Biochemistry · Molecule
Heme !
Heme is a porphyrin ring with an iron at its centre; it lets hemoglobin carry oxygen and cytochromes move electrons.
Updated 2026-11-01
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Key facts
- Protoporphyrin IX + Fe²⁺: four pyrrole rings joined by methine bridges, with the iron held by their four nitrogens.
- In hemoglobin and myoglobin the iron must be ferrous (Fe²⁺) to bind O₂; oxidised (Fe³⁺) it forms methemoglobin.
- The fifth iron position is taken by the proximal histidine; the sixth is free for O₂ (or CO).
- It is used by hemoglobin, myoglobin, respiratory-chain cytochromes, cytochromes P450, catalase, peroxidases and nitric oxide synthase.
- Also called
- Heme b, ferroprotoporphyrin IX
- Formula
- C₃₄H₃₂FeN₄O₄
- PubChem
- CID 4973
Structure
- It carries four methyls, two vinyls and two propionates; the charged propionates face the water and the rest hides in the protein pocket.
- Heme c of cytochromes c is attached to the protein by thioether bonds; heme b (this one) is not.
Clinical notes
- Intravenous hemin (heme with Fe³⁺) represses ALAS1 and is the specific treatment for acute porphyria attacks.
- Free heme is toxic (oxidant); in plasma it is caught by hemopexin, and free hemoglobin by haptoglobin (low in hemolysis).
- Heme iron from meat is absorbed better than iron from plant foods.
Pathways
Related conditions
Related
- Protoporphyrin IX
- Ferrochelatase
- Heme oxygenase
- Hemoglobin and 2,3-BPG
- Myoglobin
- Iron metabolism
- ALA synthase
Test yourself
Think of the answer, then tap the question to see it.
What oxidation state must heme iron be in to bind oxygen?
Ferrous (Fe²⁺); Fe³⁺ forms methemoglobin, which does not bind O₂.
Which plasma protein captures free heme, and which captures free hemoglobin?
Hemopexin captures heme; haptoglobin captures hemoglobin.
Why is hemin used in an acute porphyria attack?
It replaces liver heme, which represses ALAS1 and lowers ALA and porphobilinogen production.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Heme b (CID 4973) (PubChem, NCBI, 2026) · Public domain; Reactome | Heme biosynthesis (Reactome, 2026); Reactome | Heme degradation (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.