Fully active Fe-protein of the nitrogenase from Azotobacter vinelandii contains at least eight iron atoms and eight sulphide atoms per molecule.

The Fe-protein of the Azotobacter vinelandii nitrogenase enzyme complex contains a variable iron and sulphide content. The iron and sulphide content of the protein is dependent upon the specific activity. Up to a specific activity of 1000 nmol C2H4 produced X min-1 X mg Av-1(2), three iron and three sulphide atoms per molecule Av2 are found. At specific activities above 1000 nmol C2H4 produced X min-1 X mg Av-1(2), a linear relationship between specific activity and iron and sulphide content of Av2 is found. The maximum values found are 8.8 iron atoms and 8.6 sulphide atoms/molecule at a specific activity of 2250 nmol C2H4 produced X min-1 X mg Av-1(2). Also the experimental molar absorption coefficients at 430 nm of the oxidized and reduced forms depend on the specific activity. The highest values found are 15.9 mM-1 cm-1 and 9.1 mM-1 cm-1, respectively. Since occasionally the preparations with specific activities around 3000 nmol X min-1 X mg-1 are isolated which contain more than 10 iron atoms and 11 sulphide atoms per molecule, it cannot be excluded that under certain physiological conditions Av2 contains even more than two [4 Fe-4 S] clusters. The addition of MgATP induces a conformational change in the Fe-protein which results in a higher reactivity with iron chelators. But irrespective of the specific activity, the amount of iron extracted from the protein after addition of MgATP never exceeds four atoms/molecule. The results are discussed with respect to the present molecular model of the Fe-protein.