The [NiFe]-hydrogenase accessory chaperones HypC and HybG of Escherichia coli are iron- and carbon dioxide-binding proteins.

[NiFe]-hydrogenase accessory proteins HypC and HypD form a complex that binds a Fe-(CN)₂CO moiety and CO₂. In this study two HypC homologues from Escherichia coli were purified under strictly anaerobic conditions and both contained sub-stoichiometric amounts of iron (approx. 0.3 molFe/mol HypC). Infrared spectroscopic analysis identified a signature at 2337 cm⁻¹ indicating bound CO₂. Aerobically isolated HypC lacked both Fe and CO₂. Exchange of either of the highly conserved amino acid residues Cys2 or His51 abolished both Fe- and CO₂-binding. Our results suggest that HypC delivers CO₂ bound directly to Fe for reduction to CO by HypD.