Called (hupA/hydA/hupS/hoxK/vhtG) Involved in hydrogenase reactions performing different specific functions in different species eg (EC 1.12.2.1) in Desulfovibrio gigas,(EC 1.12.99.3) in Wolinella succinogenes and (EC 1.18.99.1) in E.coli and a number of other species and (EC 1.12.99.-) in the archea. [Energy metabolism, Electron transport]