# Proline
**Proline** (Pro) is a nonessential proteinogenic amino acid in which the side chain is a pyrrolidine ring covalently bonded to the backbone nitrogen, making it the only ribosomally encoded N-substituted, secondary (imino) amino acid.[1](https://journals.asm.org/doi/10.1128/mmbr.00017-26) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/) This cyclic structure restricts the backbone dihedral angles, removes the amide hydrogen-bond donor, and gives proline distinctive roles in protein structure, most visibly in collagen, while its metabolism forms a regulatory axis involved in redox control, stress responses, and cancer.[1](https://journals.asm.org/doi/10.1128/mmbr.00017-26) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf)
| Key fact | Detail |
|---|---|
| Structural distinction | The only proteinogenic amino acid whose side chain circularizes and is covalently bound to the peptide backbone nitrogen; a tertiary amide lacking a hydrogen-bond donor[1](https://journals.asm.org/doi/10.1128/mmbr.00017-26) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf) |
| Conformational effect | The pyrrolidine ring imposes unique conformational restraints on backbone and side-chain dihedral angles and undergoes cis–trans isomerization about the preceding peptide bond[4](https://pubmed.ncbi.nlm.nih.gov/31953795/) |
| Collagen content | In collagen, Xaa positions are often (2S)-proline (28%) and Yaa positions (2S,4R)-4-hydroxyproline (38%); proline is about 22% of human collagen[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846778/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf) |
| Biosynthesis | Glutamate is phosphorylated by γ-glutamyl kinase, reduced by γ-glutamyl phosphate reductase, cyclized spontaneously to Δ¹-pyrroline-5-carboxylate (P5C), and reduced to proline by P5C reductase[6](https://doi.org/10.1111/brv.12146) |
| Catabolism | Proline is oxidized to P5C by proline dehydrogenase (PRODH) in the mitochondrial matrix, then to glutamate by NAD-dependent P5C dehydrogenase[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC9676987/) |
| Physiological roles | Roles in adaptation to osmotic and dehydration stress, redox control, and apoptosis; proline metabolism is altered in cancer reprogramming[6](https://doi.org/10.1111/brv.12146) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/) |
## Structure and conformational properties
Proline's side chain is an aliphatic chain whose terminal Cδ atom is covalently linked to the backbone nitrogen, closing a five-membered pyrrolidine ring.[8](https://onlinelibrary.wiley.com/doi/10.1002/bip.20227) Because the amide nitrogen carries no hydrogen, proline residues cannot donate interstrand hydrogen bonds, and the ring restricts the backbone φ dihedral angle.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf) The peptide bond preceding a proline can adopt either the trans or the rare cis configuration, and cis–trans isomerization at prolyl bonds is a recognized regulatory mechanism in proteins, with documented examples including p53, histone H3, NF-κB, and [RNA polymerase II](https://www.edgepedia.org/rna-polymerase-ii).[4](https://pubmed.ncbi.nlm.nih.gov/31953795/) • [1](https://journals.asm.org/doi/10.1128/mmbr.00017-26)
The conformational restriction imposed by the ring makes proline prevalent in structural proteins.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC5501414/) Proline-rich sequences favor the polyproline II (PPII) helix, an extended left-handed conformation once thought to be mainly a feature of fibrous proteins such as collagen and titin but now known to occur in globular proteins and to participate in peptide–protein and protein–protein interactions.[1](https://journals.asm.org/doi/10.1128/mmbr.00017-26) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11345334) Substituted proline analogs, such as 4-fluoroproline, exploit these conformational preferences to tune the stability and folding of peptides and proteins.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC5501414/)
