Environmental Engineering Reference
In-Depth Information
Scheme 2 The MoFe protein cycle of Mo-nitrogenase, featuring the distal (a) and alternating
(b) pathway of N
2
reduction. (a) Subscripts 0-8 indicate the number of electrons transferred to the
FeMoco of the MoFe protein via the Fe protein cycle (see Scheme
1
). The distal pathway describes
one plausible sequence of events during N
2
reduction, in which the distal N is reduced first and
released as NH
3
before the M
5
stage, followed by the subsequent reduction of the proximal N and
the release of a second NH
3
at the M
8
stage. (b) The alternating pathway shares most of the steps
with the distal pathway, but differs at stages M
4
,M
5
, and M
6
(highlighted in red). It features
the alternating addition of electrons and protons at both the distal and proximal N atoms.
Consequently, the two NH
3
molecules are released at stage M
4
and M
8
, respectively, late in the
sequence. The stages unique to each pathway are colored blue and red, respectively, for the distal
pathway and the alternating pathway. Intermediates that have been identified so far are highlighted
in red boxes.
M
0
(i.e., M
3
or M
4
) (Scheme
2
a). Thus, other substrates that bind to a relatively
more oxidized state of the MoFe protein, such as acetylene and cyanide, would
become non-competitive inhibitors of N
2
and, consequently, they will be preferen-
tially reduced when there is a low electron flux through the MoFe protein [
4
,
28
,
64
]. Upon binding of N
2
, however, the MoFe protein would undergo four (if N
2
binds at M
4
) or five (if N
2
binds at M
3
) more proton/electron addition steps to yield
two molecules of NH
3
(Scheme
2
a). The successive reduction of M
0
is carried out
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