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Getting Oxygen Where It’s Needed

26 Jun

Oxygen is required by many organisms for survival, luckily it is plentiful in the air, but how does it get into all the tiny cells all over the body?

periodicFirst, Oxygen is a highly electronegative atom. This means that it attracts electrons very well and can pull them away from other molecules. Only one other atom is more electronegative and that’s the most reactive element in the periodic table, Fluorine. Electronegativity becomes useful biologically because electrons are capable to storing energy that can be passed along from one molecule to the next. But , to do this, each molecule must be more electronegative than the last. Therefore, it is not surprising that Oxygen is used as the final electron acceptor in the electron transport chain of cellular respiration. This reaction is required by many organisms, and can be highly beneficial even to some organisms that can live without Oxygen. An electron transport chain is a process by which molecules in a membrane pass en electron down the line using its energy in a controlled way to extract even more energy from sugar.

But how does Oxygen get to the cells that need it? Two molecules account for much of this action, myoglobin and hemoglobin, both illustrated below:

Image

Hb- hemoglobin, Mb-myoglobin.

What this image also elegantly portrays is the amazing similarity between the molecules that belies their evolutionary relationship.

Each of these molecules is capable of binding Oxygen, but each occurs in a different tissue. ImageHemoglobin is present in Red Blood Cells, making them capable of transporting oxygen from the lungs to other tissues. Myoglobin occurs in these ‘other’ tissues, particularly muscle cells.

Despite the fact that they both bind oxygen, they do not bind it equally well under all conditions. While hemoglobin is just as good at binding Oxygen in high concentration environments (like the lung after inhalation), it is not as good at retaining oxygen when found in less Oxygen-rich environments (such as tissues like muscle).  Under these conditions, myoglobin is much better at binding Oxygen and can pull the molecules away from hemoglobin.

Recently, several groups have published new data about how subtle differences amongst proteins involved in Oxygen transport through blood and muscle result in different binding properties. In turn, the variability in these properties underlie the amazing diversity of lifestyles found in nature, from humans to birds to giant whales capable of holding their breath for up to an hour.

Image That brings us back to the electron transport chain and Oxygen’s electronegativity, where Ois used as a ‘magnet’ for the electron traveling down the pathway from one molecule to the next. As it goes it loses some of its energy, which is converted into a new form that the cell can use. Once the electron, now at a lower energy state, gets to O2, the Oxygen splits  and takes up a Hydrogen ion to form water.

So, electronegativity and binding affinity are the forces that both transport Oxygen through the body and pulls electrons from one molecule to another. Together, the movement of electrons, like that of water through a mill, powers processes that lead to the synthesis of ATP, the energy currency of the cell (see below).

 

electron-transport-chain-cpg-notes

Note the electron traveling down the chain (in pink)

Given what we’ve discussed here, how do you think a baby ever gets to pull the Oxygen away from its mother’s blood / hemoglobin?

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6 Comments

Posted by on June 26, 2013 in Uncategorized

 

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6 responses to “Getting Oxygen Where It’s Needed

  1. ratabago

    July 10, 2013 at 1:58 am

    A nice overview. Your closing question caused a flashback in my aged brain: Dr Wallace winning the hearts and minds of every female student in his second year biochemistry class by referring to “That most efficient of all parasites, the Human foetus.”

    Spoiler Alert: My answer to the question follows.

    If I remember rightly foetal haemoglobin has a stronger affinity for Oxygen than normal haemoglobin at low partial pressures. At low partial pressure for Oxygen adult Hb has an increased affinity for 2,3-diphosphoglycerate at the 2,3-DBG binding site (I seem to recall something about that being due to a conformational change on binding carbon-di-oxide). On binding 2,3-DPG Hb undergoes a conformational change, sterically hindering Hb’s ability to bind Oxygen. Foetal Hb has an amino-acid substitution (serine replacing histadine?) in the 2,3-DBG binding site, reducing the positive charge in that site, with a reduced affinity for 2,3-DPG, and therefore having a higher affinity for Oxygen at low partial pressure.

