oxygen affinity of hemoglobin vs. myoglobin

[SaintJude]

2 questions:
Hemoglobin is composed of 4 heme binding subunits, while Myoglobin only consists of one heme binding subunit. I keep on reading that myoglobin has a stronger oxygen affinity than hemoglobin. What could account for this? (Maybe this is a minor detail, that's not necessary for the mcat? I don't know)

And also, Kaplan states that tissues during rest have a lower oxygen affinity than tissues during exercises (the affinity curve is further to the right). Why is that?

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[Erythropoietin]

2 questions:
Hemoglobin is composed of 4 heme binding subunits, while Myoglobin only consists of one heme binding subunit. I keep on reading that myoglobin has a stronger oxygen affinity than hemoglobin. What could account for this? (Maybe this is a minor detail, that's not necessary for the mcat? I don't know)

And also, Kaplan states that tissues during rest have a lower oxygen affinity than tissues during exercises (the affinity curve is further to the right). Why is that?

At the tissues, Hb releases its O2 to Mb. If Mb had a lower affinity for O2, it wouldn't be able to bind O2 at the tissues strongly. Similarly, if Hb didn't have a lower affinity for O2 at the tissues, it would keep its Hb instead of releasing it. Therefore, Mb has a higher affinity for O2.
Muscles during rest don't contract, so they don't really need O2. When you are exercising, your muscles are contracting. Contraction requires ATP, and ATP production requires O2 (electron transport chain). So when exercising, you need more ATP and therefore O2.
Fetal Hb is different than adult Hb. Mother delivers O2 to the fetus through the placenta. Therefore, fetal Hb needs to have a higher affinity for O2 than adult Hb, so it can bind to it better. (If adult Hb had a higher affinity than fetal Hb, it would less likely release its O2.) As a result, fetal Hb curve appears more on the left side (stronger affinity) than the adult Hb curve.

Hope this helps.

 

[SaintJude]

I agree that muscles during rest don't contract, and thus don't need that much O2 (supposedly?). So then wouldn't oxygen affinity to hemoglobin be higher not lower than tissues during exercise?

Why would the sigmoidal curve of "tissues during rest" be to the right of "tissues during exercise"?

I included a picture below
View attachment Picture 8.png

 

[BenzylAcetate]

I agree that muscles during rest don't contract, and thus don't need that much O2 (supposedly?). So then wouldn't oxygen affinity to hemoglobin be higher not lower than tissues during exercise?

Why would the sigmoidal curve of "tissues during rest" be to the right of "tissues during exercise"?

I included a picture below
View attachment 18395

The quality of "affinity" describes how much of a substance binds to the substrate given a certain amount of substrate outside of the tissue. (In this case oxygen (from the blood) binding to the tissue) A higher affinity means that there will be more binding, conversely lower affinity means that there will be less binding (given the same amount of "atmospheric" substance (in this case oxygen in the blood))

If the curve is shifted to the right this means that a smaller amount of oxygen will bind (to the tissue) for a given concentration of oxygen in the blood. So it makes sense that the curve describing muscle in a resting state, which has a lower affinity for oxygen, would be to the right of the curve for muscle in the active state.

Edit: The x-axis (partial pressure of Oxygen) represents the oxygen outside of the tissue, this is the pool from which the tissue can draw oxygen. For example, if we want our tissue to have a saturation of 40%, if the curve is shifted to the right it means that a greater amount of oxygen outside of the tissue is required in order to have 40% oxygen saturation inside the tissue. Requiring a greater pool of oxygen to draw from in order to maintain a similar level of oxygen inside the tissue means that the tissue has a lower affinity.

 

[Druggernaut]

I agree that muscles during rest don't contract, and thus don't need that much O2 (supposedly?). So then wouldn't oxygen affinity to hemoglobin be higher not lower than tissues during exercise?

Why would the sigmoidal curve of "tissues during rest" be to the right of "tissues during exercise"?

Elevated temperature, acidosis, and increased carbon dioxide shift the curve to the right. All three are present in exercising muscle tissue.

Think about what increased hemoglobin oxygen affinity (a left shifted curve) means: the hemoglobin will hold onto oxygen better. You don't want the blood to hang onto oxygen when it's passing through the tissues that need it, you want it to unload it into those tissues. A right shifted curve does this.

 

[SaintJude]

The quality of "affinity" describes how much of a substance binds to the substrate given a certain amount of substrate outside of the tissue. (In this case oxygen (from the blood) binding to the tissue) A higher affinity means that there will be more binding, conversely lower affinity means that there will be less binding (given the same amount of "atmospheric" substance (in this case oxygen in the blood))

If the curve is shifted to the right this means that a smaller amount of oxygen will bind (to the tissue) for a given concentration of oxygen in the blood. So it makes sense that the curve describing muscle in a resting state, which has a lower affinity for oxygen, would be to the right of the curve for muscle in the active state.

