Table 1.

Proteins involved in the maturation process of hydrogenase-2 from E. coli

Protein	Molecular mass	Function/Properties
	kDa
HybC	63.5	Large subunit of hydrogenase-2; contains the [NiFe] active site (50).
HybO	39.6	Small subunit of hydrogenase-2; contains one [3Fe-4S] and two [4Fe-4S] clusters that are involved in electron transfer from or to the active site (52).
HybF (HypA)	12.5	Involved in acquisition and insertion of the nickel into large subunit HybC (53).
HypB	31.5	GTPase activity; binds nickel and forms with SlyD the HypB–SlyD complex that activates nickel release and insertion into the large subunit (54,55).
HybG (HypC)	8.7	Small iron- and CO2-binding protein; proposed to deliver CO2 for reduction to CO by HypD (27). Forms with HypD the core complex of hydrogenase-2 maturation machinery (HybG–HypD), that is involved in delivering the completed Fe(CN)2CO moiety to the large subunit HybC (14).
HypD	41.4	The only FeS cluster-containing Hyp protein; acts as a scaffold for assembly of the Fe(CN)2CO moiety of the cofactor prior to its transfer to the large subunit (28).
HypE	35	HypE is a homodimer; forms a heterooligomer with HypCD (38) and a heterotetramer with HypF (56). HypEF heterotetramer generates CN ligands from carbamoyl phosphate in an ATP-dependent condensation reaction (25).
HypF	82	Carbamoyl phosphate phosphatase; carbamoyl-transferase; ATPase; involved in CN synthesis (26,57).
HybD	17.6	Endopeptidase; involved in C-terminal cleavage of the hydrogenase-2 large subunit (33).