Specific activities of VFe proteins at pH 7.3^a.

VFe protein	Substrates
	Protons (1 atm Ar)	N2 (1 atm) and protons
	nmol of H2 min−1mg−1	nmol of H2 min−1mg−1	nmol of NH3 min−1mg−1
VFeStr	1980 ± 50	960 ± 25	310 ± 7
VFeStrΔnifB	NDb	ND	ND
VFeStrΔnifE	1110 ± 1	690 ± 3	160 ± 5
In vitro incubated VFeStrΔnifE	1240 ± 27	700 ± 8	200 ± 9

^aAll assays were performed at 30 °C for 8 min at a molar ratio of VFe protein to VnfH of 1 : 40 (1 : 30 for VFe^StrΔnifB), and the specific activities are expressed as nmol of product per min per mg of VFe protein as an average with standard deviation.

^bND = not detected.