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

# Hydroxyproline

Hydroxyproline is a proline with an added OH group inside collagen; without vitamin C it is not formed and collagen falls apart.

Updated 2026-11-01

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## Key facts

- It has no tRNA of its own: it forms after translation, when prolyl 4-hydroxylase hydroxylates prolines already built into collagen.
- The enzyme needs vitamin C (keeps Fe²⁺ reduced), O₂ and α-ketoglutarate.
- It nearly always sits at the Y position of Gly–X–Y and stabilises the triple helix at body temperature.
- It is almost unique to collagen (and to a lesser degree elastin), so its urinary loss reflects collagen turnover.

**Also called**
4-hydroxyproline, trans-4-hydroxy-L-proline, Hyp

**Formula**
C5H9NO3

**PubChem**
[CID 5810](https://pubchem.ncbi.nlm.nih.gov/compound/5810)

## Structure

- The pyrrolidine ring of proline with a hydroxyl on carbon 4 (trans configuration).
- The OH orients the ring in a way that favours triple-helix geometry.

## Clinical notes

- Scurvy: without hydroxyproline the chains cannot form a stable triple helix at 37 °C and are degraded: fragile vessels and poor wound healing.
- Urinary hydroxyproline was used as a bone-resorption marker (Paget disease, metastases); collagen telopeptides are preferred today.
- Dietary hydroxyproline is not reused to build collagen: it is broken down.

## Related conditions

- [Scurvy (vitamin C deficiency)](https://durgical.com/en/conditions/scurvy/)
- [Osteogenesis imperfecta](https://durgical.com/en/conditions/osteogenesis-imperfecta/)

## Related

- [Collagen](https://durgical.com/en/biochemistry/enzymes/collagen/)
- [Collagen synthesis](https://durgical.com/en/biochemistry/topics/collagen-synthesis/)
- [Vitamin C (ascorbic acid)](https://durgical.com/en/biochemistry/molecules/vitamin-c/)
- [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/)

## Test yourself

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

## How is collagen hydroxyproline formed?

By hydroxylating prolines already in the chain (prolyl 4-hydroxylase with vitamin C, Fe²⁺, O₂ and α-ketoglutarate); there is no tRNA for it.

## Why is hydroxyproline important for collagen?

It stabilises the triple helix; without it collagen denatures at body temperature.

## What does urinary hydroxyproline reflect?

Collagen breakdown, mainly from bone; it rises in Paget disease and other states of high bone resorption.

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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: [L-Hydroxyproline (CID 5810)](https://pubchem.ncbi.nlm.nih.gov/compound/5810) (PubChem, US National Library of Medicine, 2026); [Collagen biosynthesis and modifying enzymes](https://reactome.org/PathwayBrowser/#/R-HSA-1650814) (Reactome, 2026); [Scurvy: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000355.htm) (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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