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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Aromatase (CYP19A1)

The enzyme that turns androgens into oestrogens; it is in the ovary, placenta, fat, bone and brain.

Updated 2026-11-01

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## Key facts

- Androstenedione → oestrone and testosterone → oestradiol: it removes carbon 19 and makes ring A aromatic.
- An endoplasmic reticulum cytochrome P450: it uses haem, O₂ and NADPH through P450 oxidoreductase (three oxidations in a row).
- In postmenopausal women and in men nearly all oestrogen comes from aromatase in fat tissue.
- Structure 3EQM is human placental aromatase with androstenedione in the active site, above the haem.

**Also called**
Oestrogen synthase

**EC number**
[1.14.14.14](https://www.enzyme-database.org/query.php?ec=1.14.14.14)

**Genes**
CYP19A1

**Structure (PDB)**
[3EQM](https://www.rcsb.org/structure/3EQM)

## Regulation

- In the ovary FSH raises its expression in granulosa cells (cAMP).
- Each tissue uses its own promoter: in fat tissue cortisol and inflammatory cytokines stimulate it, and its activity rises with age and obesity.

## Inhibitors and toxins

- Anastrozole and letrozole (non-steroidal, reversible)
- Exemestane (steroidal, irreversible inactivator)

## Clinical notes

- Aromatase inhibitors are used in hormone receptor-positive breast cancer in postmenopausal women.
- Aromatase deficiency: the mother virilises during pregnancy, a girl is born with ambiguous genitalia and adult men keep growing taller because their epiphyses do not close.
- Aromatase excess: prepubertal gynaecomastia in boys and early puberty in girls; obesity also raises aromatisation and favours gynaecomastia.

## Related conditions

- [Breast cancer](https://durgical.com/en/conditions/breast-cancer/)
- [Gynaecomastia](https://durgical.com/en/conditions/gynaecomastia/)
- [Obesity](https://durgical.com/en/conditions/obesity/)
- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)

## Related

- [Steroid hormone synthesis (overview)](https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/)
- [Adrenal enzymes and congenital adrenal hyperplasia](https://durgical.com/en/biochemistry/topics/adrenal-enzymes-and-congenital-adrenal-hyperplasia/)
- [17α-Hydroxylase/17,20-lyase (CYP17A1)](https://durgical.com/en/biochemistry/enzymes/17-hydroxylase-17-20-lyase/)
- [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/)
- [Leptin, appetite and obesity](https://durgical.com/en/biochemistry/topics/leptin-appetite-and-obesity/)
- [Nuclear receptors](https://durgical.com/en/biochemistry/topics/nuclear-receptors/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which reaction does aromatase catalyse?

It turns androgens into oestrogens: androstenedione into oestrone and testosterone into oestradiol.

## Where does most of a postmenopausal woman's oestrogen come from?

From aromatisation of adrenal androgens in fat tissue.

## Why does aromatase deficiency virilise the mother during pregnancy?

Because the placenta cannot turn fetal androgens into oestrogens, and they pass into the mother's circulation.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [CYP19A1: Aromatase (human)](https://www.uniprot.org/uniprotkb/P11511/entry) (UniProt, 2026); [CYP19A1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/cyp19a1/) (MedlinePlus, US National Library of Medicine, 2026); [3EQM: Crystal structure of human placental aromatase cytochrome P450 in complex with androstenedione](https://www.rcsb.org/structure/3EQM) (RCSB Protein Data Bank, 2026); [Aromatase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/aromatase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [Aromatase excess syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/aromatase-excess-syndrome/) (MedlinePlus, US National Library of Medicine, 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.

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