InterPro domain: IPR045006

Magnesium-chelatase is a three-component enzyme, consisting of CHLI, CHLD and CHLH subunits, that catalyses the insertion of Mg2 into protoporphyrin IX, the first unique step in the synthesis of (bacterio)chlorophyll. As a result, Mg-chelatase has an important role in channeling intermediates into the (bacterio)chlorophyll branch in response to conditions suitable for photosynthetic growth [ 1 , 2 ]. The reaction takes place in two steps including an ATP-dependent activation followed by an ATP-dependent chelation step.

This family represents CHLI subunit (38-42kDa), which possesses high affinity for ATP and play a major role in chlorophyll biosynthesis, as ATP hydrolysis by this subunit is crucial for this reaction and its activity is a determinant of the rate of Mg insertion into the tetrapyrrole ring [ 3 ]. In Arabidopsis there are two CHLI isoforms, CHLI1 and CHLI2, which share similar expression profiles but CHLI2 has a lower ATPase activity than CHLI1 and a limited role in chlorophyll biosynthesis. However, CHLI2 contributes to the assembly of the Mg-chelatase complex [ 4 ]. It does not bind abscisic acid (ABA), but is a positive regulator of ABA signaling [ 5 ].

1. Mechanism and regulation of Mg-chelatase. Biochem. J. 327 ( Pt 2), 321-33
2. Reconstitution of an active magnesium chelatase enzyme complex from the bchI, -D, and -H gene products of the green sulfur bacterium Chlorobium vibrioforme expressed in Escherichia coli. J. Bacteriol. 180, 699-704
3. The CHLI1 subunit of Arabidopsis thaliana magnesium chelatase is a target protein of the chloroplast thioredoxin. J Biol Chem 282, 19282-91
4. Functional analysis of Arabidopsis thaliana isoforms of the Mg-chelatase CHLI subunit. Photochem. Photobiol. Sci. 7, 1188-95
5. Roles of the different components of magnesium chelatase in abscisic acid signal transduction. Plant Mol. Biol. 80, 519-37