# Squalene-hopene cyclase
Squalene-hopene cyclase (SHC; EC 5.4.99.17) is a bacterial enzyme that catalyzes the cyclization of the linear triterpene squalene to hopene, the key step in the biosynthesis of hopanoids, pentacyclic membrane lipids of bacteria.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) In a single enzymatic step it forms five ring structures, alters 13 covalent bonds, and generates nine stereocenters, making it one of the most complex one-step enzymatic reactions known; the reaction has proven difficult to perform chemically.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/)
| Key fact | Detail |
|---|---|
| Reaction | Squalene (a linear C30 triterpene) cyclized to hopene, with hopanol as a byproduct, in one enzymatic step[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) |
| Complexity | Five rings, 13 covalent bonds altered, and nine stereocenters formed in a single step[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) |
| EC number | 5.4.99.17 (isomerase class)[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) |
| Model enzyme | Alicyclobacillus acidocaldarius SHC, 71.6 kDa, optimum pH 6 and 60 °C, Km of about 3–16 μM[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) |
| Structure | First solved at 2.9 Å resolution (PDB 1SQC); active site is a large central cavity lined by aromatic residues[2](https://www.rcsb.org/structure/1SQC) |
| Signature motifs | Five to eight copies of the QW motif, unique to triterpene cyclases, plus the DXDD(DTA) acid motif that initiates the polycyclization[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) |
| Distribution | Genome analysis of more than 600 bacteria found shc genes in only about 10% of them[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) |
| Biocatalysis | Used industrially for (−)-ambrox production and for scale-up enzymatic synthesis of hopene[4](https://advanced.onlinelibrary.wiley.com/doi/10.1002/adsc.201800132) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) |
## Biological role
Hopanoids are pentacyclic triterpenoids that stabilize the bacterial cytoplasmic membrane in a way comparable to the role sterols play in eukaryotes.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) Their carbon skeleton is built from isopentenyl units through either the mevalonate pathway or the mevalonate-independent DXP pathway; two farnesyl diphosphate (C15) units are condensed tail-to-tail to squalene, which SHC then cyclizes to hopene.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) Hopene is the common precursor of all known hopanoids, which carry a range of side chains.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) In bacterial hopanoid gene clusters the cyclase is encoded by the gene hpnF.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4527182/)
The enzyme is not universal among bacteria: analysis of more than 600 genome sequences found shc genes in only about 10% of species, suggesting that hopanoid biosynthesis is less widespread than once assumed.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/)
## Reaction mechanism
SHC initiates the polycyclization by protonating the 2,3-terminal double bond of squalene, generating a carbocation that undergoes a cascade of electrophilic additions to the remaining double bonds, closing rings A through E with strict stereocontrol.[6](https://openalex.org/W2066061120) The reaction ends with either proton elimination, giving hopene, or addition of water, giving hopanol in a roughly 5:1 ratio of hopene to hopanol.[7](https://ir.lib.nycu.edu.tw/bitstream/11536/39035/1/851301.pdf)
**Catalytic acid.** The crystal structure supports a mechanism in which the acid that starts the reaction by protonating a carbon-carbon double bond is an aspartate coupled to a histidine, part of the conserved DXDD(DTA) motif; a glutamate residue (E45 in the A. acidocaldarius enzyme) carries out the final deprotonation.[2](https://www.rcsb.org/structure/1SQC) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) The unusual strength of this Brønsted acid allows SHC to protonate functional groups beyond its natural substrate, a property exploited in engineered catalysts.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5599884/)
**Structural basis.** The active site sits in a large central cavity of suitable size to bind squalene in its required conformation, lined with aromatic residues.[2](https://www.rcsb.org/structure/1SQC) A cocrystal structure with the inhibitor 2-azasqualene at 2.13 Å resolution showed that the bound substrate undergoes only small conformational changes during formation of rings A through D, while ring E formation faces an entropic barrier, which may explain why steroids lack a fifth ring.[9](https://www.rcsb.org/structure/1UMP) Surface helices are connected by QW motifs (glutamine and tryptophan containing repeats) that tighten the protein structure, apparently to absorb the reaction energy without structural damage.[2](https://www.rcsb.org/structure/1SQC)
## Relationship to oxidosqualene cyclase
SHC is the prokaryotic counterpart of oxidosqualene cyclase (OSC), the eukaryotic enzyme that cyclizes 2,3-oxidosqualene toward sterols such as lanosterol.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) The two enzyme types are distantly related in sequence, probably derived from a common ancestor, and share the QW motif and a large central catalytic cavity.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) The human lanosterol synthase shares about 25% amino acid sequence identity with squalene-hopene cyclase, indicating substantial structural homology.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5599884/) They differ mainly in initiation: OSC requires the epoxidized substrate 2,3-oxidosqualene and rejects plain squalene at its entrance channel, while SHCs generally accept both squalene and 2,3-oxidosqualene, though the M. capsulatus SHC does not accept oxidosqualene.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/)
