inside our body we have two different proteins myoglobin in hemoglobin that carry out the function of bringing oxygen to the cells of our body where that oxygen can then be used in a process called cellular respiration aerobic cellular respiration to produce ATP molecules now previously we said that these su proteins contain heme groups and it's the heme group that is responsible for actually binding and holding on to the oxygen now in myoglobin we have a single polypeptide chain and so we have a single heme group but in human globin because we have four different polypeptide chains we have four different heme groups now one heme group can bind one oxygen and what that means is a single hemoglobin combine four times as many oxygen molecules as myoglobin can now if hemoglobin can bind more of these oxygen molecules why do we need myoglobin molecules in the first place and in general why does our body need to use two different proteins to carry out the same function of bringing the oxygen to the cell well as it turns out because these two proteins have slightly different structures they have slightly different properties and therefore slightly different functions as we'll see in just a moment myoglobin is used to store the oxygen while the hemoglobin is actually used to bring the oxygen continually from the lungs and to the tissues of our body and we'll see exactly why that is so in just a moment now in biochemistry we use something called the oxygen binding curve also known as the oxygen dissociation curve to basically describe the properties of myoglobin and hemoglobin and this is the graph shown on the board so the blue curve basically describes the myoglobin oxygen binding curve while the red curve describes the hemoglobin oxygen binding curve so going this way it's a binding curve going backwards it's a dissociation and so we can use Oh to buy the curve or Oh to the cessation curve interchangeably these meet these two terms mean the same exact thing now on the graph the y-axis describes the fractional saturation of the protein it tells us what fraction of the total number of proteins in our mixture is fully saturated is bound to that oxygen and ranges from a value of zero to a value of one where zero basically means none of the proteins contain oxygen while one means 100% of the proteins all the proteins are bound to oxygen now the x-axis describes the concentration of the oxygen and because oxygen is against we commonly describe the concentration of oxygen by using the partial pressure of oxygen so po2 and this is given in either millimeters of mercury or Torr these two units have the same exact quantity so one Torr is equal to one millimeter of mercury so on this particular graph we begin at zero millimeters of mercury and end up at 100 millimeters of mercury and as we'll discuss in the next lecture a value of 100 mmHg describes the partial pressure of oxygen inside our lungs while a value of 40 millimeters of mercury describes the partial pressure inside our resting tissue and we'll discuss much more of that in the next lecture so let's actually begin by describing what the meaning of these two curves are and let's begin with the blue curve that describes the oxygen binding curve for myoglobin notice for myoglobin we have a simple binding curve and what that means is it shows that as we begin to increase the concentration ever so slightly there is a sharp increase in that curve until it levels off and then essentially becomes flat now what that means is as soon as we begin to add a tiny amount of that oxygen into our mixture all that oxygen begins to bind onto the myoglobin because the myoglobin has a very high affinity for that oxygen it binds that oxygen strongly and that's why we have the sharp increase in that curve immediately after we begin increasing the concentration of that oxygen so we see that at a partial pressure of two millimeters of mercury so let's take our marker out so at this quantity here which is about two millimeters of mercury if we basically draw a straight vertical line and eventually we hit the curve we hit the curve at this value and this value describes a valley of 0.5 fractional saturation of protein so at a value of two millimeters of mercury a very small amount of oxygen 50% of all the my globin in our body in that mixture will contain oxygen balance of the heme group of the myoglobin and this means that myoglobin binds two oxygens very readily it has a very high affinity for oxygen and it also means the following so going this way the oxygen is binding onto the myoglobin but going this way the oxygen is dissociating notice as we go this way as we decrease the concentration of that oxygen that myoglobin remains bound to that oxygen and the myoglobin only begins to unload or release that oxygen when the partial pressure drops to a very low quantity and it essentially unloads it all together all the my globe and unload the oxygen very quickly and together when we drop that partial pressure to a certain value so myoglobin does not release oxygen until the partial pressure drops to a very low quantity now what is the physiological meaning behind this well what this means is our body can actually use the myoglobin for oxygen storage to basically store the oxygen until it really really needs it when the cells have a very low concentration of oxygen and that's exactly why myoglobin is the protein that is used by the muscle cells of our body to basically store the oxygen until that moment when we have very little oxygen found inside our body when the lungs can no longer deliver the oxygen to the tissue of our muscle effectively and efficiently now why