# Sterol 22-desaturase
**Sterol 22-desaturase** (also called sterol Δ22-desaturase, C-22 desaturase, or C22DES) is a membrane-bound cytochrome P450 enzyme that introduces a double bond at the C-22(23) position of the sterol side chain, converting sterols such as β-sitosterol into stigmasterol in plants and ergosta-5,7-dienol into ergosterol in fungi. Δ22-unsaturated sterols occur specifically in fungi and plants; the enzyme has no counterpart in humans, which makes it a candidate target for antifungal and agrochemical drugs.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/)
| Key fact | Detail |
|---|---|
| Reaction | Introduces a C22(23)-E double bond in the sterol side chain; in plants, β-sitosterol → stigmasterol, the last step of the 24-ethyl branch[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7972189/) |
| Enzyme class | Cytochrome P450; requires molecular oxygen and NADPH, inhibited by CO and metyrapone[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/) |
| Genes | Fungi: ERG5 (CYP61); plants: CYP710 family (CYP710A1, CYP710A2, CYP710A4 in Arabidopsis; CYP710A11 in tomato)[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) • [4](https://www.degruyterbrill.com/document/doi/10.1515/pac-2016-1216/html) |
| Kinetics (plant) | Arabidopsis CYP710A1 with β-sitosterol: Km = 1.0 μM, kcat = 0.53 min⁻¹; tomato CYP710A11: Km = 3.7 μM, kcat = 10 min⁻¹[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) |
| Kinetics (fungus) | Candida glabrata enzyme: Km = 12.5 μM, Vmax = 0.59 nmol min⁻¹ nmol⁻¹ P450 with ergosta-5,7-dienol[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725) |
| Drug sensitivity | Azole antifungals (ketoconazole, itraconazole, fluconazole) bind the heme with one-to-one stoichiometry; fluconazole Ki = 0.18 nM[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725) |
| Human relevance | No human counterpart; Δ22-unsaturated phytosterols can inhibit sterol Δ24-reductase in mammalian cells, a possible cholesterol-lowering mechanism[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/) • [6](https://portlandpress.com/biochemj/article-pdf/366/1/109/710537/bj3660109.pdf) |
## Biological role
Sterols are isoprenoid-derived lipids that maintain membrane integrity and serve as precursors of steroid hormones. One of the clearest differences in sterol composition between biological kingdoms is the presence of Δ22-sterols in fungi, where ergosterol carries the C-22 double bond, and in higher plants, where stigmasterol, (24S)-24-ethyl-cholesta-5,E-22-dien-3β-ol, does.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC3243374/) In plants the reaction is the final step of the 24-ethyl branch of sterol biosynthesis, converting β-sitosterol to stigmasterol; in the ergosterol pathway of yeast it occurs at the antepenultimate step.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7972189/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/)
The stigmasterol to β-sitosterol ratio changes during plant development and in response to stresses, and in tomato these changes correlate with induction of C22DES, the only gene encoding the enzyme in that species.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7972189/) [Infection](https://www.edgepedia.org/infection) of plant roots by the nematode *Meloidogyne incognita* increases β-sitosterol and decreases stigmasterol, a shift possibly facilitated by the C22-desaturase gene CYP710A11, one line of evidence linking the enzyme to biotic stress responses.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC11591786/)
## Enzyme identity and evolution
The fungal enzyme is encoded by ERG5, the gene for cytochrome P450 CYP61, the second P450 of the ergosterol pathway after sterol 14α-demethylase (CYP51).[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725) • [4](https://www.degruyterbrill.com/document/doi/10.1515/pac-2016-1216/html) In plants the reaction is catalyzed by the CYP710 family: CYP710A1, CYP710A2, and CYP710A4 in *Arabidopsis thaliana* and CYP710A11 in tomato have been biochemically characterized, and CYP710A genes occur from gymnosperms to monocots and dicots, with CYP710B sequences in red and green algae and a partial sequence in the moss *Physcomitrella patens*.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) • [9](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01886/full) Because C22DES acts downstream of CYP51 in the sterol pathway and is phylogenetically related to it, the enzyme has been proposed to have evolved from a CYP51 gene duplication.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7972189/)
Plant and fungal sterol desaturases share more than 43% sequence identity and a unique stretch, FLFASQDASS, containing a highly conserved alanine considered a substrate-binding contact residue; the mature *A. thaliana* protein has a reported molecular mass of 56,344 D.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/) A plant-like CYP710C1 gene in the protozoan parasite *Leishmania donovani* also encodes a functional C-22 desaturase producing stigmasterol from β-sitosterol in vitro.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC6464246/)
## Catalytic properties
The reaction is a cytochrome P450-dependent dehydrogenation: it requires molecular oxygen and NADPH and is inhibited by carbon monoxide and metyrapone.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/) Early work testing 23-hydroxysterols as intermediates in yeast microsomes found no evidence for a two-stage mechanism of desaturation via hydroxylation and dehydration.[11](https://pubmed.ncbi.nlm.nih.gov/2256937/) Kinetic studies of cloned yeast CYP61 revealed a random bi-bi mechanism with NADPH donating electrons through NADPH-cytochrome P450 oxidoreductase.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/)
