Each red cell is donut shaped and carries many copies of a special protein called hemoglobin. Proteins come in three varieties, "globular", "membrane", and "structural". The "globin" in hemoglobin indicates hemoglobin is a globular protein. Globular proteins can be crystallized to determine their structure. There are over 250 million hemoglobin molecules inside the average red cell. Can you guess what "hemo" means?
Each of the 250 million hemoglobin molecules carries four oxygen molecules to the hungry cells that need it. That means that one red cell can carry a billion oxygen molecules! Lets look more closely at the hemoglobin molecule: Can you see the rings that carry the oxygens?
The molecule above is called the hemoglobin tetramer. "Tetra" means it consists of four similar parts or "mers". You might have heard the word "dimer" which means "two parts", or the word "polymer", which means "many parts". In sickle cell disease, a small change in the hemoglobin enables the tetramers to stick together in long rods or polymers. This spells trouble! More on that below.
It turns out that each hemoglobin molecule actually consists of four "monomers" that look something like this:
Each one of the smaller molecules can carry one oxygen. If you look carefully you can see a small ring molecule in the lower part of the figure. It is that little ring that holds the oxygen molecule with the help of an iron molecule right in the center.
If we zoom in a little further we can see the ring itself. This ring is called called a "porphyrin ring". It is the iron molecule held by this ring that gives blood its red color. A similar porphyrin ring in plants holds a magnesium which gives leaves their green color. So the job of the hemoglobin protein is to hold this porphyrin ring, and the job of the porphyrin ring is to hold an iron atom. The job of the iron atom is to hold one molecule of oxygen for hungry cells. Isn't it amazing that such a large protein as hemoglobin has the job of carrying such a small molecule as oxygen to the cells where it is used in cellular respiration. During cellular respiration cells convert the fuel of sugar or glucose into pyruvate. Pyruvate supplies the energy to convert ADP to ATP and this is where cells get their energy. But we're getting ahead of ourselves...
Hemoglobin and Sickle Cell Anemia
Everyone is a unique individual. Yet they have five fingers on each hand. Even though the hand has five fingers, each finger has its own unique shape and fingerprint. So everyone's fingers are a little different but they still work fine for picking things up and dropping things off. Hemoglobin is the same way. Proteins like hemoglobin can be a little different from person to person and still work just fine. But sometimes hemoglobin can be just different enough to cause problems.
We call everyday, vanilla hemoglobin by the name hemoglobin A. Since the word hemoglobin A is so long, we shorten it to HbA save time. HbA is the normal variety. Most people have HbA as their hemoglobin.
There are many variations of HbA that work well for picking up oxygen in the lungs and dropping it off in the tissues. Everybody is born with their own hemoglobin type, just like they are born with their own fingerprint. The information for how to make hemoglobin is stored in the genes. There is one kind of hemoglobin that can make life difficult however.
This type of hemoglobin that makes some people sick is called hemoglobin S, where S stands for "sickle". We will call it HbS for short. It looks like this, and you can see it is "goofed up" compared to normal hemoglobin.
Hemoglobin S has a very annoying habit of forming those polymers we talked about above, long tubular rods. This tends to happen at the worst time and in the worst place: in the capillaries: when and where the cells are hungriest for oxygen. These long rods of HbS are very stiff, like a fishing pole sticking into the side of a pup tent. They form as shown in the picture below, and grow very long until they distort the cell.
When red cells sickle they can't squeeze through the capillaries anymore. They start sticking to the walls and to each other and clogging up the plumbing. This is called a sickle cell crisis and it can be very dangerous.