# Ent-kaurene synthase
Ent-kaurene synthase (KS, EC 4.2.3.19) is a class I diterpene synthase that converts ent-copalyl diphosphate into the tetracyclic hydrocarbon ent-kaur-16-ene, releasing diphosphate in the reaction ent-copalyl diphosphate = ent-kaurene + diphosphate.[1](https://iubmb.qmul.ac.uk/enzyme/EC4/2/3/19.html) All land plants produce ent-kaur-16-ene; in vascular plants it is an intermediate in gibberellin phytohormone biosynthesis, while in bryophytes it appears to be a precursor to an undefined signaling molecule.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) Beyond plants, KS enzymes or KS activities occur in rhizobial bacteria and in fungi.[3](https://doi.org/10.1128/jb.01031-13) • [4](https://doi.org/10.1074/jbc.272.35.21706)
| Key fact | Detail |
|---|---|
| EC number and reaction | EC 4.2.3.19: ent-copalyl diphosphate = ent-kaurene + diphosphate[1](https://iubmb.qmul.ac.uk/enzyme/EC4/2/3/19.html) |
| Pathway position | Second committed cyclization step in gibberellin biosynthesis, after ent-copalyl diphosphate synthase (CPS)[5](https://doi.org/10.1104/pp.15.01727) |
| Occurrence | All land plants; rhizobia (conserved operon); fungi such as Phaeosphaeria sp. L487[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) • [3](https://doi.org/10.1128/jb.01031-13) • [4](https://doi.org/10.1074/jbc.272.35.21706) |
| First structure | Bradyrhizobium japonicum BjKS (gene blr2150), PDB 4W4R, with a bisphosphonate inhibitor[6](https://www.nature.com/articles/srep06214) • [7](https://www.rcsb.org/structure/4W4R) |
| Product specificity | A conserved isoleucine controls the final rearrangement; Ile→Thr mutants yield mostly ent-pimaradiene[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) |
| Flux control | Ent-kaurene levels change more than 1,000-fold through CPS manipulation but less than 2-fold from KS over-expression[5](https://doi.org/10.1104/pp.15.01727) |
| Derived metabolites | More than 800 known natural products derive from ent-kaurene, including maize kauralexins[5](https://doi.org/10.1104/pp.15.01727) |
## What ent-kaurene synthase does
KS performs the second of two cyclizations that build ent-kaurene from the linear precursor geranylgeranyl diphosphate (GGPP). In flowering plants, ent-copalyl diphosphate synthase (ent-CPS) first cyclizes GGPP to ent-copalyl diphosphate (ent-CPP, also called ent-CDP), and KS then converts ent-CPP to ent-kaurene.[8](https://doi.org/10.1042/bj20140134) The IUBMB nomenclature records KS as part of a bifunctional ent-kaurene biosynthesis enzyme in some organisms, alongside [EC 5.5.1.13](https://www.edgepedia.org/ent-copalyl-diphosphate-synthase) (ent-copalyl diphosphate synthase).[1](https://iubmb.qmul.ac.uk/enzyme/EC4/2/3/19.html)
The reaction is a controlled carbocation cascade. The accepted mechanism begins with Mg2+-catalyzed loss of the diphosphate group from ent-CPP, generating an allylic cation that cyclizes to the pimaren-8-yl cation. A secondary cyclization follows, with a Wagner-Meerwein migration (carbocation rearrangement shifting a carbon bond) of carbon C-12 to C-16, forming the ent-kauran-16-yl cation; loss of a proton from the neighboring methyl group then yields the alkene ent-kaur-16-ene.[6](https://www.nature.com/articles/srep06214) • [2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) Downstream of KS, gibberellin biosynthesis proceeds through cytochrome P450 and 2-oxoglutarate-dependent dioxygenase steps, including a ring contraction that converts the 6-6-6-5 ring system of ent-kaurene into the 6-5-6-5 arrangement characteristic of the gibberellins.[5](https://doi.org/10.1104/pp.15.01727)
Ent-kaurene is not only a hormone precursor. It serves as an intermediate in specialized diterpenoid metabolism, with more than 800 known derived natural products; among these are the maize kauralexins, antibiotic diterpenoid defenses.[5](https://doi.org/10.1104/pp.15.01727)
## Mechanism and structure
KS belongs to the class I terpene synthases, which ionize a prenyl diphosphate substrate using metal ions and then shape the resulting carbocation cascade. The clearest structural picture comes from the bacterium *Bradyrhizobium japonicum*. The first X-ray crystal structure of a class I bacterial diterpene cyclase was that of ent-kaurene synthase from *B. japonicum* (BjKS, gene blr2150), reported together with site-directed mutagenesis of its catalytic residues.[6](https://www.nature.com/articles/srep06214) The structure is deposited as PDB 4W4R.[7](https://www.rcsb.org/structure/4W4R)
The structure shows the ent-CPP substrate binding in a hydrophobic pocket near a cluster of Asp and Arg residues that are essential for catalysis, with the carbocations formed on ionization protected by Leu, Tyr, and Phe residues; a bisphosphonate inhibitor binds to the same site.[6](https://www.nature.com/articles/srep06214)
