Mechanism and regulation of Mg-chelatase.
Mg-chelatase catalyses the insertion of Mg into protoporphyrin IX (Proto). This seemingly simple reaction also is potentially one of the most interesting and crucial steps in the (bacterio)chlorophyll (Bchl/Chl)-synthesis pathway, owing to its position at the branch-point between haem and Bchl/Chl synthesis. Up until the level of Proto, haem and Bchl/Chl synthesis share a common pathway. However, at the point of metal-ion insertion there are two choices: Mg2+ insertion to make Bchl/Chl (catalysed by Mg-chelatase) or Fe2+ insertion to make haem (catalysed by ferrochelatase). Thus the relative activities of Mg-chelatase and ferrochelatase must be regulated with respect to the organism's requirements for these end products. How is this regulation achieved? For Mg-chelatase, the recent design of an in vitro assay combined with the identification of Bchl-biosynthetic enzyme genes has now made it possible to address this question. In all photosynthetic organisms studied to date, Mg-chelatase is a three-component enzyme, and in several species these proteins have been cloned and expressed in an active form. The reaction takes place in two steps, with an ATP-dependent activation followed by an ATP-dependent chelation step. The activation step may be the key to regulation, although variations in subunit levels during diurnal growth may also play a role in determining the flux through the Bchl/Chl and haem branches of the pathway.
Walker
C J
CJ
Department of Biological Sciences, Clemson University, Clemson, SC 29634-1903, USA.
Willows
R D
RD
D000595:Amino Acid Sequence; D002736:Chloroplasts / Q000201:enzymology; D016384:Consensus Sequence; D005294:Ferrochelatase / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D008190:Lyases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D046911:Macromolecular Substances; D008961:Models, Structural; D008969:Molecular Sequence Data; D010944:Plants / Q000201:enzymology* / Q000235:genetics; D016415:Sequence Alignment
D046911:Macromolecular Substances; D008190:Lyases; C086788:magnesium chelatase; D005294:Ferrochelatase