SCIENCE OF HEMOGLOBIN
PETER STARR NORTHROP
If you’ve got a minute—I want to take you on one of the most important journeys that happens in your body every second of every day.
It’s the journey of this goofy little shape-shifting molecule with a goofy little shape-shifting name; and the way it works is so mind blowing and so surprisingly easy to understand that it might be one of the most beautiful ways of demonstrating the profound and nearly artistic mechanics of your body’s smallest inner workings.
Today folks, I’d really like you to learn about HEMOGLOBIN, the shape-shifting wizard in your blood.
So yea, when you start out learning Biology—so much of what you memorize is the WHAT of Bio that it’s easy to gloss over the WHY.
For instance, pretty much everyone knows that your blood carries delicious, life-giving Oxygen to your muscles and tissues and deadly, gross Carbon Dioxide away from them. This happens mostly because Red Blood Cells are chock-full of this amazing protein called Hemoglobin that makes your blood red, your poop brown and all sorts of other amazing stuff.
Without hemoglobin, every cell in your body wouldn’t have the oxygen it needs to turn food into energy (so you’d die) and poisonous levels of CO2 would build up in all your organs pretty much immediately (so you’d die). So its a pretty important part of your day-to-day, right?
But wait—how the heck does Hemoglobin even know which cells to drop off oxygen at and which ones to suck up carbon dioxide from? How can a single molecule be such a genius that it can do two of the most important chemical process in our body at once?
Well, the answer is one of perfectly organized chemistry, and it’ll give you a quick glimpse into how gorgeously designed even the smallest systems in your body are.
So, Hemoglobin is a protein, right? It takes up about 35% of the volume in each of your Red Blood Cells. Just in case you’re not with me: a protein is a biological molecule made up a tightly folded chain of amino acids. Because there’s 21 kinds of amino acids they can be built from Proteins can get huge and have really complicated shapes.
A protein’s shape determines its function. If a protein isn’t exactly the right shape, it can’t do its job at all. And, since proteins are so complicated—their shape can screw up really easily if the temperature around them shifts or their pH goes all outta whack or whatever. Protein’s are powerful enough to be the very thing that you are built out of—but individually they’re even more delicate than snowflakes.
That’ll be important later—wait for it.
So take a look at this guy. Hemoglobin is built out of four subunits, each centered around an Iron compound called a heme group (hence the name). That’s how I remember what the structure is: hemaglobin is a globe’a hemes.
Oxygen binds to an Iron ion in the middle of the heme group because each subunit of hemoglobin makes a little dimple the perfect shape for carrying an O2 molecule. The image to the right is the actual chemical structure, but that’s way too complicated to conceptualize right now.
Think of a partially deflated beach ball with four big’ol dimples pushed into it. In Hemoglobin’s normal state, it can readily take up 1 oxygen molecule (O2) in each little dimple. And since there’s four of those— you get four oxygen molecules for each little bit of Hemoglobin.
So, when Hemoglobin is in its normal state—it picks up 4 O2 molecules in the pulmonary arteries near your lungs—and then it treks off to find areas that desperately need that sweet, sweet Oxygen to help keep your cells respiring.
And this is the cool part.
See, when your body uses oxygen to make ATP during Respiration, the chief by product is Carbon Dioxide. So, as your cells use up more and more oxygen, more and more CO2 builds up in your cells.
But isn’t just CO2 bouncing around all gas like and random in your intercellular fluid—CO2 dissolves in water to form Carbonic Acid, thanks to an enzyme chilling in your blood stream. Actual bubbles of CO2 in your blood would be ULTRA BAD NEWS. Just ask anyone you know who’s ever gotten the bends while scuba diving.
But there’s a problem: This acid builds up in the fluid in your cells and starts lowering the pH of EVERYTHING. This is also no good, because your body has LUDICROUSLY SPECIFIC pH needs. If the Carbonic Acid kept building up, it would start breaking everything happening nearby.
However, this acid seeps into your blood—where all your red blood cells carrying all your hemoglobin are hanging out.
And DONT FORGET: Even the slightest change of pH can change the shape of a protein—especially one as big and fancy as Hemoglobin.
Now, Carbonic acid is a pretty-weak acid—but its just strong enough that it MESSES UP the shape of Hemoglobin. The acid makes hemoglobin shrivel and collapse in on it self a tiny bit, making those dimples in it WAY larger. This size change allows the oxygen molecules to escape from the iron in the dimples. So, oxygen BUSTS out of your red blood cells, and rushes down its concentration gradient like an NFL running back tripping on jet fuel—straight into the cells that need it.
AT THE SAME TIME, Hemoglobin is now a shape that looks pretty cool to any undissolved CO2 molecules in your blood and Carbonic acid—so the tips of each Hemoglobin’s 4 subunits soak up 1 CO2 molecule each, making a totally NEW molecule with a totally awesome name: Carbaminohemoglobin.
It’s such a lyrical name that I can’t help almost singing it: “Carb-a-MEAN-O-hee-MUH-glo-BIN” (!)
But ANYWAY: At the same time, Hemoglobin can help mop up the Carbonic acid as well—effectively acting as a buffer for your blood! This stuff is also called ACID Hemoglobin. ‘Cause its Hemoglobin plus a little acid. Cool!
Hemoglobin holds on to the CO2 and carbonic acid until it gets back to your lungs, where the pH is back to normal. Hemoglobin WAY prefers to bind with oxygen at normal blood pH, so it shoves off those freeloadin’ CO2 punks, scrunches back up into its regular shape, and picks up a fresh batch of oxygen.
So, in the areas where your body needs Oxygen, there will be enough CO2 to lower the pH of your blood enough to change Hemoglobin from an Oxygen-loving protein to a Carbon Dioxide moving powerhouse. In areas where there’s a bunch of orxygen (read: Your lungs), Hemoglobin can change into the oxygen-loving champ it always was. This is SUPER CONVENIENT, considering that the lungs are the exact spot where your body needs to offload all its excess CO2.
It all works because Hemoglobin is the exact perfect shape for it to do this job. Its all in the structure. Don’t forget: in biochemistry, structure determines function.
This is a pretty fundamental principle to how a lot of biochemistry works. Your body is this elegant system of perfectly shaped proteins and hormones and fatty acids and phospholipids and its like a giant puzzle where all the pieces fit together so perfectly that it allows us to do all the amazing things we do in life—all while living off of nothing but air, water and pizza pockets.
I mean, we didn’t go through billions of years of evolution for nothing, folks!
This is the fact that bound me forever to science—the fact that made my cynical 19 year old punk self believe in magic again. That’s the power of studying science: if you take the time to study truly in depth on any subject—you can find magic and glory even in the smallest bits of your blood.