# Chelatase

A chelatase is an enzyme that catalyzes the insertion of a divalent metal ion into a tetrapyrrole (ring of four pyrrole units forming heme and chlorophyll) ring, the chemical step that converts organic tetrapyrrole precursors into the metalated cofactors heme, chlorophyll, cobalamin (vitamin B\(_{12}\)), siroheme, and coenzyme F\(_{430}\)<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>. The metal determines the cofactor's function: Fe\(^{2+}\) in heme and siroheme, Mg\(^{2+}\) in chlorophyll and bacteriochlorophyll, Co\(^{2+}\) in cobalamin, and Ni\(^{2+}\) in coenzyme F\(_{430}\)<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>. All of these cofactors arise from a common tetrapyrrole precursor, uroporphyrinogen III, and each requires a specific chelatase to insert its metal ion in response to cellular needs<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>.

| Key fact | Detail |
|---|---|
| Reaction | Insertion of a divalent metal ion (Fe\(^{2+}\), Mg\(^{2+}\), Co\(^{2+}\), or Ni\(^{2+}\)) into a tetrapyrrole macrocycle, with loss of two protons from the pyrrole nitrogens<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup> |
| Main classes | Class I, ATP-dependent heteromeric complexes; Class II, ATP-independent monomeric or homodimeric proteins; Class III, multifunctional dehydrogenases with iron chelation ability<sup>[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9133952/)</sup> |
| Ferrochelatase | ATP-independent; catalyzes the terminal step of heme synthesis in nearly every living organism; monomeric size 38–51 kDa with no more than six residues perfectly conserved across evolution<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup> |
| Magnesium chelatase | ATP-dependent AAA+ enzyme of three subunits (ChlI/BchI, ChlD/BchD, ChlH/BchH); requires roughly 12–15 ATP per reaction cycle; catalyzes the first committed step of (bacterio)chlorophyll synthesis<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup><sup> • </sup><sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup> |
| Catalytic power | ATP hydrolysis shifts the product-to-substrate ratio for Mg chelation from the equilibrium value of 10\(^{-6}\) to about 500, a shift of 10\(^{8}\), equivalent to a free energy change of 53 kJ·mol\(^{-1}\)<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)</sup> |
| Branch point | At protoporphyrin IX, Mg\(^{2+}\) insertion by Mg-chelatase leads to (bacterio)chlorophyll while Fe\(^{2+}\) insertion by ferrochelatase leads to heme, so the two enzymes compete for a shared substrate<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup> |

## The three classes of chelatases

On the basis of structural, functional, and genetic studies together with genome sequencing, three main classes of chelatases are recognized<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>.

**Class I** enzymes are ATP-dependent heteromeric complexes. They include protoporphyrin IX magnesium chelatase (ChlH-I-D, or BchH-I-D in bacteria), which is involved in chlorophyll and bacteriochlorophyll biosynthesis, and the aerobic cobaltochelatase (CobN-S-T) associated with cobalamin biosynthesis<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>. A nickel chelatase for cofactor F\(_{430}\) has also been assigned to this class<sup>[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9133952/)</sup>.

**Class II** enzymes are ATP-independent. They include protoporphyrin IX ferrochelatase (HemH) of heme biosynthesis, sirohydrochlorin ferrochelatase (SirB) of siroheme biosynthesis, and the anaerobic sirohydrochlorin cobaltochelatases (CbiK and CbiX) involved in cobalamin biosynthesis<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>.

**Class III** enzymes are multifunctional homodimeric proteins associated with [siroheme biosynthesis](https://www.edgepedia.org/siroheme-biosynthesis), namely siroheme synthase (CysG) and the yeast precorrin-2 dehydrogenase and sirohydrochloride ferrochelatase (Met8p); each houses two enzymatic activities, a dehydrogenase and a chelatase<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>.

The classes are unrelated by sequence. There is no sequence similarity among chelatases as a whole, and even within the ATP-independent class members share less than 12 percent overall sequence identity, yet they adopt a similar fold with two Rossmann-type alpha/beta domains forming an active-site cleft<sup>[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)</sup>.

## The branch point between heme and chlorophyll

Up to protoporphyrin IX, heme and (bacterio)chlorophyll synthesis share a common pathway. At that point the cell faces two choices: Mg\(^{2+}\) insertion to make (bacterio)chlorophyll, catalyzed by Mg-chelatase, or Fe\(^{2+}\) insertion to make heme, catalyzed by ferrochelatase<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup>. The relative activities of the two enzymes must therefore be regulated according to the organism's requirements for these end products<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup>. In photosynthetic organisms the two chelating enzymes compete directly for protoporphyrin IX<sup>[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC58950/)</sup>.

