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
- Genes
- CYP19A1
- Structure (PDB)
- 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
Related
- Steroid hormone synthesis (overview)
- Adrenal enzymes and congenital adrenal hyperplasia
- 17α-Hydroxylase/17,20-lyase (CYP17A1)
- Cholesterol
- Leptin, appetite and obesity
- 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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: CYP19A1: Aromatase (human) (UniProt, 2026); CYP19A1 gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 3EQM: Crystal structure of human placental aromatase cytochrome P450 in complex with androstenedione (RCSB Protein Data Bank, 2026); Aromatase deficiency: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Aromatase excess syndrome: MedlinePlus Genetics (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.