This family includes HycB, the FeS subunit of a membrane-associated formate hydrogenlyase system (FHL-1) in Escherichia coli that breaks down formate, produced during anaerobic fermentation, to H2 and CO2. FHL-1 consists of formate dehydrogenase H (FDH-H) and the hydrogenase 3 complex (Hyd-3). HycB is thought to code for the [4Fe-4S] ferredoxin subunit of hydrogenase 3, which functions as an intermediate electron carrier protein between hydrogenase 3 and formate dehydrogenase. HydN codes for the [4Fe-4S] ferredoxin subunit of FDH-H; a hydN in-frame deletion mutation causes only weak reduction in hydrogenase activity, but loss of more than 60% of FDH-H activity. This pathway is only active at low pH and high formate concentrations, and is thought to provide a detoxification/de-acidification system countering the buildup of formate during fermentation.