Purification and kinetic characterization of the magnesium protoporphyrin IX methyltransferase from Synechocystis PCC6803.
Magnesium protoporphyrin IX methyltransferase (ChlM), catalyses the methylation of magnesium protoporphyrin IX (MgP) at the C(6) propionate side chain to form magnesium protoporphyrin IX monomethylester (MgPME). Threading methods biased by sequence similarity and predicted secondary structure have been used to assign this enzyme to a particular class of S-adenosyl-L-methionine (SAM)-binding proteins. These searches suggest that ChlM contains a seven-stranded beta-sheet, common among small-molecule methyltransferases. Steady-state kinetic assays were performed using magnesium deuteroporphyrin IX (MgD), a more water-soluble substrate analogue of MgP. Initial rate studies showed that the reaction proceeds via a ternary complex. Product (S-adenosyl-L-homocysteine; SAH) inhibition was used to investigate the kinetic mechanism further. SAH was shown to exhibit competitive inhibition with respect to SAM, and mixed inhibition with respect to MgD. This is indicative of a random binding mechanism, whereby SAH may bind productively to either free enzyme or a ChlM-MgD complex. Our results provide an overview of the steady-state kinetics for this enzyme, which are significant given the role of MgP and MgPME in plastid-to-nucleus signalling and their likely critical role in the regulation of this biosynthetic pathway.
Shepherd
Mark
M
Robert Hill Institute for Photosynthesis, Department of Molecular Biology and Biotechnology, Firth Court, Western Bank, University of Sheffield, Sheffield S10 2TN, UK.
Reid
James D
JD
Hunter
C Neil
CN
D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000378:metabolism; D003001:Cloning, Molecular; D000458:Cyanobacteria / Q000201:enzymology*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D007700:Kinetics; D008780:Methyltransferases / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D012436:S-Adenosylmethionine / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
D002352:Carrier Proteins; D012436:S-Adenosylmethionine; D008780:Methyltransferases; C060234:magnesium-protoporphyrin methyltransferase