**Substrate specificity differs between kingdoms.** In yeast, the size and direction of the C-24 substituent strongly affect Δ22-bond formation: 24β-ethyl sterols gave 65%, 24β-methyl 50%, 24α-ethyl 10%, 24α-methyl 5%, and unsubstituted (24H) 7% in the sterol-auxotrophic GL7 mutant studies.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/) Consistently, sterols with alkyl substituents in the 24α position were poor substrates for the yeast enzyme.[11](https://pubmed.ncbi.nlm.nih.gov/2256937/) The higher plant enzyme instead recognizes 24α-ethyl and 24β-methyl groups equally well, while the 24α-methyl sterol is not a suitable substrate; in the moss enzyme, 24α-ethyl sterol was preferred but neither 24α- nor 24β-methyl sterols were accepted.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/)
## Characterized plant enzymes
Recombinant Arabidopsis CYP710A1 and tomato CYP710A11 convert β-sitosterol to stigmasterol, with CYP710A11 roughly twenty times faster (kcat 10 min⁻¹ versus 0.53 min⁻¹).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) Arabidopsis CYP710A2 produces both brassicasterol and stigmasterol, from 24-epi-campesterol and β-sitosterol respectively, and a T-DNA insertion disrupting CYP710A2 eliminated brassicasterol and crinosterol production, showing that no other desaturase compensates for this reaction in [Arabidopsis](https://www.edgepedia.org/arabidopsis).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) Overexpression of CYP710A1 or CYP710A11 in Arabidopsis raised stigmasterol levels 6- to 32-fold, and each CYP710A gene shows strict tissue- and organ-specific regulation, implying differential tissue distributions of Δ22-unsaturated sterols.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/) Genome-wide identification of C-22 desaturases has since extended to crop genera such as *Asparagus*, where candidate genes complemented an erg5 disruption in *Neurospora crassa*.[12](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2025.1690526/full)
## Drug target and inhibition
Sterol Δ22-desaturase purified from the pathogenic yeast *Candida glabrata* showed Km = 12.5 μM and Vmax = 0.59 nmol min⁻¹ nmol⁻¹ P450 for ergosta-5,7-dienol, and an apparent monomeric molecular mass of about 59,000, close to the 58,000 reported for *Saccharomyces cerevisiae* CYP61.[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725) The azole antifungals ketoconazole, itraconazole, and fluconazole formed low-spin complexes with the ferric heme and inhibited the reconstituted enzyme with one-to-one stoichiometry; the azole amount needed for 50% inhibition was approximately 0.1 nmol for all three, and fluconazole acted competitively with a Ki of 0.18 nM, comparable to its inhibition of *Candida albicans* CYP51.[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725) Disruption of CYP61 in *S. cerevisiae* is not lethal, but inhibition of the enzyme in the cereal pathogen *Rhynchosporium secalis* correlates with growth arrest in azole-resistant strains, so its role in antifungal therapy remains under study.[5](https://journals.asm.org/doi/10.1128/aac.43.7.1725)
Because the enzyme is absent from humans, Δ22-unsaturated sterols themselves have pharmacological interest: they inhibit sterol Δ24-reductase in mammalian cells at physiologically relevant concentrations, which may contribute to the cholesterol-lowering action of phytosterols and suggests possible hypocholesterolemic agents.[6](https://portlandpress.com/biochemj/article-pdf/366/1/109/710537/bj3660109.pdf)
## References
1. [Morikawa et al. Cytochrome P450 CYP710A Encodes the Sterol C-22 Desaturase in Arabidopsis and Tomato. Plant Cell (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC1425849/)
2. [Biosynthesis of Cholesterol and Other Sterols. Chemical Reviews (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3191736/)
3. [Structural and functional analysis of tomato sterol C22 desaturase. BMC Plant Biology (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7972189/)
4. [P450 catalysed dehydrogenation. Pure and Applied Chemistry](https://www.degruyterbrill.com/document/doi/10.1515/pac-2016-1216/html)
5. [Lamb et al. Purification, Reconstitution, and Inhibition of Cytochrome P-450 Sterol Δ22-Desaturase from Candida glabrata. Antimicrobial Agents and Chemotherapy](https://journals.asm.org/doi/10.1128/aac.43.7.1725)
6. [Inhibition of cholesterol biosynthesis by Δ22-unsaturated phytosterols via competitive inhibition of sterol Δ24-reductase. Biochemical Journal](https://portlandpress.com/biochemj/article-pdf/366/1/109/710537/bj3660109.pdf)
7. [Sterol Metabolism (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3243374/)
8. [Loss of Sterol Biosynthesis in Economically Important Plant Pests and Pathogens (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC11591786/)
9. [Triterpene Structural Diversification by Plant Cytochrome P450 Enzymes. Frontiers in Plant Science](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01886/full)
10. [A Plant-like Cytochrome P450 Subfamily CYP710C1 Gene in Leishmania donovani Encodes Sterol C-22 Desaturase (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6464246/)
11. [Giner & Djerassi. Inhibition and substrate specificity of yeast delta 22-desaturase. Biochem Biophys Res Commun](https://pubmed.ncbi.nlm.nih.gov/2256937/)
12. [Genome-wide identification and functional validation of sterol C-22 desaturases and C-24 methyltransferases in Asparagus. Frontiers in Plant Science](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2025.1690526/full)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Terpene, sterol, and prenyltransferase synthases › Sterol biosynthesis enzymes › Sterol desaturases and oxidases*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 11, 2026; 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