oxygen, the low concentration in the tissues draws dissolved oxygen out of the blood and into the tissues, leaving very little oxygen dissolved in the blood. Without a high concentration in the blood, oxygen bound to hemoglobin is free to go back into solution in the blood. In addition, active tissues have a high concentration of carbon dioxide, because it is produced as a waste product of cellular respiration. Hemoglobin can also bind to carbon dioxide when it is in the T-conformation, but a single subunit cannot bind to carbon dioxide and oxygen at the same time, so the oxygen gets released by the hemoglobin subunit as it changes to the T-structure and binds to carbon dioxide. As a result of the cooperativity of hemoglobin, once one subunit binds carbon dioxide and adopts the T-structure, the other three subunits will also change to the T-structure and release any oxygen that they hold. In this way, the release of oxygen is facilitated not only by the lack of oxygen in the bloodstream, but also by the high concentration of carbon dioxide in active tissues and the cooperativity of the hemoglobin subunits.
The iron in hemoglobin is the primary reason that hemoglobin is red in color, and in turn, why blood itself is red, but another feature of hemoglobin is that it changes color when it changes its conformation. The R-structure is bright red in color, and the T-structure is darker and more purplish in color. Why then, you might ask, is it that whenever you get cut and bleed, the blood is always red? The answer is that when we bleed, the blood is exposed to the air, and the oxygen in the air enters the blood and binds to the hemoglobin, causing it to adopt the bright red R-structure. The only time we see blood when the hemoglobin is in the T-structure is in our intact veins. Go ahead and look at the inside of your wrist or forearm. Do you see those blue to purplish blood vessels? Those are veins with oxygen-poor blood in them. The hemoglobin in these veins is in the T-conformation and appears more blue to us rather than purple because we’re looking at it through our skin and the walls of our veins.
Let’s review: Hemoglobin is an iron-containing protein that can bind to either oxygen or carbon dioxide. The hemoglobin in red blood cells acts like a massive oxygen storage facility that takes oxygen out of solution in the blood, allowing yet more oxygen to enter the blood from the lungs. The end result is that the blood transports over thirty times more oxygen than could be in solution at one time. Hemoglobin is a tetramer composed of four nearly-identical subunits, each of which contains an iron atom at a specific site called the heme group, which is the part of the hemoglobin protein that can bind an oxygen molecule. Each hemoglobin subunit is capable of changing its shape or conformation between two basic forms, the relaxed or R-structure and the tense or T-structure. Oxygen binds easily to the R-structure, and when one subunit of the tetramer binds to oxygen it causes the other three to change to the R-structure as well. This positive reinforcement for oxygen binding is called cooperativity, the ability of one subunit of a protein to positively influence the activity of another subunit of the same protein. In oxygen-poor and carbon dioxide-rich tissues that are using lots of oxygen, this cooperativity works in the reverse direction, when carbon dioxide binds to hemoglobin and forces it to change all four subunits into the T-structure and release all of its oxygen.