oxygen hemoglobin dissociation

[Dencology]

hey guys. i had a question on Destroyer Bio#242. how come A and E could not be the right answer? Obviously, as the pH decreases the blood becomes more acidic and hemoglobin would dump more oxygen (oxygen saturation becomes lower for hemoglobin) so obviously the curve would shift to the right as well and it will become linear.

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[creative8401]

Hi Dencology. This is a tough one. Let me try to explain.

In order to understand this problem, you need to be aware of the structure of hemoglobing. Hemoglobin, yes an "enzyme" you can say, is not simply composed of a single polypeptide chain. It is as a matter of fact consisting of 4 tetramers bound together. Hemoglobin has a special property - it is an allosteric enzyme. What does that mean? An allosteric enzyme increases its binding affinity towards a molecule as the number of that molecule increase. For example, when a single oxygen molecule binds to the first subunit on hemoglobin, hemoglobin increases its affinity for the 2nd oxygen molecule. Once the 2nd one binds, hemoglobin increases its affinity to the 3rd oxygen molecule, etc...until four oxygen molecules have attached. Great! (This is why carbon monoxide poisoining is so dangerous)

Now lets go to the choices. First, choice E will not work because hemoglobin has a cooperativity of binding. It does not have a 1:1 relationship, but simply increases its affinity as explained above. Linear graph curves are best for enzymes that follow a Michaelis-Menton curve. In other words, their binding affinity is not related to the number of molecules bonded to the enzyme.

Choice A is very poorly written. It is like saying "when you give someone money, you will have more money"...makes no sense. Because hemogloin is an allosteric enzyme, it has a sigmoidal curve, which is shown in the graph. It already is sigmoidal, and rather impossible to change from hyperbolic to sigmoidal. Again hyperbolic graphs follow a Michaelis-Menton curve.

Your understanding for the shift in the curve is correct. Hope this makes more sense.

Thanks.

 

[userah]

so when it says "curve will shift to the right" does it mean that the curve will become more angled while still maintaining the sigmoidal...ness? lol sorry might be poorly phrased, i'm just trying to understand what the answer choice means by saying the curve will shift to the right...

 

[creative8401]

Hi Userah. =) I totally understand your question. Let's take a look at it.

When it says shift to the right, that means the entire graph moves right. In other words, imagine the line as being a hard rigid metal line (you cant bend it at all!), lol, and when you shift to the right, the entire curve is displaced to the right. That means every point moves in the same proportion. (This is my best explanation, if you want a visual, check this out: http://www.bio.davidson.edu/Courses/anphys/1999/Dickens/Oxygendissociation.htm

When the line shifts to the right, it means that at a given condition, the amount of oxygen present in the blood will be higher. What are these conditions?

1) An increase in temperature causes a shift to the right. Why? Remember that proteins are sensitive to temperature, a too high or too low a temperature, will affect their structure. When you increase the temperature of the body, you can think of it as the hemoglobin molecule becomes a bit unstable in its structure and no longer can carry the oxygen molecules affectively.

Other conditions include a rise in H+ concentration which means that the amount of carbon dioxide levels are high. When CO2 is high, the body wants more delivery of oxygen, hence the curve is shifted to the right.

Hope this clears it up a bit. Let me know.

 

[userah]

ooooo okay. that makes a lot of sense now. thanks for clarifying so quickly =). i'm currently sitting with my schaum's book out and going over everything again haha.