Active-site residues determine the product. Substituting threonine for a conserved isoleucine short-circuits the complex bicyclization and rearrangement reaction catalyzed by KSs after initial cyclization, leading to predominant production of ent-pimaradiene, largely ent-pimara-8(14),15-diene; this single-residue effect extends even to the bryophyte bifunctional enzyme.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) Ile→Ala or Ser mutants also produce significant amounts of 8α-hydroxy-ent-pimara-15-ene, formed when water adds to the cation instead of deprotonation.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) Conversely, in the moss *Physcomitrella patens* kaurene synthase, smaller active-site residues in place of Leu and Tyr allow water to quench the ent-kauranyl cation, so the enzyme yields 16α-hydroxy-ent-kaurane alongside kaurene.[6](https://www.nature.com/articles/srep06214)
The BjKS structure resembles the α domains of modern plant terpene cyclases, supporting the proposal that many plant terpene cyclases arose from bacterial diterpene cyclases.[6](https://www.nature.com/articles/srep06214)
## Where it occurs: plants, fungi and bacteria
**Land plants.** Every surveyed group of embryophytes produces ent-kaur-16-ene, but the enzymatic arrangement differs by lineage. The ancestral fused, bifunctional CPS/KS enzyme is retained only in certain early diverging plants such as bryophytes; in most plants, gene duplication and sub-functionalization produced distinct ent-CPP synthases and ent-kaurene synthases.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) The *Physcomitrella patens* bifunctional enzyme PpCPS/KS is a 101-kDa polypeptide that carries out both the class II and class I reactions in one protein.[9](https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2006.10.018) • [8](https://doi.org/10.1042/bj20140134)
Gene family size varies widely among grasses. Rice contains four CPS/CPS-like genes and eleven KS/KS-like genes involved in producing a large variety of labdane-type diterpenes.[10](https://link.springer.com/article/10.1186/s12870-015-0647-6) Maize holds a tandem array of three plastid-targeted ent-kaurene synthase genes, ZmTPS1, ZmKSL3, and ZmKSL5, all producing ent-kaurene from ent-CPP.[5](https://doi.org/10.1104/pp.15.01727) [Brachypodium distachyon](https://www.edgepedia.org/brachypodium-distachyon) carries one ent-CPS gene homolog (BdCPS) and four tandemly arrayed KS-like genes (BdKS1, KSL2, KSL3, KSL4), a pattern described in a 2024 characterization.[11](https://doi.org/10.1093/bbb/zbad146)
**Bacteria.** The nitrogen-fixing symbiotic bacterium *Bradyrhizobium japonicum* carries adjacent genes encoding two sequentially acting diterpene cyclases that together transform GGPP to ent-kaurene, the olefin precursor to the gibberellin plant hormones.[3](https://doi.org/10.1128/jb.01031-13) Homologous terpene synthase genes occur across all four major rhizobial genera, and cloned representatives from each genus were confirmed to produce ent-kaurene.[3](https://doi.org/10.1128/jb.01031-13) These diterpene synthases sit in a conserved operon that includes an adjacent isoprenyl diphosphate synthase producing the GGPP precursor; the operon is selectively expressed during nodulation, and its scattered distribution is attributable to independent horizontal gene transfer within symbiotic plasmids or genomic islands.[3](https://doi.org/10.1128/jb.01031-13)
**Fungi.** Ent-kaurene synthase activity has been characterized from the fungus Phaeosphaeria sp. L487; comparison of this KS with diterpene cyclases from *Gibberella fujikuroi* and other fungi is needed to understand the evolution of gibberellin biosynthesis in fungi.[4](https://doi.org/10.1074/jbc.272.35.21706)
## Role in gibberellin and specialized metabolism
Gibberellin biosynthesis begins with the CPS- and KS-catalyzed cyclization of GGPP to ent-CPP and then ent-kaurene, after which oxidative steps take over.[5](https://doi.org/10.1104/pp.15.01727) Because ent-kaurene feeds the hormone pathway, loss of KS activity produces gibberellin-deficient dwarfism. In maize, the dwarf-5 (d5) mutant maps to a region containing the tandem array ZmTPS1, ZmKSL3, and ZmKSL5; all three enzymes are targeted to plastids and produce ent-kaurene from ent-CPP, but only the expression of ZmKSL3 is abolished in d5 plants, making ZmKSL3 the non-redundant KS for gibberellin metabolism.[5](https://doi.org/10.1104/pp.15.01727) This explains a general pattern: dwarf mutants often map to KS or CPS loci because removing either cyclization step starves the plant of gibberellin.
