title: Evidence for a covalent intermediate in the S-adenosyl-L-methionine-dependent transmethylation reaction catalysed by sirohaem synthase.
abstract: CysG, also known as uroporphyrinogen III methylase and sirohaem synthase (CysG; EC 2.1.1.107), is a multifunctional enzyme that is able to transform uroporphyrinogen III into sirohaem via two S-adenosyl-L-methionine (AdoMet)-dependent transmethylations, an NAD(+)-dependent dehydrogenation and a ferrochelation. The apparent tight binding of AdoMet to this multifunctional enzyme is investigated. The use of a rapid AdoMet binding assay demonstrates that CysG becomes labelled with both [methyl-3H]AdoMet and [carboxyl-14C]AdoMet. Further experiments show that the CysG-AdoMet complex is subsequently able to methylate uroporphyrinogen III. CysG remains associated with the labelled constituents of the AdoMet even after denaturation with urea and SDS/PAGE, suggesting that the AdoMet has become covalently linked to the protein. A rapid examination of some of the other transmethylases involved in corrin biosynthesis reveals that they bind the AdoMet in a similar fashion. A multistep transmethylation mechanism is proposed to explain the observed results.
authors.affiliation: Department of Molecular Genetics, Institute of Ophthalmology, London, UK.
authors.forename: S C
authors.initials: SC
authors.lastname: Woodcock
authors.forename: M J
authors.initials: MJ
authors.lastname: Warren
mesh_terms: D002384:Catalysis; D002942:Circular Dichroism; D004926:Escherichia coli / Q000201:enzymology; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry / Q000378:metabolism*; D012436:S-Adenosylmethionine / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D012436:S-Adenosylmethionine; D008780:Methyltransferases; C060508:uroporphyrin-III C-methyltransferase