## Proline and collagen
Collagen, the most abundant protein in the mammalian body and essential to the structure and strength of bones, skin, cartilage, and blood vessels, is a right-handed bundle of three parallel, left-handed polyproline II-type helices built on a Gly–Xaa–Yaa repeat in which glycine occupies every third residue.[11](https://link.springer.com/content/pdf/10.1007/s00018-019-03363-3.pdf) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846778/) The Xaa and Yaa positions are frequently proline and its post-translationally hydroxylated form, 4-hydroxyproline: in the canonical analysis, proline occupies about 28% of Xaa positions and 4-hydroxyproline about 38% of Yaa positions, and proline constitutes roughly 22% of human collagen overall.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846778/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf) The enzyme prolyl hydroxylase converts proline residues to hydroxyproline, a modification considered necessary for the integrity of the triple helix; the hydroxyl group of hydroxyproline stabilizes the triple helix at body temperature.[12](https://link.springer.com/article/10.1007/s42452-025-07780-x) • [13](https://academic.oup.com/nutritionreviews/article/80/6/1497/6380930?login=false)
Proline availability matters for collagen biosynthesis. Proline makes up about 10% of the total amino acids in collagen, and studies indicate that endogenous synthesis alone can be insufficient for maximal growth and collagen production, for example during wound healing.[11](https://link.springer.com/content/pdf/10.1007/s00018-019-03363-3.pdf) [Glutamine](https://www.edgepedia.org/glutamine), converted to glutamate and then through the proline biosynthetic pathway, is considered a regulatory amino acid of proline availability for collagen biosynthesis.[11](https://link.springer.com/content/pdf/10.1007/s00018-019-03363-3.pdf) Collagen in the extracellular matrix, connective tissue, and bone also serves as an abundant reservoir of proline, released when matrix metalloproteinases degrade collagen under stress.[14](https://www.annualreviews.org/doi/pdf/10.1146/annurev.nutr.012809.104638)
## Metabolism
Proline is synthesized from glutamate. In the prevalent pathway, γ-glutamyl kinase phosphorylates glutamate to γ-glutamyl phosphate, γ-glutamyl phosphate reductase reduces it to glutamic-γ-semialdehyde, which cyclizes spontaneously to Δ¹-pyrroline-5-carboxylate (P5C), and P5C reductase reduces P5C to proline.[6](https://doi.org/10.1111/brv.12146) In higher plants and animals the first two steps are catalyzed by the bifunctional enzyme P5C synthase (P5CS).[6](https://doi.org/10.1111/brv.12146) Alternative routes use the arginine biosynthetic pathway: ornithine can be converted to P5C by ornithine aminotransferase and then reduced to proline, or converted directly to proline by ornithine cyclodeaminase.[6](https://doi.org/10.1111/brv.12146) Three P5C reductase isozymes exist in humans: PYCR1 and PYCR2 preferentially use NADH and are located in mitochondria, while PYCRL prefers NADPH and is cytosolic.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/)
Catabolism runs through the mitochondrial matrix: proline dehydrogenase (PRODH, also called proline oxidase), a flavoenzyme bound to the inner mitochondrial membrane, oxidizes proline to P5C with reduction of FAD, and P5C dehydrogenase converts P5C to glutamate in an NAD⁺-dependent step.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC9676987/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/) Because P5C is both the committed precursor and the first degradative product of proline, the paired reactions have been described as a proline–P5C cycle that can transfer redox potential; proline-derived electrons enter the electron transport chain at complex II and can either reduce oxygen to water or generate superoxide at complex III.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/)
## Regulation, stress, and disease
Proline metabolism responds to stress. In plants, biosynthesis is enhanced and degradation diminished during osmotic or salt stress, and proline accumulates on dehydration through abscisic acid-dependent and independent signaling; genetic manipulation that increases proline accumulation has improved osmotic stress tolerance in bacteria and yeast.[6](https://doi.org/10.1111/brv.12146) In humans, the degradative enzyme PRODH/POX is induced by stress signals including p53, PPARγ, and AMPK, while the synthetic enzymes are upregulated by the oncogenes c-MYC and PI3K.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/) PRODH/POX was identified as P53-induced gene 6 (PIG-6) and generates reactive oxygen species that participate in signaling for apoptosis, autophagy, and senescence, and proline metabolism is now recognized as part of metabolic reprogramming in cancer.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/)
