Cleaving the N,N Triple Bond: The Transformation of Dinitrogen to Ammonia by Nitrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 155

molecules and undergoes a conformational change, which allows it to complex
with the MoFe protein (i.e., stage (i) in Scheme 1 ). The formation of the
Mo-nitrogenase complex then facilities the inter-protein, one-electron transfer
from the [Fe 4 S 4 ] 1+ cluster of the Fe protein to the P-cluster of the MoFe protein,
which occurs concomitantly with the hydrolysis of two MgATP molecules.
Although the exact order of ATP hydrolysis and electron transfer is not clear [ 4 ],
it has been proposed that, upon the transfer of electrons, the inorganic phosphates
are released from the FeP ox {MgADP-P i } 2 MoFeP red complex (where FeP ox refers to
the oxidized Fe protein and MoFeP red refers to the reduced MoFe protein), followed
by the dissociation of the Fe protein from the MoFe protein (Scheme 1 ). The release
of the inorganic phosphates signifies that the energy derived from the Fe protein is
relayed to the MoFe protein; whereas the dissociation of the Fe protein allows the
re-reduction of its [Fe 4 S 4 ] 1+ cluster following the release of MgADP and the start of
another round of electron transfer (Scheme 1 ).
Scheme 1 The Fe protein cycle of Mo-nitrogenase. This cycle describes (i) the complex forma-
tion of the MgATP-bound Fe protein (FeP) with the MoFe protein (MoFeP); (ii) the ATP
hydrolysis-dependent electron transfer from FeP to MoFeP; (iii) the dissociation of the complex
upon the release of the inorganic phosphate (P i ); and (iv) the re-reduction of FeP and the
replacement of MgADP by MgATP. Superscripts: red, reduced; ox, oxidized.
Excitingly, structural insights into the Fe protein cycle have been provided by a
number of Fe protein-MoFe protein complexes generated by (i) chemical
crosslinking; (ii) addition of the non-hydrolyzable MgATP analog, MgAMPPCP;
(iii) addition of the non-hydrolyzable “MgADP-P i ” analog, MgADP-AlF 4 ; and