    Well, that’s a bit foggy, but thanks for the mental workout and nostalgia binge.:)

     
  2. downhousesoftware

    July 10, 2013 at 7:49 am

    Exactly. And, although we might not like to think of the fetus as a parasite, viewing it that way makes its relationship with the mother all the easier to understand.

    An interesting condition occurs when some people fail to turn off their fetal hemoglobin production due to a number of causes. What’s interesting is that there is no real condition associated. The only noticeable difference is that this may occur in places where diseases such as malaria are common, and that this persistence may help protect the individual from disease.

    Something I found (or, did not find) was that there does not appear to be any problem in pregnancy associated with a mother’s persistent fetal hemoglobin. One would expect that if both mother and child are making fetal hemoglobin, this would make it more difficult for the fetus to survive owing to its lack of advantage in binding oxygen. This may be the case, but I found no record of it in a quick search.

     
    • ratabago

      July 10, 2013 at 10:09 am

      That is interesting, and raises a few questions. I’ve just spent a little time trying to find info on problems with persistent foetal haemoglobin and pregnancy, and also got nowhere. But I do know there is an Oxygen concentration gradient across the placenta, with concentrations lower on the foetal side. Foetal blood also usually has a higher red blood cell count than adult blood.

      So I went and looked up proportions of HbF typical in thalassemia minor, and for sickle cell anaemia. The Journal for the American Hematology Society’s hematology library gives a range for HbF in African Americans suffering sickle cell anaemia of 5% to 8%. The Mayo Clinic gives 5% to 15% HbF in thalassemia minor. They also give reference values for HbF in neonates as 22.8-92.0%. So I’m speculating that there may be enough spare capacity, so to speak, that the higher proportion of HbF in the foetus may be sufficient that nobody has noticed any problems.

      But I do wonder if in these circumstances:
      1/. The foetus produces a higher than normal proportion of HbF throughout the pregnancy?
      2/. And/or a still higher red blood cell count?
      3/. If there is something else unusual about the maternal blood, say maybe a lower pH than usual, that helps compensate?
      4/. If there really is a higher rate of miscarriage in these cases, and it has been masked by other problems (or I just missed them in my quick search)?
      5/. If this becomes more of a concern with increased altitude?

      (And as a pointless aside, my spell checker keeps trying to internationalise the spelling for The American Hematology Society. It is kind of annoying).

       
      • downhousesoftware

        July 10, 2013 at 10:23 am

        Thanks for taking the time to looks that up and share it, ratabago. I was wondering about the proportion of HbF in these people too, so I’m glad you found those data. #3 hadn’t occurred to me, but that could be an interesting solution. It may also be a similar (I suppose, inverse) change in the fetus, such that its HbF has the more favorable environment for binding.
        I had thought about the miscarriage question, but I agree that it would be hard to see unless it was very dramatic.
        The altitude question is a good one. I know that animals that live at high altitude have Hb that binds O2 ‘better’ (at lower partial pressure).
        Further, I found something that suggests that Sherpas living at high altitude have shifted O2 dissociation curves – I’ll post that graph on a new post. Interestingly, the advantage goes away and they have lower than normal O2 binding capacity at sea level, leaving them anemic.

        I’ll have to incorporate some of this into my fall lecture. I’m so glad you wrote and then followed up.

         
  3. stampdxer

    June 10, 2016 at 3:12 pm

    MOOC I’m watching says there are 4 protein complexes in the chain..

     
    • downhousesoftware

      June 10, 2016 at 3:31 pm

      Dear Stampdxer:

      Thank you for the comment, that’s a good catch. I had scavenged the figure in my post mostly because it clearly depicted the required role of oxygen without paying much attention to the protein complexes. In the diagram above, complex II is de-emphasized – it’s where FADH2 enters (succinate dehydrogenase). Now that you point it out, I may go and find a better illustration, or use my powerpoint drawing from my own lectures, to make sure everything gets properly represented.
      -Jack

       

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