# Ergosterol biosynthesis enzymes (ERG genes)
Ergosterol biosynthesis enzymes are the set of proteins encoded by ERG genes that fungi use to build ergosterol, starting from acetyl-CoA via the mevalonate pathway and the isoprenoid intermediate farnesyl pyrophosphate (farnesyl-PP). In *Saccharomyces cerevisiae* the pathway comprises 24 reactions catalyzed by 25 enzymes, counting the two Hmg1/Hmg2 isozymes, and falls into three phases: mevalonate biosynthesis, farnesyl-PP biosynthesis, and the final conversion of farnesyl-PP to ergosterol.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
| Key fact | Detail |
|---|---|
| Pathway size in *S. cerevisiae* | 24 reactions, 25 enzymes (including Hmg1/Hmg2 isozymes)[1](https://journals.asm.org/doi/10.1128/aem.02273-24) |
| Farnesyl-PP to ergosterol | 16 enzymes, from farnesyl-PP through squalene and lanosterol[1](https://journals.asm.org/doi/10.1128/aem.02273-24) |
| Essential ERG genes (aerobic) | ERG9, ERG1, ERG7, ERG11, ERG24, ERG25, ERG26, ERG27[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) |
| Non-essential ERG genes | ERG28, ERG2, ERG3, ERG4, ERG5, ERG6[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) |
| Main transcriptional regulators | Upc2 and Ecm22, binding sterol regulatory elements[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) |
| Known kinetic parameter set | Erg6p: \(K_{\mathrm{d}}\) ≈ 4 µM for ATP, SAM, and zymosterol; \(k_{\mathrm{cat}}\) 0.6 to ~5 min−1[3](https://doi.org/10.3390/biom14030249) |
| Fungal-specific segment | The ergosterol phase is largely fungal, but farnesyl-PP biosynthesis also occurs in plants and animals, and ergosterol biosynthesis occurs in some nonfungal eukaryotes, including trypanosomes and some algae[1](https://journals.asm.org/doi/10.1128/aem.02273-24) • [9](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.665206/full) |
## The pathway at a glance
The route begins in the mevalonate phase, where Hmg1 and Hmg2 reduce Hmg-CoA to mevalonate, and Erg12, Erg8, Erg19, Idi1, and Erg20 then convert mevalonate successively into farnesyl-PP.[1](https://journals.asm.org/doi/10.1128/aem.02273-24) The committed sterol step is ERG9, which encodes squalene synthase condensing two farnesyl-PP molecules into one molecule of squalene, the first sterol-structured molecule, and the direct precursor of ergosterol biosynthesis.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) ERG1 (squalene epoxidase) oxidizes squalene, and ERG7 (lanosterol synthase) cyclizes the product to lanosterol.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
From lanosterol the pathway performs demethylation, reduction, alkylation, and desaturation steps. ERG11 encodes lanosterol 14-alpha demethylase (Cyp51, a cytochrome P450), and ERG24 encodes the C-14 reductase.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Later, ERG2 encodes sterol 8-isomerase, which moves the B-ring double bond of fecosterol from position 8 to position 7 to form episterol, the Δ7,8-isomerisation step of the late pathway.[4](https://doi.org/10.3390/jof7030237)
**Compartment shifts** organize the three phases in *S. cerevisiae*: mevalonate biosynthesis occurs mainly in the vacuole and mitochondria, farnesyl-PP biosynthesis in the vacuole, and ergosterol biosynthesis in the endoplasmic reticulum.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
## The ERG gene inventory and essentiality
Eight ERG genes are essential aerobically in *S. cerevisiae*: ERG9, ERG1, ERG7, ERG11, ERG24, ERG25, ERG26, and ERG27; the remaining late and accessory genes, ERG28, ERG2, ERG3, ERG4, ERG5, and ERG6, are non-essential.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Mutants in the essential genes are mostly temperature-sensitive and grow only in medium supplied with exogenous ergosterol under aerobic conditions, which shows that the essentiality of these steps is conditional on oxygen availability.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Under low oxygen cells cannot synthesize ergosterol at all and instead absorb sterols from the external environment.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
