Catalytic reduction of CN-, CO, and CO2by nitrogenase cofactors in lanthanide-driven reactions

Article abstract of DOI:10.1002/anie.201410412

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^-), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6-lutidinium triflate (Lut-H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^- or CO to C₁-C₄ hydrocarbons, and CO₂ to CO and C₁-C₃ hydrocarbons. The C-C coupling from CO₂ indicates a unique Fischer-Tropsch-like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^-, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase-based electrocatalysts for the production of hydrocarbons from these carbon-containing compounds.

Full text of DOI:10.1002/anie.201410412

Angewandte
Chemie
DOI: 10.1002/anie.201410412
Enzyme Catalysis
ꢀ
Catalytic Reduction of CN , CO, and CO by Nitrogenase Cofactors in
2
Lanthanide-Driven Reactions**
Chi Chung Lee, Yilin Hu,* and Markus W. Ribbe*
[
11–13]
Abstract: Nitrogenase cofactors can be extracted into an
organic solvent to catalyze the reduction of cyanide (CN ),
substitution of Fe for Mo and homocitrate at one end.
ꢀ
The structural homology between the L-cluster and the two
cofactors is striking; more importantly, it suggests a close
resemblance of these clusters to one another in their catalytic
capacities.
carbon monoxide (CO), and carbon dioxide (CO ) without
2
using adenosine triphosphate (ATP), when samarium(II)
iodide (SmI ) and 2,6-lutidinium triflate (Lut-H) are employed
2
as a reductant and a proton source, respectively. Driven by
Such a resemblance indeed exists between the M- and V-
clusters, as both cofactors can be extracted from protein into
ꢀ
SmI , the cofactors catalytically reduce CN or CO to C –C
2
1
4
[
10]
hydrocarbons, and CO to CO and C –C hydrocarbons. The
an organic solvent, N-methylformamide (NMF),
and
2
1
3
ꢀ
CꢀC coupling from CO indicates a unique Fischer–Tropsch-
directly used as a catalyst to reduce CN or CO to hydro-
carbons in the presence of a strong reductant, europium(II)
2
like reaction with an atypical carbonaceous substrate, whereas
ꢀ
II
[14]
the catalytic turnover of CN , CO, and CO2 by isolated
diethylenetriaminepentaacetate (Eu -DTPA).
Driven by
II
0
cofactors suggests the possibility to develop nitrogenase-based
electrocatalysts for the production of hydrocarbons from these
carbon-containing compounds.
Eu -DTPA (E ’ = ꢀ1.14 V at pH 8), both cofactors generate
ꢀ
alkanes and alkenes of varying lengths as products of CN or
CO reduction at comparable efficiencies. Additionally, they
ꢀ
both display a strong preference of CN over CO as
N
itrogenase is a uniquely versatile metalloenzyme that
a substrate, which may originate from a stabilizing effect of
ꢀ
[14]
catalyzes the reduction of various substrates, such as nitrogen
CN on certain oxidation states of the two cofactors.
ꢀ
II
(
N ), carbon monoxide (CO), and cyanide (CN ), at its
2
However, Eu -DTPA is not a strong enough reductant to
drive the catalytic turnover of CO by either cofactor, as the
turnover numbers (TON) of CO by both cofactors are less
[
1–4]
cofactor site.
The molybdenum (Mo) and vanadium (V)
nitrogenases, two homologous members of this enzyme
family, contain homologous cofactors, the molybdenum–iron
cofactor (designated the M-cluster) and the vanadium–iron
cofactor (designated the V-cluster), respectively, at their
respective active sites.
a [MoFe S C] cluster that can be viewed as [Fe S ] and
[15]
than one. Moreover, this reductant does not support the
reduction of CO by the cofactors, an event that requires more
2
ꢀ
[16]
reducing power than the reduction of CN or CO. This
observation prompts the questions of 1) whether CO and CO2
can be catalytically turned over by these clusters in the
presence of an appropriate reductant; and 2) if the L-cluster
