Oxygen Binding Curve for Myoglobin and Hemoglobin

inside our body we have two different proteins myoglobin and 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 two proteins contain heem groups and it's the heem group that is responsible for actually binding and holding on to the oxygen now now in myoglobin we have a single polypeptide chain and so we have a single hem group but in hemoglobin because we have four different polypeptide

chains we have four different heem groups now one heem group can bind one oxygen and what that means is a single hemoglobin can bind four times as many oxygen molecules as myoglobin can now if hemoglobin combined 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 oxy oxygen while the hemoglobin is actually used to bring the oxygen continually from the lungs into 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 Des cries 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 curve and so we can use O2 binding curve or o to dissociation curve interchangeably these me these two terms mean the same exact thing now on the graph the Y AIS 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 it 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 a gas we commonly describe the concentration of oxygen by using the partial pressure of oxygen so P2 and this is given in either millimeters of mercury or t these two units have the same exact quantity so one Tor is equal to 1 millim of mercury so on this particular graph we begin at 0 millimet of mercury and end up at 100 mm 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 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's 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 2 m

MIM of mercury so let's take our marker out so at this quantity here which is about 2 mm 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 value of 0.5 fractional saturation of protein so at a value of 2 mm of mercury a very small amount of o oxygen 50% of all the My globin in our body in that mixture will contain oxygen bound to the heem group of the myoglobin and this means that myoglobin binds to

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 dis associating 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 mlob 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 it turn turns out that myoglobin has these properties because it only consists of a single polypeptide chain because it consist of only a single chain it only consists of a single hem 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 myoside 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 qu to a very small value and when it drops below a certain value ultimately all these myoglobin molecules in the 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 uh unlike myoglobin the curve for hemoglobin is not as sharp in fact we have this s-shaped curve and this s shaped is known as the sigmo shaped 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 2 mm 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 point five Mark and we draw a vertical line down this will give us uh a partial pressure of about 26 mm of mercury so a concentration of 26 mm of mercury is needed for exactly half of those hemoglobin molecules to become saturated to bind that oxygen compared to the 2 mm of mercury for the myoglobin and what that means is that hemoglobin binds oxygen much less likely than the my globin and so it has a lower affinity for oxygen than my globin and once

again if we read the curve backwards so going this way the protein is binding the 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 steadily than myoglobin and that's exactly what makes hemoglobin much better at carrying the oxygen from the lungs and releasing it at the tissues and 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 into the tissues and cells of our body and only when the hemoglobin isn't able to actually bring enough oxygen to our cells only then the myoglobin begin begin to release that oxy to the muscle cells of our body so the sigmoidal curve is produced as a result of hemoglobin's ability to bind oxygen in a Cooperative fashion so earlier we said that myoglobin binds oxygen in a non-cooperative fash fion 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 heem group at one side on the hemoglobin makes the other unoccupied sides much more likely to bind oxygen so what that means is the different hem groups inside our hemoglobin actually interact with one another another and so for example if we have a hemoglobin that contains fully unoccupied sides when one of those he groups binds oxygen that will make the other three unoccupied sides 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 four hem 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 o oxygen so what that means is at at the lungs when our hemoglobin is in the lungs it can easily bind that oxygen but when it gets the cells

and tissues of our body it has no problem actually unloading and releasing that oxygen to the cells of the tissues of our body so we conclude that myoglobin does not bind oxygen cooperatively and this basically makes it a great storing molecule F so it stores the oxygen in the muscle cells of our body because it only releases it when the concentration drops to very low value on the other hand because the hemoglobin consists of these four individual polypeptide chains these polypeptide chains the Hem groups can interact with one another and they can basically induce the

release or The Binding of the oxygen and so this means it 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 hemoglobin's Cooperative Behavior makes it a great oxygen carrier it can readily bind that O2 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 mobin that is used as the oxygen carrier inside the red blood cells of our body