Non-essential genes still matter physiologically. **ERG6**, the C-24 methyltransferase, is non-lethal when mutated but severely growth-deficient, and its overexpression increases cellular ergosterol content.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) The erg3 mutant is viable but cannot grow on media lacking a fermentable carbon source; some erg3 mutants are sensitive to low temperature yet insensitive to sterol synthesis inhibitors, a pattern that suggests a compensatory ergosterol-synthesis branch.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Mutations in non-essential genes alter membrane composition in ways that affect membrane potential, salt tolerance, and drug resistance.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
Pathogenic fungi add redundancy at the C-4 demethylation step: in *Aspergillus fumigatus* and *Candida albicans* two C-4 sterol methyl oxidases, Erg25p and Erg251p, exist, with Erg251p serving as the dominant enzyme in the *C. albicans* alternative pathway; dysfunction of Erg251p causes accumulation of pathway intermediates.[5](https://www.mdpi.com/2076-2607/13/4/862)
## Pathway-level regulation
Under hypoxic conditions the transcription factors Ecm22 and Upc2 bind sterol regulatory elements in the promoters of sterol biosynthesis genes to promote their expression.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Upc2 additionally induces the ABC transporters Aus1 and Pdr11, which import environmental sterols.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) Under low oxygen, cells switch from synthesis to uptake, absorbing sterols from the culture medium.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
In the Ceratocystidaceae, HMGCR catalyzes a likely rate-limiting step of the mevalonate phase, while ERG11 and ERG1 are likely rate-limiting in the second phase.[4](https://doi.org/10.3390/jof7030237) In engineering terms, boosting pathway dosage does not reliably help: in *Aspergillus oryzae*, overexpression of most ergosterol synthases delayed growth, with AoErg12- and AoIdi1A-overexpression strains showing significantly reduced colony diameters.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
## Enzyme organization: the ergosome question
ERG25 (C-4 sterol methyl oxidase), ERG26 (sterol 4-α-carboxylate 3-dehydrogenase) and ERG27 (3-keto sterol reductase) are ER-localized oxidoreductases that work together as enzymatic complexes catalyzing the final steps of zymosterol synthesis.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/) • [4](https://doi.org/10.3390/jof7030237) ERG28 encodes a proposed scaffold protein mediating formation of the ERG25/ERG26/ERG27 complex, and its disruption slows yeast growth.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
The scaffold role of Erg28 remains a matter of debate: the disruption phenotype is established, but whether and how Erg28 organizes the three enzymes into a functional ergosome is not settled in the literature.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/)
## How it compares with cholesterol and phytosterol routes
The mevalonate phase is conserved across all eukaryotes, including plants and animals; farnesyl-PP biosynthesis likewise occurs in plants, animals, and fungi, while the ergosterol biosynthetic phase that follows is largely fungal but also occurs in nonfungal eukaryotes, including trypanosomes and some algae.[1](https://journals.asm.org/doi/10.1128/aem.02273-24) • [9](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.665206/full) [Ergosterol](https://www.edgepedia.org/ergosterol) and cholesterol share a tetracyclic backbone with a hydroxyl group at C-3 and a double bond at C-5,6; ergosterol is distinguished by an additional methyl group at C-24 on its side chain, the product of the ERG6 methyltransferase step that animals lack.[5](https://www.mdpi.com/2076-2607/13/4/862)
