Magnesium chelatase

 

Magnesium chelatase has a vital role in chlorophyll biosynthesis, using the energy from ATP hydrolysis to insert a Mg(II) ion into a porphyrin ring. It is part of a generic family of cellular ATPases known as AAA, displaying homology in particular to Cobalt Chelatase. Three subunits make up the overall protein in bacteria, BchI, BChlH and BChlD, also conserved in higher organisms. BChlI has ATPase activity and BChlH binds to the protoporphyrin group while BChlD has an regulatory allosteric role. Currently, there is limited experimental evidence to confirm a formal mechanism of magnesium chelatase with Arg 289 in BChlI being the only identifiable catalytic residue.

 

Reference Protein and Structure

Sequence
P26239 UniProt (6.6.1.1) IPR011775 (Sequence Homologues) (PDB Homologues)
Biological species
Rhodobacter capsulatus SB 1003 (Bacteria) Uniprot
PDB
1g8p - CRYSTAL STRUCTURE OF BCHI SUBUNIT OF MAGNESIUM CHELATASE (2.1 Å) PDBe PDBsum 1g8p
Catalytic CATH Domains
1.10.8.80 CATHdb (see all for 1g8p)
Click To Show Structure

Enzyme Reaction (EC:6.6.1.1)

ATP(4-)
CHEBI:30616ChEBI
+
water
CHEBI:15377ChEBI
+
magnesium(2+)
CHEBI:18420ChEBI
+
protoporphyrin(2-)
CHEBI:57306ChEBI
→
ADP(3-)
CHEBI:456216ChEBI
+
hydron
CHEBI:15378ChEBI
+
magnesium protoporphyrin(2-)
CHEBI:60492ChEBI
+
hydrogenphosphate
CHEBI:43474ChEBI
Alternative enzyme names: Mg-chelatase, Mg-protoporphyrin IX magnesio-lyase, Magnesium-chelatase, Magnesium-protoporphyrin IX chelatase, Magnesium-protoporphyrin chelatase, Protoporphyrin IX Mg-chelatase, Protoporphyrin IX magnesium-chelatase, Mg-protoporphyrin IX chelatase,

Enzyme Mechanism

Introduction

Binding of the porphyrin.BChlH complex and the Magnesium triggers the release of the ATP binding site of the BChI subunit which is able to hydrolyse ATP. The hydrolysis of ATP proceeds via the nucleophilic attack of a water molecule on the gamma phosphate, to form a pentavalent transition state stabilised by Arg 289. This collapses to release the products ADP and Pi, causing subunit motion which results in the transfer of Mg2+ to the porphyrin.

Catalytic Residues Roles

UniProt PDB* (1g8p)
Arg289 Arg289A The protonated side chain of Arg 289 acts to stabilise the pentavalent phosphate transition state through electrostatic interaction. The transition state then collapses to release the products resulting in ATP hydrolysis. electrostatic stabiliser
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

References

  1. Fodje MN et al. (2001), J Mol Biol, 311, 111-122. Interplay between an AAA module and an integrin I domain may regulate the function of magnesium chelatase. DOI:10.1006/jmbi.2001.4834. PMID:11469861.
  2. Adams NB et al. (2016), FEBS Lett, 590, 1687-1693. The catalytic power of magnesium chelatase: a benchmark for the AAA(+) ATPases. DOI:10.1002/1873-3468.12214. PMID:27176620.
  3. Chen X et al. (2015), Nat Plants, 1, 15125-. Crystal structure of the catalytic subunit of magnesium chelatase. DOI:10.1038/nplants.2015.125. PMID:27250678.

Step 1.

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Catalytic Residues Roles

Residue Roles
Arg289A electrostatic stabiliser

Chemical Components

Step 1.

Contributors

Peter Sarkies, Gemma L. Holliday, Morwenna Hall