Flux into ent-kaurene is controlled primarily at the CPS step rather than at KS. Manipulating CPS changes ent-kaurene levels by more than 1,000-fold, whereas over-expression of Arabidopsis AtKS on top of that adds only a minimal increase (below 2-fold).[5](https://doi.org/10.1104/pp.15.01727)
## How it compares with related diterpene synthases
Class I terpene synthases that do not mediate ent-kaurene formation but generate other labdane-related diterpenes are designated kaurene synthase-like (KSL) enzymes.[10](https://link.springer.com/article/10.1186/s12870-015-0647-6) The distinction is functional, not structural: KSL enzymes act on the same or closely related substrates but abbreviate or divert the KS reaction. Rice illustrates this divergence within one genome: OsKS1 is required for gibberellin production, while a separate ent-isokaurene synthase, OsKSL6, cyclizes ent-CPP to a different skeleton.[5](https://doi.org/10.1104/pp.15.01727) In poplar, a single amino acid difference determines whether a diterpene synthase produces ent-kaurene or 16α-hydroxy-ent-kaurane.[10](https://link.springer.com/article/10.1186/s12870-015-0647-6)
Compared with the class II CPS reaction, which forms the bicyclic ent-CPP ring system from GGPP, KS performs the additional ionization, second cyclization, Wagner-Meerwein rearrangement, and final deprotonation. In most plants the two reactions are carried by separate enzymes, but fusion persists in bryophytes and separate adjacent genes serve the same two-step sequence in rhizobia.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full) • [3](https://doi.org/10.1128/jb.01031-13) Even canonical KS enzymes are not always perfectly specific: BRENDA documents multi-product behavior such as PpCPS/KS producing 16-hydroxy-ent-kaurene via three carbocation intermediates, and minor products of some KSs including ent-beyerene, ent-sandaracopimaradiene, and ent-kaur-15-ene.[12](https://www.brenda-enzymes.org/enzyme.php?ecno=4.2.3.19)
## Recent developments
Two recent results extend the picture. A 2024 study showed that the F72Y mutation of *Bradyrhizobium japonicum* ent-kaurene synthase redirects the multistep carbocation cyclization to produce the tricyclic ent-rosa-5(10),15-diene and ent-pimara-8,15-diene in addition to tetracyclic ent-kaurene, with combined computational and experimental work suggesting that Tyr72 serves as a general base for reshaping the catalytic function.[13](https://pubs.rsc.org/en/content/articlelanding/2024/cy/d3cy00115f) This demonstrates that a single active-site substitution can open routes to entire alternative diterpene scaffolds. Also in 2024, characterization of the *Brachypodium distachyon* diterpene synthase genes confirmed the one-CPS/four-KSL arrangement in that grass genome.[11](https://doi.org/10.1093/bbb/zbad146)
## References
1. [EC 4.2.3.19 — IUBMB Enzyme Nomenclature](https://iubmb.qmul.ac.uk/enzyme/EC4/2/3/19.html)
2. [Extending a Single Residue Switch for Abbreviating Catalysis in Plant ent-Kaurene Synthases (Frontiers in Plant Science)](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.01765/full)
3. [Functional Conservation of the Capacity for ent-Kaurene Biosynthesis and an Associated Operon in Certain Rhizobia (Journal of Bacteriology)](https://doi.org/10.1128/jb.01031-13)
4. [ent-Kaurene Synthase from the Fungus Phaeosphaeria sp. L487 (Journal of Biological Chemistry)](https://doi.org/10.1074/jbc.272.35.21706)
5. [A Tandem Array of ent-Kaurene Synthases in Maize with Roles in Gibberellin and More Specialized Metabolism (Plant Physiology)](https://doi.org/10.1104/pp.15.01727)
6. [Structure, function and inhibition of ent-kaurene synthase from Bradyrhizobium japonicum (Scientific Reports)](https://www.nature.com/articles/srep06214)
7. [RCSB PDB 4W4R: Crystal structure of ent-kaurene synthase BjKS](https://www.rcsb.org/structure/4W4R)
8. [Molecular evolution of the substrate specificity of ent-kaurene synthases to adapt to gibberellin biosynthesis in land plants (Biochemical Journal)](https://doi.org/10.1042/bj20140134)
9. [Identification and functional analysis of ent-kaurene synthase (PpCPS/KS) from Physcomitrella patens (FEBS Letters)](https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2006.10.018)
10. [One amino acid makes the difference: the formation of ent-kaurene and 16α-hydroxy-ent-kaurane by diterpene synthases in poplar (BMC Plant Biology)](https://link.springer.com/article/10.1186/s12870-015-0647-6)
11. [Characterization of diterpene synthase genes in Brachypodium distachyon (Bioscience, Biotechnology, and Biochemistry)](https://doi.org/10.1093/bbb/zbad146)
12. [BRENDA Enzyme Database — EC 4.2.3.19 ent-kaurene synthase](https://www.brenda-enzymes.org/enzyme.php?ecno=4.2.3.19)
13. [Harnessing the catalytic plasticity of the ent-kaurene synthase from Bradyrhizobium japonicum to produce the ent-rosane and ent-pimarane scaffolds (Catalysis Science & Technology, 2024)](https://pubs.rsc.org/en/content/articlelanding/2024/cy/d3cy00115f)
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*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 › Gibberellin-biosynthetic diterpene synthases*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license