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

# 17α-Hydroxylase/17,20-lyase (CYP17A1)

One enzyme with two activities that decide whether pregnenolone goes to cortisol, to androgens or only to aldosterone.

Updated 2026-11-01

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## Key facts

- 17α-Hydroxylase: pregnenolone → 17-hydroxypregnenolone and progesterone → 17-hydroxyprogesterone (needed for cortisol).
- 17,20-Lyase: cuts the side chain to form DHEA (and, to a lesser degree, androstenedione): needed for androgens and oestrogens.
- It is in the fasciculata, reticularis and gonads but not in the glomerulosa; lyase activity rises with cytochrome b5 and phosphorylation.
- Structure 3RUK is human CYP17A1 with abiraterone bound to the haem iron.

**Also called**
P450c17

**EC number**
[1.14.14.19](https://www.enzyme-database.org/query.php?ec=1.14.14.19)

**Genes**
CYP17A1

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

## Regulation

- ACTH in the adrenal and LH in Leydig and theca cells raise its expression.
- Cytochrome b5 and serine phosphorylation favour 17,20-lyase activity (adrenarche).

## Inhibitors and toxins

- Abiraterone (castration-resistant prostate cancer)
- Ketoconazole

## Clinical notes

- Its deficiency blocks cortisol, androgen and oestrogen production; 11-deoxycorticosterone and corticosterone pile up: hypertension, low potassium and absent puberty.
- Abiraterone blocks adrenal and tumour androgen production in advanced prostate cancer; it is given with a glucocorticoid.
- Increased 17,20-lyase activity in ovary and adrenal contributes to the androgen excess of polycystic ovaries.

## Pathways

- [Adrenal cortisol synthesis](https://durgical.com/en/biochemistry/pathways/adrenal-cortisol-synthesis/)

## Related conditions

- [Congenital adrenal hyperplasia due to 17α-hydroxylase deficiency](https://durgical.com/en/conditions/congenital-adrenal-hyperplasia-due-to-17-hydroxylase-deficiency/)
- [Prostate cancer](https://durgical.com/en/conditions/prostate-cancer/)
- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)

## Related

- [Adrenal enzymes and congenital adrenal hyperplasia](https://durgical.com/en/biochemistry/topics/adrenal-enzymes-and-congenital-adrenal-hyperplasia/)
- [Aromatase (CYP19A1)](https://durgical.com/en/biochemistry/enzymes/aromatase/)
- [Cortisol](https://durgical.com/en/biochemistry/molecules/cortisol/)
- [17-Hydroxyprogesterone (17-OHP)](https://durgical.com/en/biochemistry/molecules/17-hydroxyprogesterone/)
- [21-Hydroxylase (CYP21A2)](https://durgical.com/en/biochemistry/enzymes/21-hydroxylase/)
- [Steroid hormone synthesis (overview)](https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/)

## Test yourself

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

## Which two activities does CYP17A1 have?

17α-hydroxylase (the road to cortisol) and 17,20-lyase (the road to androgens).

## Why does the zona glomerulosa make only aldosterone?

Because it does not express CYP17A1, so pregnenolone cannot go towards cortisol or androgens.

## Which drug inhibits CYP17A1 in prostate cancer?

Abiraterone.

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

Sources: [CYP17A1: Steroid 17-alpha-hydroxylase/17,20 lyase (human)](https://www.uniprot.org/uniprotkb/P05093/entry) (UniProt, 2026); [CYP17A1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/cyp17a1/) (MedlinePlus, US National Library of Medicine, 2026); [3RUK: Human Cytochrome P450 CYP17A1 in complex with Abiraterone](https://www.rcsb.org/structure/3RUK) (RCSB Protein Data Bank, 2026); [17 alpha-hydroxylase/17,20-lyase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/17-alpha-hydroxylase-17-20-lyase-deficiency/) (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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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/17-hydroxylase-17-20-lyase/