title: S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase from Rhodobacter capsulatus: mechanistic insights and stimulation with phospholipids.
abstract: The enzyme BchM (S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase) from Rhodobacter capsulatus catalyses an intermediate reaction in the bacteriochlorophyll biosynthetic pathway. Overexpression of His(6)-tagged protein in Escherichia coli resulted in the majority of polypeptide existing as inclusion bodies. Purification from inclusion bodies was performed using metal-affinity chromatography after an elaborate wash step involving surfactant polysorbate-20. Initial enzymatic assays involved an in situ generation of S-adenosyl-L-methionine substrate using a crude preparation of S-adenosyl-L-methionine synthetase and this resulted in higher enzymatic activity compared with commercial S-adenosyl-L-methionine. A heat-stable stimulatory component present in the S-adenosyl-L-methionine synthetase was found to be a phospholipid, which increased enzymatic activity 3-4-fold. Purified phospholipids also stabilized enzymatic activity and caused a disaggregation of the protein to lower molecular mass forms, which ranged from monomeric to multimeric species as determined by size-exclusion chromatography. There was no stimulatory effect observed with magnesium-chelatase subunits on methyltransferase activity using His-BchM that had been stabilized with phospholipids. Substrate specificity of the enzyme was limited to 5-co-ordinate square-pyramidal metalloporphyrins, with magnesium-protoporphyrin IX being the superior substrate followed by zinc-protoporphyrin IX and magnesium-deuteroporphyrin. Kinetic analysis indicated a random sequential reaction mechanism. Three non-substrate metalloporphyrins acted as inhibitors with different modes of inhibition exhibited with manganese III-protoporphyrin IX (non-competitive or uncompetitive) compared with cobalt II-protoporphyrin IX (competitive).
authors.affiliation: Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, NSW 2109, Australia.
authors.forename: Artur
authors.initials: A
authors.lastname: Sawicki
authors.forename: Robert D
authors.initials: RD
authors.lastname: Willows
mesh_terms: D004789:Enzyme Activation / Q000187:drug effects*; D007700:Kinetics; D008780:Methyltransferases / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D010743:Phospholipids / Q000494:pharmacology*; D011524:Protoporphyrins / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D012436:S-Adenosylmethionine / Q000378:metabolism*
chemical_list: D010743:Phospholipids; D011524:Protoporphyrins; C038654:manganese protoporphyrin; D012436:S-Adenosylmethionine; D008780:Methyltransferases; C060234:magnesium-protoporphyrin methyltransferase