## Properties of characterized SHCs
Only a few SHCs have been purified and studied biochemically. The enzyme from the thermophilic bacterium [Alicyclobacillus](https://www.edgepedia.org/alicyclobacillus) acidocaldarius, first purified in 1986, became the model enzyme of the field.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) It is a membrane-associated protein of 71.6 kDa that can be solubilized with nonionic detergents, with a pH optimum of 6 and a temperature optimum of 60 °C.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) SHCs are classified as monotopic membrane proteins, submerged into the nonpolar part of the phospholipid bilayer from one side without protruding through it.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) Optimal temperatures vary from 30 °C to 60 °C depending on the source species, roughly tracking the growth temperature of the host organism.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) Characterized SHCs include those of [Bradyrhizobium](https://www.edgepedia.org/bradyrhizobium) japonicum (76.3 kDa, optimum 28 °C), Methylococcus capsulatus (74.1 kDa, 40 °C), [Rhodopseudomonas](https://www.edgepedia.org/rhodopseudomonas) palustris, Streptomyces peucetius (35 °C), and Zymomonas mobilis (30 °C).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/)
## Biocatalytic applications
[Chemical synthesis](https://www.edgepedia.org/chemical-synthesis) of hopene remains inefficient, and extraction from plants or bacteria is complicated and environmentally unfriendly, so enzymatic production with SHC is an attractive alternative.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) A 2020 study identified a novel soluble SHC, OUC-SaSHC, from [Streptomyces](https://www.edgepedia.org/streptomyces) albolongus ATCC 27414; the gene encodes 691 amino acids, and the enzyme, with one DXDDTA motif and seven QW motifs, showed a specific activity of 1138 U/mg.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/) Because it is soluble rather than membrane-bound, it can be purified simply by Ni2+-NTA chromatography. Under optimized conditions (100 mM sodium phosphate buffer, pH 7.0, 0.5% Tween 80, 20 mM squalene, 0.14 mg/mL enzyme, 30 °C), a 100 mL scale-up reaction converted up to 99% of squalene in 36 hours and produced 8.07 mg/mL hopene.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/)
**Beyond the natural substrate.** SHC tolerates many terpenoid substrates and analogs, a property with industrial potential.[10](https://pubmed.ncbi.nlm.nih.gov/37055654/) A biocatalytic process using squalene-hopene cyclase has been developed for production of the fragrance compound (−)-ambrox.[4](https://advanced.onlinelibrary.wiley.com/doi/10.1002/adsc.201800132) Engineered SHCs have also been used for asymmetric cation-olefin monocyclizations, locking linear terpenoid substrates in defined chiral conformations to achieve polyene cyclizations with stereocontrol.[11](https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cssc.202102180) The enzyme's ability to perform Brønsted acid activation of diverse biological and nonbiological functional groups suggests a broad scope for developing new protein-engineered catalysts for organic synthesis.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5599884/)
## References
1. [Siedenburg, G., Jendrossek, D. (2011). Squalene-Hopene Cyclases. Applied and Environmental Microbiology / PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3131620/)
2. [RCSB PDB 1SQC: Squalene-hopene cyclase from Alicyclobacillus acidocaldarius (Wendt, Poralla, Schulz, Science 1997)](https://www.rcsb.org/structure/1SQC)
3. [Liu, Z. et al. (2020). A Novel Soluble Squalene-Hopene Cyclase and Its Application in Efficient Synthesis of Hopene. Frontiers in Bioengineering and Biotechnology](https://pmc.ncbi.nlm.nih.gov/articles/PMC7232578/)
4. [Eichhorn, E. et al. (2018). Biocatalytic Process for (−)-Ambrox Production Using Squalene Hopene Cyclase. Advanced Synthesis & Catalysis](https://advanced.onlinelibrary.wiley.com/doi/10.1002/adsc.201800132)
5. [Biosynthesis of Squalene from Farnesyl Diphosphate in Bacteria: Three Steps Catalyzed by Three Enzymes. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4527182/)
6. [Rationally designed inhibitors as tools for comparing the mechanism of squalene-hopene cyclase with oxidosqualene cyclase](https://openalex.org/W2066061120)
7. [National Chiao Tung University thesis: Squalene-Hopene Cyclase (SHC) reaction scheme](https://ir.lib.nycu.edu.tw/bitstream/11536/39035/1/851301.pdf)
8. [Structural and Chemical Biology of Terpenoid Cyclases. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5599884/)
9. [RCSB PDB 1UMP: Geometry of triterpene conversion to pentacarbocyclic hopene (Reinert, Balliano, Schulz, 2004)](https://www.rcsb.org/structure/1UMP)
10. [Squalene hopene cyclases and oxido squalene cyclases: potential targets for regulating cyclisation reactions. PubMed](https://pubmed.ncbi.nlm.nih.gov/37055654/)
11. [Asymmetric Cation-Olefin Monocyclization by Engineered Squalene-Hopene Cyclases. ChemSusChem](https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cssc.202102180)
---
*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Terpene, sterol, and prenyltransferase synthases › Terpene synthase families and mechanisms › Di- and triterpene synthases › Squalene-hopene cyclases and bacterial triterpene cyclases*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license