[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Heme

[Biochemistry](https://durgical.com/en/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

Loading the 3D model…

## 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](https://pubchem.ncbi.nlm.nih.gov/compound/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

- [Heme synthesis](https://durgical.com/en/biochemistry/pathways/heme-synthesis/)
- [Heme breakdown and bilirubin](https://durgical.com/en/biochemistry/pathways/heme-breakdown-and-bilirubin/)

## Related conditions

- [Acute intermittent porphyria](https://durgical.com/en/conditions/acute-intermittent-porphyria/)
- [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)

## Related

- [Protoporphyrin IX](https://durgical.com/en/biochemistry/molecules/protoporphyrin-ix/)
- [Ferrochelatase](https://durgical.com/en/biochemistry/enzymes/ferrochelatase/)
- [Heme oxygenase](https://durgical.com/en/biochemistry/enzymes/heme-oxygenase/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Myoglobin](https://durgical.com/en/biochemistry/enzymes/myoglobin/)
- [Iron metabolism](https://durgical.com/en/biochemistry/topics/iron-metabolism/)
- [ALA synthase](https://durgical.com/en/biochemistry/enzymes/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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/biochemistry/molecules/heme/)

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)](https://pubchem.ncbi.nlm.nih.gov/compound/4973) (PubChem, NCBI, 2026) · Public domain; [Reactome | Heme biosynthesis](https://reactome.org/PathwayBrowser/#/R-HSA-189451) (Reactome, 2026); [Reactome | Heme degradation](https://reactome.org/PathwayBrowser/#/R-HSA-189483) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

[← All of biochemistry](https://durgical.com/en/biochemistry/)

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/biochemistry/molecules/heme/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/heme/