heme
Meaning of heme: The iron-holding ring that lets blood carry oxygen, and the same ring that turns a bruise yellow as it is dismantled.
Definition of heme
Heme is a flat ring-shaped molecule with a single atom of iron held at its center. Locked inside a protein, that iron can grip and release oxygen without being permanently changed, which is what allows blood to carry oxygen. Heme also sits at the working center of enzymes that pass electrons along, including those the liver uses to break down medicines.
What is it part of?
Most heme in the body is inside hemoglobin, the protein that fills red cells, where four heme groups give each molecule four places to hold oxygen. Muscle carries its own version, myoglobin, with one heme apiece, and that is what makes red meat red. Beyond oxygen carrying, heme is the business end of the cytochromes, enzymes that shuttle electrons and that the liver uses to dismantle drugs. One chemical trick serves all of them: an iron atom that can gain and lose an electron over and over without falling apart.
Why does carbon monoxide matter here?
The iron in heme is not fussy about what it binds. Carbon monoxide attaches at the same site as oxygen and holds on far more tightly, so even a small amount in the air takes red cells out of service, and the cells still carrying oxygen give it up less readily. That double effect starves the tissues while the blood itself stays bright red rather than turning dusky, which is why the poisoning is easy to miss. Cyanide attacks heme too, but the heme inside the cytochromes that let cells use oxygen at all.
What happens when it is made or broken down?
| Stage | What can go wrong |
|---|---|
| Assembly, in eight enzyme steps | A blocked step causes a porphyria, with attacks of pain or skin blistering |
| Iron supply | Too little iron leaves small pale red cells |
| Breakdown after a red cell dies | The ring is opened and becomes bilirubin, a yellow pigment |
| Clearance of bilirubin | If it builds up, the skin and eyes turn yellow |
The colors of a bruise follow the same path outside a vessel, running from red through green to yellow as the ring is taken apart. Lead poisoning works partly by jamming two of the assembly steps.
Also known as
Used in a sentence
Iron is incorporated into heme during red cell production in the marrow.
Good to know
Heme is not confined to red cells; it is also the working center of the cytochrome enzymes that transfer electrons and metabolize drugs, which is why disorders of heme reach well beyond the blood.
Word origin
Where “heme” comes from
Heme comes from the Greek haima, meaning blood, the same root behind hemoglobin and hemorrhage.