The Function of Two Radical‐SAM Enzymes, HcgA and HcgG, in the Biosynthesis of the [Fe]‐Hydrogenase Cofactor

In the biosynthesis of the iron‐guanylylpyridinol (FeGP) cofactor, 6‐carboxymethyl‐5‐methyl‐4‐hydroxy‐2‐pyridinol (1) is 3‐methylated to form 2, then 4‐guanylylated to form 3, and converted into the full cofactor. HcgA‐G proteins catalyze the biosynthetic reactions. Herein, we report the function of two radical S‐adenosyl methionine enzymes, HcgA and HcgG, as uncovered by in vitro complementation experiments and the use of purified enzymes. In vitro biosynthesis using the cell extract from the Methanococcus maripaludis ΔhcgA strain was complemented with HcgA or precursors 1, 2 or 3. The results suggested that HcgA catalyzes the biosynthetic reaction that forms 1. We demonstrated the formation of 1 by HcgA using the 3 kDa cell extract filtrate as the substrate. Biosynthesis in the ΔhcgG system was recovered by HcgG but not by 3, which indicated that HcgG catalyzes the reactions after the biosynthesis of 3. The data indicated that HcgG contributes to the formation of CO and completes biosynthesis of the FeGP cofactor. The FeGP cofactor is the prosthetic group of [Fe]‐hydrogenase, which catalyzes reversible hydride transfer from H2 to an organic substrate. HcgA and HcgG are involved in the biosynthesis of the FeGP cofactor. In vitro biosynthesis experiments indicated the functions of these two radical S‐adenosyl methionine enzymes. HcgA biosynthesizes the initial pyridinol precursor, and HcgG contributes to the formation of the CO and acyl ligands.