A Cobalamin‐Dependent Radical SAM Enzyme Catalyzes the Unique C <sub>α</sub> ‐Methylation of Glutamine in Methyl‐Coenzyme M Reductase
Jana Gagsteiger, Sören Jahn, Lorenz Heidinger, Lukas Gericke, Jennifer N. Andexer, Thorsten Friedrich, Christoph Loenarz, Gunhild Layer
Abstract
Abstract Methyl‐coenzyme M reductase, which is responsible for the production of the greenhouse gas methane during biological methane formation, carries several unique posttranslational amino acid modifications, including a 2‐( S )‐methylglutamine. The enzyme responsible for the C α ‐methylation of this glutamine is not known. Herein, we identify and characterize a cobalamin‐dependent radical SAM enzyme as the glutamine C‐methyltransferase. The recombinant protein from Methanoculleus thermophilus binds cobalamin in a base‐off, His‐off conformation and contains a single [4Fe‐4S] cluster. The cobalamin cofactor cycles between the methyl‐cob(III)alamin, cob(II)alamin and cob(I)alamin states during catalysis and produces methylated substrate, 5′‐deoxyadenosine and S ‐adenosyl‐ l ‐homocysteine in a 1 : 1 : 1 ratio. The newly identified glutamine C‐methyltransferase belongs to the class B radical SAM methyltransferases known to catalyze challenging methylation reactions of sp 3 ‐hybridized carbon atoms.