Edit: The x-axis (partial pressure of Oxygen) represents the oxygen outside of the tissue, this is the pool from which the tissue can draw oxygen. For example, if we want our tissue to have a saturation of 40%, if the curve is shifted to the right it means that a greater amount of oxygen outside of the tissue is required in order to have 40% oxygen saturation inside the tissue. Requiring a greater pool of oxygen to draw from in order to maintain a similar level of oxygen inside the tissue means that the tissue has a lower affinity.

Yes! That makes sense. Your "edit" made all the difference. 👍 Thank you!
So the graph is indicating that resting tissues has a greater oxygen pull that the tissue can draw from.

 

[Ssina]

loooool cheesy but helps me visualize stuff!

 

[Erythropoietin]

I agree that muscles during rest don't contract, and thus don't need that much O2 (supposedly?). So then wouldn't oxygen affinity to hemoglobin be higher not lower than tissues during exercise?

Why would the sigmoidal curve of "tissues during rest" be to the right of "tissues during exercise"?

I included a picture below
View attachment 18395

So I don't think this image is quite correct (or someone disprove me) for the following reasons:
The graph is showing Hb-saturation on the y-axis. If the curve is on the right side, that means the tissue requires more oxygen. During exercise, metabolic rate goes up and the body requires more O2 use. As a result we produce more CO2 and lactic acid and our body temperature goes up; all of these three reasons (low pH, high temp, more CO2) cause the Hb-curve to shift to the right.
So where is the inconsistency in this thread I wonder?

Edit: I also wanted to add in the following sentence from my previous post:
Muscles during rest don't contract, so they don't really need O2. When you are exercising, your muscles are contracting. Contraction requires ATP, and ATP production requires O2 (electron transport chain). So when exercising, you need more ATP and therefore O2.

 

[dranet]

I couldn't see if anyone actually answered your first question. The difference between Myoglobin and Haemoglobin with regards to oxygen affinity is Haemoglobin behaves allosterically with regard to oxygen whereas Myoglobin does not..Allosteric being an important word in enzyme kinetics meaning "other located site" from the active site of the enzyme.
At high oxygen tensions e.g Lungs, the more oxygen loaded onto the Haem unit the greater is the affinity for more oxygen binding. The reverse is true in low oxygen tensions site, the more it unloads the more it wants to unload-loss of affinity. All of this is due to the changes in quaternary structure of Haemoglobin that occur each time oxygen is loaded or unloaded from the haem sub units. The sub units interact with eachother in this process. The result is a sigmoidal curve. Myoglobin does not behave that way. It has a different quaternary structure with a single oxygen binding site that is composed of Fe 2+, oxygen binds it becomes Fe3+ and there it ends its oxygenation because it can't do it with that Fe3+ there. However Fe3+ is smaller than Fe2+ so it has room to bind O2. You get a hyperbolic curve which results in a the opposite picture at low oxygen tensions than what you see in Haemoglobin. Myoglobin is in striated muscle to service the intracellular needs of these busy cells (cardiac and skeletal). It snatches oxygen away from the major O2 transport protein Haemoglobin and keeps it bound tightly specifically for the lower ranges of O2 that are found within cellsthat you don't get in the ECF. Striated muscles need a lot of oxygen so its their selfish fail safe to make sure they have reserves rather than just depending upon passer by oxyhaemoglobin. I tried to find what actually makes it off load but it looks like it is just that low oxygen tension in the ICF . The Fe remains as Fe3+ so it won't bind O2 again after it just released it and it is the cytochrome b5 that reduces it back to Fe2+ so it can pick up oxygen. Hope that helps. I bold typed some words that are MCAT topics. So you can see it is infact a good question that helps you revise some other things.

 

[Druggernaut]

So I don't think this image is quite correct (or someone disprove me) for the following reasons:
The graph is showing Hb-saturation on the y-axis. If the curve is on the right side, that means the tissue requires more oxygen. During exercise, metabolic rate goes up and the body requires more O2 use. As a result we produce more CO2 and lactic acid and our body temperature goes up; all of these three reasons (low pH, high temp, more CO2) cause the Hb-curve to shift to the right.
So where is the inconsistency in this thread I wonder?

There's no inconsistencies. At the far right of the curve, they show the lungs. This is where the partial pressure of oxygen is highest, both in tissue and the blood. At the left side of the curve, you have hypoxic tissue (person exercising). You need to consider the pressure gradient. Hemoglobin doesn't unload oxygen in the lungs in the pulmonary circulation because the tissue it's in contact with has equal or greater oxygen pressure. Contrast that with hypoxic tissue (depicted with a pressure of about 20 mm Hg, much lower than the lung), and the partial pressure of oxygen in blood can be much lower and still be unloaded to tissues. This is why it's on the left side of the figure, and has lower values for the x- and y-axes: oxygen can be unloaded to the tissue at a lower hemoglobin saturation and lower oxygen partial pressure.