 The addition of the second electron results in the [Fe(I)Fe(II)]-[4Fe4S]+ configuration and a protonated azadithiolate bridge.Hydrogenases are found in prokaryotes, lower eukaryotes, and archaea.This broad category of metalloenzymes can be divided into [NiFe], [FeFe], and [Fe] variants based on the transition metals found in their active sites.