## Magnesium chelatase: an AAA+ motor

Magnesium chelatase (EC 6.6.1.1) performs the initial step of chlorophyll biosynthesis that commits the pathway, inserting an Mg\(^{2+}\) ion into the macrocycle of protoporphyrin IX<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)</sup>. In all photosynthetic organisms studied, it is a three-component enzyme<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup>. The subunits have molecular weights of roughly 40, 70, and 140 kDa; the first two form a characteristic two-tiered ring complex with six subunits in each ring, while the third, ChlH/BchH, is the catalytic subunit that binds the porphyrin substrate<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. Twelve to fifteen ATP molecules are required for each reaction cycle<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>, and the reaction proceeds in two steps, an ATP-dependent activation followed by an ATP-dependent chelation step<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)</sup>.

Only the ChlI subunit has confirmed ATPase activity, although ChlD is also a member of the AAA+ superfamily and allosterically regulates the enzyme<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)</sup>. In the presence of both ATP and Mg\(^{2+}\), an AAA+ motor complex is assembled from a hexameric or heptameric BchI ring connected to a hexameric BchD ring; protoporphyrin IX binds to the BchH subunit, and its transient interaction with the motor complex leads to metal insertion<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC3595025/)</sup>. Structural work on ChlH shows two linked domains in an initially open conformation binding protoporphyrin IX, with rearranging loops enveloping the substrate to form an active-site cavity<sup>[8](https://pubmed.ncbi.nlm.nih.gov/33257858/)</sup>.

**Why ATP is needed.** The chelation reaction is thermodynamically unfavorable, with a ΔG°′ of roughly 25–33 kJ·mol\(^{-1}\)<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)</sup>. Simply accelerating the rate is inadequate because the reverse reaction would speed up proportionally. By coupling metalation to ATP hydrolysis, magnesium chelatase shifts the product-to-substrate ratio from the equilibrium value of 10\(^{-6}\) to about 500 at low porphyrin concentrations, a displacement of 10\(^{8}\) corresponding to 53 kJ·mol\(^{-1}\); the ratio decays back to equilibrium once ATP is consumed<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)</sup>. The enzyme's rate enhancement, \( k_{\mathrm{cat}}/k_{\mathrm{uncat}} \), is 400 × 10\(^{6}\) M, rising to 550 × 10\(^{6}\) M in the presence of the activator protein GUN4<sup>[9](https://eprints.whiterose.ac.uk/id/eprint/101272/1/Adams_et_al-2016-FEBS_Letters.pdf)</sup>. The auxiliary factor GUN4 is required for full activity of the complex<sup>[10](https://www.researchgate.net/publication/401731031_Filament_Formation_by_ChlI_Challenges_the_Current_View_of_Magnesium_Chelatase_Architecture)</sup>.

## Ferrochelatase: the ATP-independent counterpart

Ferrochelatase catalyzes the terminal reaction in heme synthesis in nearly every living organism, inserting ferrous iron into protoporphyrin IX with the expulsion of two protons<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. It requires no ATP or other high-energy compound<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC1077671/pdf/plntphys00817-0223.pdf)</sup>. In eukaryotes the enzyme is peripherally associated with the inner mitochondrial membrane with its active site facing the membrane; in prokaryotes it is membrane-associated in some species and soluble in others<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. Mammalian ferrochelatase is the only chelatase in mammalian cells and is itself a metalloprotein carrying a cysteine-coordinated [2Fe-2S] iron-sulfur cluster whose biological significance remains unclear, although it is essential for the catalytic reaction<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup><sup> • </sup><sup>[12](https://www.sciencedirect.com/science/article/pii/S0010854522000595)</sup>.

The enzyme is highly specific for ferrous iron in vivo, but in vitro it inserts a wide variety of divalent metal ions into porphyrins, some at rates comparable to iron<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. Loss of ferrochelatase activity in humans leads to accumulation of protoporphyrin IX, erythropoietic protoporphyria, liver damage, and possibly death<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>.

**Mechanism.** [Porphyrin](https://www.edgepedia.org/porphyrin) metalation involves approximation of metal and porphyrin, desolvation of the metal ion, distortion of pyrrole rings out of the macrocyclic plane, formation of metal-nitrogen bonds, and removal of the two remaining protons; ferrochelatase has been shown or proposed to catalyze each of these events<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. Multiple lines of evidence support porphyrin distortion as a central feature of the mechanism: N-alkylated porphyrins, which are distorted, metallate non-enzymatically 10\(^{3}\) to 10\(^{5}\) times faster than undistorted porphyrins, and the B. subtilis enzyme crystallized with N-methyl mesoporphyrin shows ring A distorted by 36°<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>. The rate-limiting step of the overall reaction, at least in vitro, is product release, which in the human enzyme is accompanied by unwinding of an active-site π-helix<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)</sup>.

