FIG 9.
Frh. Frh is a heterotrimeric enzyme, consisting of a large subunit (FrhA), small subunit (FrhG), and a flavin adenine dinucleotide (FAD)-containing subunit (FrhB). FrhA contains the [NiFe] active site for formation or oxidation of H2, and FrhB has an FAD-containing active site for coenzyme F420 oxidation or reduction. Electrons are transmitted between active sites by four cubane Fe-S clusters ([4Fe4S]) located in FrhG and FrhB. Bidirectional arrows indicate Frh is a reversible enzyme, and brown lines trace the putative pathway of electron transport.