Environmental Engineering Reference
In-Depth Information
(a)
[2Fe-2S]
[4Fe-4S]
Mo +HC
Fe-
protein
α
α
α
β
β
β
P-cluster
NifS
NifS
Precursor
FeMo-co
FeMo-co
[2Fe-2S]
[4Fe-4S]
Fe-S core
Cys Fe
2+
Cys Fe
2+
Precursor
FeMo-co
FeMo-co
β
β
β
α
α
α
P-cluster
NifU
NifU
NifB
NifEN
NifEN
MoFe-protein
Stage
1
2
3
4
5
6
HC
S
o
, ? X (C, O or N)
HC
Fe
[2Fe-2S]
(NifU)
[4Fe-4S]
(NifU)
Fe-S core
(NifB)
Precursor
(NifEN)
FeMo-co
(NifEN)
FeMo-co
(MoFe-protein)
(b)
(1)
(2)
(3)
(4)
(5)
+ Mn
2+
+ Ca
2+
+ Mn
2+
hυ
hυ
(dark)
apo-WOC
Mn(II)
Mn(III)
Mn(III)
Mn(II)Mn(III)
Mn(III)Mn(III)
figure 25.5
Sequence of events during FeMo cofactor assembly. (a) The biosynthetic flow of FeMo cofactor is NifU→ NifS →
NifB → NifeN → MoFe-protein. HC, homocitrate. Structures of intermediates during FeMo cofactor assembly. Shown are the cluster types
that have been identified (on NifU, NifeN, and MoFe-protein) or proposed (for NifB). Hypothetically, NifB could bridge two [4Fe-4S] clus-
ters by inserting a sulfur atom along with the central atom, X, thereby generating a FeeS scaffold that could be rearranged into a precursor
closely resembling the core structure of the mature FeMo cofactor. In the case of the NifeN-associated precursor, only the 8Fe model is
shown. The potential presence of X in the intermediates of FeMo cofactor biosynthesis is indicated by a question mark. reprinted with per-
mission from ref. [60]. © 2009, Nature Publishing group. (b) assembly of the WOC in Photosystem II. The proposed stepwise binding of
Mn(II) (1), Ca(II) to the apo-WOC (2), as well as the photo-oxidation of Mn(II) to Mn(III) are shown in this figure. Initially, a single Mn(II)
is bound to the apo-WOC and oxidized by a first quantum of light. after binding of Ca(II) and a subsequent light-independent structural rear-
rangement (3) a second Mn(II) is bound (4) and photooxidized (5). The binding of the two missing manganese ions and chloride is not
depicted in this scheme as it could not be kinetically resolved. reprinted with permission from ref. [59]. © 2011, elsevier.
amine groups of poly(allylamine hydrochloride) to the surface of Mn sites of MnO
2
electrodes effectively stabilized the Mn(III)
species, resulting in approximately 500 mV negative shift of the onset potential for water oxidation at neutral pH [65]. It showed
that additional organic groups of manganese, which exist in Photosystem II, may increase the catalytic activity of these
clusters.
In conclusion, from a certain viewpoint, an enzyme is considered a nano-sized inorganic core in a protein matrix. a model
system by BSa and manganese oxide was selected and it was observed that when nanoparticles form in the presence of pro-
teins, the protein not only induces nucleation, but can also inhibit the further growth of nanominerals, and numerous and very