Cross-kingdom ortholog mapping connects the fungal genes to plant and animal counterparts: ERG3 corresponds to the plant DWF7 and animal SC5DL sterol C-5 desaturase (EC 1.14.19.20, an oxo-diiron/NADPH enzyme); ERG4 corresponds to DWF1/[DHCR24](https://www.edgepedia.org/24-dehydrocholesterol-reductase), the Δ24 reductase; and ERG6 corresponds to the SMT sterol C24/28 methylases (EC 2.1.1.41/43, SAM-dependent), for which no animal ortholog exists.[3](https://doi.org/10.3390/biom14030249)
Even the order of steps differs by lineage. In *S. cerevisiae*, C14-demethylation occurs first, followed by C4-demethylation to yield zymosterol, then C24-methylation; in most pathogenic fungi such as *Cryptococcus* and *C. albicans*, C24-methylation comes first, followed by C14- then C4-demethylation.[3](https://doi.org/10.3390/biom14030249) Phylogenomic analysis indicates that the last eukaryotic common ancestor harbored a large panel of sterol-synthesis enzymes, with subsequent evolution across the eukaryotic tree proceeding mainly by gene losses.[6](https://pubmed.ncbi.nlm.nih.gov/20333205/)
## By the numbers
- **24 reactions, 25 enzymes** for the full *S. cerevisiae* route, including the Hmg1/Hmg2 isozymes as two of the 25.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
- **16 enzymes** catalyze the conversion of farnesyl-PP to ergosterol, beginning with farnesyl-PP to squalene and cyclization to lanosterol.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
- **Erg6p kinetics**: binding affinities for ATP, SAM, and zymosterol each give \(K_{\mathrm{d}}\) values of approximately 4 µM; the enzyme is slow-acting with a basal \(k_{\mathrm{cat}}\) of 0.6 min⁻¹ that rises through allosteric regulation to a maximum of approximately 5 min⁻¹, supporting a rate-limiting role in some organisms.[3](https://doi.org/10.3390/biom14030249)
- **Overexpression cost**: in *A. oryzae*, all transgenic overexpression strains except AoHmgB, AoErg8, and AoErg26A showed delayed growth, with smaller colony diameters than the control strain.[1](https://journals.asm.org/doi/10.1128/aem.02273-24)
One source describes ergosterol biosynthesis as facilitated by 25 coding genes in Ceratocystidaceae, versus the 25-enzyme figure (including two isozymes) for *S. cerevisiae*.[1](https://journals.asm.org/doi/10.1128/aem.02273-24) • [4](https://doi.org/10.3390/jof7030237)
## References
1. Expression pattern, subcellular localization of Aspergillus oryzae ergosterol synthases, and their effects on ergosterol and fatty acid metabolism. Applied and Environmental Microbiology, 2024. https://journals.asm.org/doi/10.1128/aem.02273-24
2. Recent Advances in Ergosterol Biosynthesis and Regulation Mechanisms in Saccharomyces cerevisiae. https://pmc.ncbi.nlm.nih.gov/articles/PMC5574775/
3. Druggable Sterol Metabolizing Enzymes in Infectious Diseases: Cell Targets to Therapeutic Leads. Biomolecules, 2024. https://doi.org/10.3390/biom14030249
4. Characterization of the Ergosterol Biosynthesis Pathway in Ceratocystidaceae. Journal of Fungi, 2021. https://doi.org/10.3390/jof7030237
5. Regulation of Ergosterol Biosynthesis in Pathogenic Fungi: Opportunities for Therapeutic Development. Microorganisms, 2025. https://www.mdpi.com/2076-2607/13/4/862
6. Phylogenomics of sterol synthesis: insights into the origin, evolution, and diversity of a key eukaryotic feature. https://pubmed.ncbi.nlm.nih.gov/20333205/
7. Outline of the biosynthesis and regulation of ergosterol in yeast. World Journal of Microbiology and Biotechnology, 2019. https://link.springer.com/article/10.1007/s11274-019-2673-2
8. Saccharomyces cerevisiae superpathway of ergosterol biosynthesis I. Saccharomyces Genome Database. https://pathway.yeastgenome.org/YEAST/new-image?object=ERGOSTEROL-SYN-PWY&type=PATHWAY
9. frontiersin.org, https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.665206/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 › Ergosterol-pathway enzyme sets (ERG genes)*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026 · Edited: Oct 11, 2026; Oct 15, 2026 · Last review: Sep 30, 2026*
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