hemoglobin and myoglobin
/ HEE-muh-glow-bin, MY-uh-glow-bin /
Every breath you take ends with oxygen being handed to a single iron atom buried inside a protein. Hemoglobin, the red pigment of blood, picks up oxygen in your lungs and ferries it to every tissue; myoglobin, packed into muscle, takes that oxygen and stores it until the muscle needs it. Both are built around the same chemical gadget: an iron ion held in a flat ring called a heme, and the deep red of blood and the dark red of steak both come from this iron-heme unit absorbing light.
The heme is a porphyrin — a large flat organic ring with four nitrogen atoms pointing inward — and it grips an iron(II) ion in the center like a hand holding a coin. Iron normally wants six neighbors in an octahedron; four of those are the porphyrin nitrogens in the plane, the fifth is a nitrogen from a histidine of the protein reaching up from below, and the sixth site, on top, is left open. That open spot is where an O2 molecule docks, binding end-on to the iron. Myoglobin is a single such unit and simply stores oxygen. Hemoglobin is four of them clipped together into one four-subunit protein, and that teamwork is the whole point: the four sites talk to each other so that hemoglobin grabs oxygen greedily in the oxygen-rich lungs and lets go of it readily in the oxygen-poor tissues, a behavior called cooperative binding.
There is a beautiful piece of coordination chemistry hidden here. In deoxy-hemoglobin the iron(II) is high-spin, slightly too big to fit neatly in the porphyrin plane, so it sits a little out of the ring. When O2 binds, the iron flips to low-spin, shrinks, and snaps into the plane — and that tiny movement of the iron tugs on the attached histidine and ripples through the whole protein, nudging the other subunits to bind oxygen more eagerly. This is why carbon monoxide is so deadly: it binds the same iron site far more tightly than O2, locks it up, and oxygen can no longer load. An honest caveat: free heme in water is quickly and irreversibly oxidized to useless iron(III); it is the surrounding protein pocket that keeps the iron as iron(II) and lets it bind oxygen reversibly without rusting.
Myoglobin gives a diver's muscles an oxygen reserve, which is why deep-diving whales and seals carry so much of it that their muscle is almost black; hemoglobin in the same animals does the long-haul delivery from lungs to that store.
Same heme-iron tool, two jobs: hemoglobin transports, myoglobin stores.
The iron stays iron(II) throughout reversible O2 binding; it is not oxidized to iron(III). If it is (forming methemoglobin), it can no longer carry oxygen.