Increasing nitrogenase catalytic efficiency for MgATP by changing serine 16 of its Fe protein to threonine: Use of Mn2+ to show interaction of serine 16 with Mg2+

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Abstract
MgATP‐binding and hydrolysis are an integral part of the nitrogenase catalytic mechanism. We are exploring the function of MgATP hydrolysis in this reaction by analyzing the properties of the Fe protein (FeP) component of Azotobacter vinelandii nitrogenase altered by site‐directed mutagenesis. We have previously (Seefeldt, L.C., Morgan, T.V., Dean, D.R., & Mortenson, L.E., 1992, J. Biol. Chem. 267, 6680–6688) identified a region near the N‐terminus of FeP that is involved in interaction with MgATP. This region of FeP is homologous to the well‐known nucleotide‐binding motif GXXXXGKS/T. In the present work, we examined the function of the four hydroxyl‐containing amino acids immediately C‐terminal to the conserved lysine 15 that is involved in interaction with the β‐phosphate of MgATP. We have established, by altering independently Thr 17, Thr 18, and Thr 19 to alanine, that a hydroxyl‐containing residue is not needed at these positions for FeP to function. In contrast, an hydroxyl‐containing amino acid at position 16 was found to be critical for FeP function. When the strictly conserved Ser 16 was altered to Ala, Cys, Asp, or Gly, the FeP did not support N2 fixation when expressed in place of the wild‐type FeP in A. vinelandii. Altering Ser 16 to Thr (S16T), however, resulted in the expression of an FeP that was partially active. This S16T FeP was purified to homogeneity, and its biochemical examination allowed us to assign a catalytic function to this hydroxyl group in the nitrogenase mechanism. Of particular importance was the finding that the S16T FeP had a significantly higher affinity for MgATP than the wild‐type FeP, with a measured Km of 20 μM compared to the wild‐type FeP Km of 220 μM. This increased kinetic affinity for MgATP was reflected in a significantly stronger binding of the S16T FeP for MgATP. In contrast, the affinity for MgADP, which binds at the same site as MgATP, was unchanged. The catalytic efficiency (kcat/Km) of S16T FeP was found to be 5.3‐fold higher than for the wild‐type FeP, with the S16T FeP supporting up to 10 times greater nitrogenase activity at low MgATP concentrations. This indicates a role for the hydroxyl group at position 16 in interaction with MgATP but not MgADP. The site of interaction of this residue was further defined by examining the properties of wild‐type and S16T FePs in utilizing MnATP compared with MgATP. The S16T FeP was severely compromised in its interaction with MnATP, suggesting a mechanism where the hydroxyl group of amino acid 16 interacts with the Mg2+ of bound MgATP.