Inborn errors affect both directions of the pathway. Deficiency of P5CS, the first enzyme in the route from glutamate to proline, was the first reported inborn error of proline biosynthesis; affected siblings showed low proline, ornithine, citrulline, and arginine, with hypotonia, dysmorphic signs, pes planus, and clonic seizures.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/) On the degradative side, PRODH deficiency causes type I hyperprolinemia, with plasma proline reported at 2 to 10 times normal values in affected families, and P5C dehydrogenase deficiency causes type II hyperprolinemia.[15](https://www.jianhaidulab.com/wp-content/uploads/2016/08/Inborn-errors-of-proline-metabolism.pdf) • [16](https://www.cur.ac.rw/mis/main/library/documents/book_file/digital-666431634ee798.62542156.pdf) Deficiencies of ornithine aminotransferase, which links proline and arginine metabolism, cause gyrate atrophy of the choroid and retina, leading to blindness by the fifth decade of life.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/)
## References
1. [Translation factors that keep the ribosome Pro-active, Microbiology and Molecular Biology Reviews](https://journals.asm.org/doi/10.1128/mmbr.00017-26)
2. [Phang, J. M. (2018). Proline Metabolism in Cell Regulation and Cancer Biology: Recent Advances and Hypotheses, Antioxidants & Redox Signaling](https://pmc.ncbi.nlm.nih.gov/articles/PMC6338564/)
3. [The Chemistry and Biology of Collagen Hybridization](https://pmc.ncbi.nlm.nih.gov/articles/PMC10789224/pdf/nihms-1957560.pdf)
4. [Conformational landscape of substituted prolines, PubMed](https://pubmed.ncbi.nlm.nih.gov/31953795/)
5. [Collagen Structure and Stability, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2846778/)
6. [Fichman et al. (2015). Evolution of proline biosynthesis, Biological Reviews](https://doi.org/10.1111/brv.12146)
7. [Proline Metabolism in Neurological and Psychiatric Disorders, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9676987/)
8. [Rath (2005). The structure of "unstructured" regions in peptides and proteins, Peptide Science](https://onlinelibrary.wiley.com/doi/10.1002/bip.20227)
9. [4-Fluoroprolines: Conformational Analysis and Effects on the Stability and Folding of Peptides and Proteins, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5501414/)
10. [Proline, a unique amino acid whose polymer, polyproline II helix, and its analogues are involved in many biological processes: a review](https://pmc.ncbi.nlm.nih.gov/articles/PMC11345334)
11. [Proline-dependent regulation of collagen metabolism, Cell and Molecular Life Sciences](https://link.springer.com/content/pdf/10.1007/s00018-019-03363-3.pdf)
12. [Collagen and collagen-derived materials: synthesis, structure, classification, fundamental properties and biomedical applications, Discover Applied Sciences](https://link.springer.com/article/10.1007/s42452-025-07780-x)
13. [Impact of collagen protein ingestion on musculoskeletal connective tissue remodeling, Nutrition Reviews](https://academic.oup.com/nutritionreviews/article/80/6/1497/6380930?login=false)
14. [Phang et al. (2010). Proline Metabolism and Microenvironmental Stress, Annual Review of Nutrition](https://www.annualreviews.org/doi/pdf/10.1146/annurev.nutr.012809.104638)
15. [Inborn errors of proline metabolism](https://www.jianhaidulab.com/wp-content/uploads/2016/08/Inborn-errors-of-proline-metabolism.pdf)
16. [Textbook of Biochemistry for Medical Students, 6th Edition](https://www.cur.ac.rw/mis/main/library/documents/book_file/digital-666431634ee798.62542156.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Amines and nitrogen functional groups › Amino acids and derivatives › Proteinogenic amino acid classes › Aliphatic and hydroxyl-containing amino acids*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license