[NiFe]-hydrogenase maturation: Isolation of a HypC-HypD complex carrying diatomic CO and CN− ligands

► Anaerobic isolation of a HypCD complex revealed it has six Fe ions and four S. ► FTIR spectroscopy showed that the HypC-HypD complex has CN−, CO and CO2 ligands. ► HypD isolated aerobically lacks the CN−, CO and CO2 ligands and two of the Fe ions. ► A Cys41Ala variant of HypD lacks the CN−, CO and CO2 ligands. ► The complex containing a Cys41Ala variant of HypD has five of the six Fe ions.