Table 1.

Comparison of the properties of the hydrogenase isoenzymes

Enzyme	Subunit composition/electron pathway	Metals and cofactors	Function	Dye specificity	App. Km for H2	References
Hydrogenase-1	α2β2γ2 (HyaB-HyaA-HyaC)2	[NiFe]; [4Fe-3S], [3Fe-4S], [4Fe-4S]; cyt b	H2 oxidation, O2 tolerance	Nitroblue tetrazolium, Benzyl viologen (BV)	9 μM (PFEa)2 μM (BV)	(141, 154, 156)
Hydrogenase-2	αβγδ (HybC-HybO-HybA-HybB)	[NiFe]; 2 [4Fe-4S], [3Fe-4S]; 4 [FeS]	H2 oxidation, energy conservation	Benzyl viologen	17 μM (PFE)3.7 μM (BV)	(13, 144, 154, 158)
Hydrogenase-3 (FHL complex)	FdhH–HycB-G–HycE	Mo-bis-PGD,b Selenocysteine, [4Fe-4S]; 7 [4Fe-4S]; [NiFe]	Proton reduction	Benzyl viologen	34 μM (PFE)	(28, 29)
Hydrogenase-4 (E. coli)	?–HyfA-I	predicted FHL ortholog	Proton reduction?	Unknown	Unknown	(7)
Hydrogenase-5 (Salmonella Typhimurium)	α2β2γ2 (HydB-HydA-HydC)2	[NiFe]; [4Fe-3S], [3Fe-4S], [4Fe-4S]; cyt b	H2 oxidation, O2 tolerance	Nitroblue tetrazolium, Benzyl viologen	9 μM (PFE)	(11, 12)

^a PFE, protein film electrochemistry.

^b Mo-bis-PGD, molybdopterin guanine dinucleotide.