does my globin observe this behavior well it turns out because it turns out that my globin has these properties because it only consists of a single polypeptide chain because it consists of only a single chain it only consists of a single heme group and what that means is it does not bind oxygen cooperatively and we'll see exactly what that means in just a moment so what is the physiological consequence of the of this property of myoglobin well these properties of myoglobin as described here makes it a perfect molecule to store the oxygen inside our cells in fact myoglobin and that's why it's called Myo Myo means myocyte or muscle cell myoglobin is used to store oxygen inside our muscle cells myoglobin only releases that oxygen to the muscle cells when the concentration drops to a very small quantity very small value and when it drops below a certain value ultimately all these myoglobin molecules in a Cell will release that oxygen together and that's exactly what the sharp increase or decrease in that blue curve actually means now let's move on to hemoglobin notice unlike hemoglobin the unlike my globe and the curve for hemoglobin is not a sharp in we have this s-shaped curve and this S shape is known as the sigmoidal shape curve and what the sigmoidal shape curve basically means is the binding affinity of hemoglobin for oxygen is much smaller than that for myoglobin so we said earlier that a concentration of only two millimeters of mercury is needed to actually bind 50% of that myoglobin to oxygen now in the case of hemoglobin if we draw a straight line from the 50% mark from the 0.5 mark and we draw a vertical line down this will give us a partial pressure of about 26 millimeters of mercury so a concentration of 26 millimeters of mercury is needed for exactly half of those hemoglobin molecules to become saturated to bind that oxygen compared to the two millimeters of mercury for the myoglobin and what that means is that hemoglobin binds oxygen much less likely than the myoglobin and so it has a lower affinity for oxygen than myoglobin and once again if we read the curve backwards so going this way the protein is binding to oxygen but going backwards the protein is dissociating that oxygen and so if we go this way we see that our hemoglobin releases that oxygen much more readily than myoglobin and that's exactly what makes hemoglobin much better at carrying the oxygen from the lungs and releasing it efforts issues in cells of our body and that's exactly what the function of hemoglobin is so myoglobin is used to actually store that oxygen in the muscle cells while hemoglobin is used to actually carry the oxygen from the lungs through the bloodstream and to the tissues and cells of our body and only when the hemoglobin isn't able to actually bring enough oxygen to our cell only done thus my globin begin begin to release that ox into the muscle cells of our body so the sigmoidal curve is produced as a result of hemoglobins ability to bind oxygen in a cooperative fashion so earlier we said that myoglobin binds oxygen in a non cooperative fashion while hemoglobin binds it cooperatively so what do we mean by that and why does that actually occur well this means that the binding of oxygen at one heme group at one side on the hemoglobin makes the other unoccupied sites much more likely to bind oxygen so what that means is the different heme groups inside our hemoglobin actually interact with one another and so for example if we have a hemoglobin that contains fully unoccupied sites when one of those heme groups binds oxygen that will make the other three unoccupied sites much more likely to bind an oxygen and conversely the release of oxygen from one side on the hemoglobin molecule makes the other occupied sites much more likely to unload those oxygen so because hemoglobin consists of these four polypeptide chains the for heme groups found on the four different chains can actually interact together they can cooperate with one another to basically either release or bind that oxygen manner that oxygen molecule in a cooperative fashion now what this means physiologically as I said earlier it makes the hemoglobin a great carrier of oxygen so what that means is at at the lungs when our hemoglobin is in the lungs it can easily by that oxygen but when it gets the cells and tissues of our body it has no problem actually unloading and releasing that out into the cells of the tissues of our body so we can food that myoglobin does not bind oxygen cooperatively and this basically makes it a great storing molecule so it stores the oxygen in the muscle cells of our body because it only releases it when the concentration drops to a very low value on the other hand because the hemoglobin consists of these four individual polypeptide chains these polypeptide chains the heme groups can interact with one another and they can basically induce their release or the binding of the oxygen and so this means it it makes it a perfect molecule to act as a carrier of oxygen inside our blood system so that's exactly why hemoglobin is used by our body to basically take that oxygen in the lungs and bring it to the tissues of our body so we see that hemoglobin cooperatively uh we see that hemoglobins cooperative behavior makes it a great oxygen carrier it can readily bind that o to in the lungs and has no problem releasing that oxygen inside the tissues of our body and we'll discuss in much more detail what the physiological consequence is of hemoglobin and myoglobin in the next lecture we're going to see exactly why it's hemoglobin and not myoglobin that is used as the oxygen carrier inside the red blood cells of our body