Protein Ligand Interactions

Protein Ligand Interactions


Hi my name is Caitlyn. I am an SI leader for BCH4024 at UF. And today we’ll be examining the protein ligand interactions. Specifically in the context of hemoglobin and myoglobin binding to oxygen, which should be from your notes. So first we should examine why we have this interaction between oxygen, the ligand, binding to a protein, like hemoglobin or myoglobin. So, in order for you body to function, you undergo metabolism. For metabolism to occur, your body needs oxygen. However, oxygen is a nonpolar molecule. Whereas water is a polar molecule. This means that oxygen is not soluble in water, and thus, would not be soluble in your bloodstream. Instead, we have to have a protein carry oxygen to your muscles in your body that are undergoing metabolism. Now to do that, we need hemoglobin. So, as I’ve written up, hemoglobin will carry oxygen from you lungs to the peripheral tissues of your body. From there, it dumps the oxygen into the tissues to your myoglobin. Similarly, myoglobin is located in the muscle and I think an easy way to remember that is to think of the “myo” prefix of “myoglobin,” which should sound similar to “myosin.” A protein that’s located in your tissues of your muscles. As you should have learned in Biology 2. All right, so now we will examine the structures of our proteins of interest which are, hemoglobin and myoglobin. Now, what both of these structures have in common is this 8-helix polypeptide. The first thing to note is that hemoglobin is a tetramer, which means it has 4 subunits, and each subunit consists of this 8-helix polypeptide. So it has 4 of those, and you’ll see your notes that it kind of looks like a little cube, where in each corner there is a polypeptide subunit. Myoglobin, however, is a monomer. Meaning that it only has one 8-helix polypeptide structure. It only has one, and no more than that. Now for you tetramer and hemoglobin you’ll note that 2 of them are alpha subunits, and 2 are beta. You’ll definitely want to remember that for the exam. Now, as far as cooperativity, goes, you’ll see in hemoglobin, there’s positive cooperativity. This means that as each consecutive oxygen molecule binds to the protein, it increases either the affinity or the availability of those subunits. Of oxygen to bind to the protein. However, myoglobin, since it only consists of 1 unit, there’s nothing for it to “cooperate” with. So in this case, we’ll note that there is not cooperativity of myoglobin to oxygen. This is reflected in both of the curves for each protein. You’ll see that a sigmoidal curve or the s-shaped is characteristic of hemoglobin. Now this shows that as partial pressure of oxygen increases, the binding of oxygen to protein increases dramatically as we start increasing the partial pressure of oxygen.