Evidence for a covalent intermediate in the S-adenosyl-l-methionine-dependent transmethylation reaction catalysed by sirohaem synthase

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.

The Escherichia coli cysG gene encodes S-adenosylmethionine-dependent uroporphyrinogen III methylase

The Escherichia coli cysG gene was successfully subcloned and over-expressed to produce a 52 kDa protein that was purified to homogeneity. This protein was shown to catalyse the S-adenosylmethionine-dependent methylation of uroporphyrinogen III to give a product identified as sirohydrochlorin on the basis of its absorption spectra, incorporation of 14C label from S-adenosyl[Me-14C]methionine and mass and 1H-n.m.r. spectra of its octamethyl ester. Further confirmation of the structure was obtained from a 14C-n.m.r. spectrum of the methyl ester produced by incubation of the methylase with uroporphyrinogen III, derived from [4.6-13C2]porphobilinogen, and S-adenosyl[Me-13C]methionine.