Fig. 1: Active site of Hmd and reactions catalysed by Hmd.

a, The FeGP cofactor (magenta) and methenyl-H4MPT+ substrate (green) in the closed active site. The proposed H2 binding site is highlighted in blue, and the hydride acceptor C14a is marked in methenyl-H4MPT+. His14 and Glu207 mark the start of the putative proton relay network29, highlighted in light blue. b, Methenyl-H4MPT+ reduction reaction, catalysed by Hmd with hydride transfer20,32. For methenyl-H4MPT+ and methylene-H4MPT, only the imidazolinium and imidazolidine rings are depicted. c, Hydrogen/deuteron isotope exchange reaction, catalysed by Hmd33.