Isolation of two forms of the nitrogenase VFe protein from Azotobacter vinelandii

When Q-Sepharose was used in the purification of the V nitrogenase proteins from Azotobacter vinelandii, an increase in resolution was observed that resulted in a separation of the nitrogenase component 1 protein (Av1') into two forms, labeled Avl'(A) and Av1'(B). Even though both forms possessed the same enzymatic behavior, Av1'(A) exhibited a lower specific activity and migrated during gel filtration with an apparent lower molecular weight than Av1'(B). Furthermore, SDS-polyacrylamide gel, electrophoresis showed different relative compositions of the two major subunits of both forms, with Av1'(A) possessing a trimer (alpha beta(2)) pattern compared to the more typical tetramer (alpha(2) beta(2)) pattern found for Av1'(B). Metal analysis indicated a V-to-Fe ratio of 1:19 for Av1'(A) and 1:15 (or 2:30) for Av1'(B), while acid-labile sulfide analysis showed that Av1'(A) possessed about half as much sulfide as Av1'(B). EPR spectroscopy revealed that both proteins retained the S = 3/2 and S = 1/2 signals observed in earlier isolations, with an additional S = 1/2 signal present in the spectrum of protein A. These results suggest that Av1'(A) is an incomplete form of the VFe protein, containing only one cofactor and one P duster with an additional [Fe-4-S-4]-like cluster. The presence of a V storage protein in A. vinelandii was also investigated. Although no V storage protein was found, two V-binding proteins were observed.