The FeMoco-deficient MoFe Protein Produced by a nifH Deletion Strain of Azotobacter vinelandii Shows Unusual P-cluster Features

The His-tag MoFe protein expressed by the nifH deletion strain Azotobacter vinelandii DJ1165 (Delta nifH MoFe protein) was purified in large quantity. The alpha 2beta 2 tetrameric Delta nifH MoFe protein is FeMoco-deficient based on metal analysis and the absence of the S = 3/2 EPR signal, which arises from the FeMo cofactor center in wild-type MoFe protein. The Delta nifH MoFe protein contains 18.6 mol Fe/mol and, upon reduction with dithionite, exhibits an unusually strong S = 1/2 EPR signal in the g approx 2 region. The indigo disulfonate-oxidized Delta nifH MoFe protein does not show features of the P2+ state of the P-cluster of the Delta nifB MoFe protein. The oxidized Delta nifH MoFe protein is able to form a specific complex with the Fe protein containing the [4Fe-4S]1+ cluster and facilitates the hydrolysis of MgATP within this complex. However, it is not able to accept electrons from the [4Fe-4S]1+ cluster of the Fe protein. Furthermore, the dithionite-reduced Delta nifH MoFe can be further reduced by Ti(III) citrate, which is quite unexpected. These unusual catalytic and spectroscopic properties might indicate the presence of a P-cluster precursor or a P-cluster trapped in an unusual conformation or oxidation state