## Evolution

The two best-studied chelatases differ profoundly in architecture despite catalyzing similar reactions. In *Rhodobacter sphaeroides*, ferrochelatase is a single subunit of 115 kDa, whereas Mg-chelatase is a 1.1–1.2-MDa complex of 2–3 copies of BchH, 6 copies of BchI, and 6 copies of BchD; ferrochelatase belongs to a non-homologous class that includes the anaerobic cobaltochelatases CbiK and CbiX, while Mg-chelatase is related to AAA+ chelatases such as the aerobic cobaltochelatase CobNST<sup>[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC3121458/)</sup>.

Evolutionary reconstructions of the ATP-independent class propose a route from an ancestral CbiX-like cobaltochelatase to a sirohydrochlorin ferrochelatase by a change in metal specificity to Fe\(^{2+}\), and then to a protoporphyrin ferrochelatase by a change in tetrapyrrole substrate specificity from sirohydrochlorin to protoporphyrin<sup>[14](https://www.jbc.org/article/S0021925820862772/pdf)</sup>. A monomeric class II enzyme designated Mch4, with four histidine residues at its active site, has been described as an evolutionary-intermediate type that functions as a sirohydrochlorin ferrochelatase in vivo and can use Ni\(^{2+}\), Co\(^{2+}\), and Fe\(^{2+}\) with sirohydrochlorin in vitro<sup>[15](https://www.sciencedirect.com/science/article/pii/S0162013425001904)</sup>.

## Recent findings

A 2020 study combining crystallography, modeling, mutagenesis, and enzymology established how the ChlH subunit envelops its porphyrin substrate, coupling ATP hydrolysis by the ChlID motor complex to chelation within ChlH<sup>[8](https://pubmed.ncbi.nlm.nih.gov/33257858/)</sup>. In 2025, researchers demonstrated chelatase activity in bacterial magnesium dechelatase, an enzyme that normally extracts the central magnesium ion from chlorophyll; the recombinant protein inserted metal ions such as zinc into a chlorophyll derivative, with residues H32, D34, and D62 implicated in metal coordination and proton extraction, and the bacterial gene complemented *Escherichia coli* ferrochelatase mutants<sup>[16](https://pubmed.ncbi.nlm.nih.gov/40527589/)</sup>.

## References

1. [Chelatases: distort to select? (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2997100/)
2. [Ferrochelatase: The Convergence of the Porphyrin Biosynthesis and Iron Transport Pathways (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3156484/)
3. [Ferrochelatase: Mapping the Intersection of Iron and Porphyrin Metabolism in the Mitochondria (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9133952/)
4. [Mechanism and regulation of Mg-chelatase (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC1218797/)
5. [The catalytic power of magnesium chelatase: a benchmark for the AAA+ ATPases (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4982103/)
6. [Role of Magnesium Chelatase Activity in the Early Steps of the Tetrapyrrole Biosynthetic Pathway (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC58950/)
7. [Chlorophyll Biosynthesis Gene Evolution Indicates Photosystem Gene Duplication, Not Photosystem Merger, at the Origin of Oxygenic Photosynthesis (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3595025/)
8. [The active site of magnesium chelatase (PubMed)](https://pubmed.ncbi.nlm.nih.gov/33257858/)
9. [The catalytic power of magnesium chelatase (FEBS Letters PDF, White Rose)](https://eprints.whiterose.ac.uk/id/eprint/101272/1/Adams_et_al-2016-FEBS_Letters.pdf)
10. [Filament Formation by ChlI Challenges the Current View of Magnesium Chelatase Architecture (ResearchGate preprint)](https://www.researchgate.net/publication/401731031_Filament_Formation_by_ChlI_Challenges_the_Current_View_of_Magnesium_Chelatase_Architecture)
11. [Walker and Weinstein, Plant Physiol. Vol. 95, 1991 (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC1077671/pdf/plntphys00817-0223.pdf)
12. [Metal ion coordination sites in ferrochelatase (ScienceDirect)](https://www.sciencedirect.com/science/article/pii/S0010854522000595)
13. [A bchD (Magnesium Chelatase) Mutant of Rhodobacter sphaeroides Synthesizes Zinc Bacteriochlorophyll (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3121458/)
14. [A Story of Chelatase Evolution (Journal of Biological Chemistry)](https://www.jbc.org/article/S0021925820862772/pdf)
15. [Monomeric class II chelatase Mch4 (Journal of Inorganic Biochemistry)](https://www.sciencedirect.com/science/article/pii/S0162013425001904)
16. [Chelatase activity of magnesium dechelatase associated with chlorophyll degradation (PubMed)](https://pubmed.ncbi.nlm.nih.gov/40527589/)

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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Elemental and cofactor metabolism › Cofactor and coenzyme biosynthesis › Metallocofactor assembly › Metal insertion and chelatase mechanisms*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*

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