Biochemical basis of the Bohr effect - and other ways Hemoglobin O2-binding is affected (by CO etc.)
Hemoglobin’s the reason your blood can take oxygen from your lungs to your toe & the Bohr effect helps hemoglobin know when & when not to let go! I hope it wouldn’t bore you if I discuss the Bohr effect - it’s the reason the hemoglobin in your blood chooses whether to oxygen bind or eject! Yawning? Then you’re taking in oxygen which hemoglobin will take to tissues in need, those with lots of CO₂ but little O₂. And it’s thanks to the Bohr effect that this molecular magic it can do!
blog form (originally posted a couple years ago, but I’m busy with some very non-boring stuff): http://bit.ly/bohreffect • Cooperative binding & Allostery - Hem...
The textbook-y explanation, and then a deeper, bumblier look. The Bohr effect describes how low pH (acidity) lowers the affinity of hemoglobin for oxygen, making hemoglobin more likely to offload oxygen in areas of low pH, which for reasons I’ll get into, tissues in need of oxygen tend to have. So how does it work? Well first, what is hemoglobin?
Hemoglobin is this protein in your blood that picks up oxygen from your lungs and takes it all the way to your toes. We’ve talked about it a lot in terms of when it doesn’t work - like in sickle cell disease where mutations cause it to clump up and block blood vessels, cutting off circulation to tissues and organs, and causing pain and organ damage. http://bit.ly/sicklecelldiseases
But we haven’t talked much about hemoglobin when it does work. So that’s what I want to tell you about today - how hemoglobin “knows” when to take or give up oxygen - and how we know how it “knows.” It’s a really cool protein - it has 4 subunits which can each take an oxygen and they work as a team so that one binding makes it easier for the others to bind and one letting go makes it easier for the others to let go. This is called cooperativity - and we’ll talk more about what causes it in a bit. But this kind of take-all or dump-all approach with respect to oxygen means that it has to be tightly regulated so that it doesn’t all get dumped too soon. And the hemoglobin doesn’t just retake what it dumps. And, as we’ll see, the Bohr effect helps with both of these.
Often “respiration” is used to describe breathing, but biochemists often talk about respiration in terms of the processes that take place in your cells to use the oxygen (O₂) you get when you breath to make energy. The basic idea of cellular respiration is that you breathe in oxygen, combine it with breakdown products of things like glucose (blood sugar) to make the energy storage molecule ATP, and generate carbon dioxide (CO₂) that you breathe out as a waste product in the process. That has to occur in cells all throughout your body. So oxygen has to be able to get to all of those cells. And you have CO₂ being produced in all of those cells. But you only have oxygen entering your body in one place - your lungs.
Your lungs might not seem that big, but they use their allocated space wisely. They have a branching structure with the branches ending in tiny grape-like parts called alveoli which are covered with tiny little blood vessels called capillaries. They’re tiny in terms of diameter, but huge in terms of surface area, so they can let lots of oxygen in. It’s an easy route for free oxygen to get in - it just has to diffuse through the cell membrane, which is easy for it because it’s so small. Diffusion’s basically just random molecule moving - it leads to a net movement of molecules from where they’re more cramped (areas of high concentration) to places where they’re less cramped (areas of low concentration) until the concentration is equal everywhere. The molecules still move around randomly but because their movement is random, for each molecule “moving left” there’s another one “moving right” so there’s no net movement and no net change in concentration anywhere once a system reaches equilibrium.
When it comes to reporting concentrations of gases, people frequently talk in terms of partial pressures. The “partial” comes from the fact that when you have a gas or a mixture of gases, the total pressure is proportional to the number of gas molecules, not their identity (e.g. either 1000 CO₂ gas molecules OR 1000 O₂ gas molecules would produce the same pressure). So, instead of counting individual gas molecules, you can get information about how many gas molecules there are by measuring the pressure. And since the identity of the gas doesn’t matter, you can add the pressure contribution of different gases (their partial pressures) together to get the total pressure (e.g. a mixture of 1000 CO₂ gas molecules AND 1000 O₂ gas molecules TOGETHER would have a total pressure proportional to 2000 gas molecules).
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