resembles the M- and V-clusters in the conversion of carbon-
containing compounds to hydrocarbons.
[
1,5]
The M-cluster (Figure S1A) is
7
9
4 3
[
MoFe S ] subclusters bridged by three equatorial m sulfides
3 3 2
and one interstitial m carbide. In addition, this cofactor has an
endogenous compound, homocitrate, attached to its Mo
end.
6
[
6–8]
II
The V-cluster (Figure S1B) is nearly identical to the
The answer to both questions is yes. When Eu -DTPA is
M-cluster in structure, except for the substitution of V for Mo
and a slight elongation of the metal–sulfur core of this
replaced by a stronger reductant, samarium(II) iodide
[17]
(SmI ), the NMF-extracted M-, V-, and L-clusters are all
2
[9,10]
ꢀ
cluster.
Apart from the two cofactors, a third cluster
capable of turning over CN , CO, and CO under ambient
2
0
species has been identified both as a biosynthetic intermedi-
ate and as a structural homolog of the M-cluster. Designated
as the L-cluster (Figure S1C), this [Fe S C] cluster represents
conditions in organic solvents. Driven by SmI (E ’ = ꢀ1.55 V
2
in THF) and using protons supplied by 2,6-lutidinium triflate
[
18]
ꢀ
(Lut-H),
the three clusters not only reduce CN (Fig-
8
9
an all-iron version of the cofactor, as it closely resembles the
core structure of the mature M-cluster except for the
ure 1A, upper part; Table S1) and CO (Figure 1B, upper
part; Table S1) to CH , C H , C H , C H , C H , 1-C H , and
4
2
4
2
6
3
6
3
8
4
8
n-C H , but also reduce CO to CO, CH , C H , C H , C H ,
4
10
2
4
2
4
2
6
3
6
[
*] Dr. C. C. Lee, Prof. Dr. Y. Hu, Prof. Dr. M. W. Ribbe
Department of Molecular Biology and Biochemistry
University of California, Irvine
Irvine, CA 92697-3900 (USA)
E-mail: yilinh@uci.edu
and C H (Figure 1C, upper part; Table S1). Gas chromatog-
3 8
raphy–mass spectrometry (GC-MS) analysis confirms CN ,
ꢀ
CO, and CO as the carbon sources for the hydrocarbons
2
generated in these reactions, as all products display the
1
3
ꢀ
13
expected mass shifts upon substitution of CN , CO, and
mribbe@uci.edu
1
3
12
ꢀ
12
CO , for CN (Figure 1A, lower part), CO (Figure 1B,
2
Prof. Dr. M. W. Ribbe
Department of Chemistry
University of California, Irvine (USA)
1
2
lower part), and CO (Figure 1C, lower part), respectively.
2
Activity analysis further demonstrates that all three clusters
ꢀ
turn over CN , CO, and CO catalytically (i.e., TON > 1) in
[
**] This work was supported by NIH grant GM-67626 (M.W.R.).
2
the presence of SmI , with the M-, V-, and L-clusters showing
TONs of 15, 13, and 13, respectively, for CN (Figure 2A),
3.0, 2.7, and 4.5, respectively, for CO (Figure 2B), and 1.4, 1.8,
Supporting information for this article (including experimental
procedures, Table S1, and Figures S1 and S2) is available on the
WWW under http://dx.doi.org/10.1002/anie.201410412.
2
ꢀ
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1219

.
Angewandte
Communications
ꢀ
Figure 1. Reduction of CN , CO, and CO by nitrogenase cofactors. Shown are the activity (upper part) and GC-MS (lower part) analyses of
2
ꢀ
hydrocarbon (HC) formation in the reductions of A) CN , B) CO, and C) CO by M-, V-, and L-clusters.
2
and 2.3, respectively, for CO (Figure 2C). While the prefer-
ence of CN as a substrate is preserved by all three clusters in
detected if CO is supplied directly as a substrate at the
same concentration as the maximum amount of CO gener-
ated from CO2 reduction (Figure S2). Furthermore, the
2
ꢀ
reactions driven by SmI , the catalytic turnover of CO and
2
CO by these clusters in the presence of this reductant is
reduction of CO to CO and hydrocarbons is carried out by
2
2
+
particularly exciting, as it not only illustrates the impact of
redox potential on the catalytic efficiency and substrate range
of nitrogenase cofactors, but also defines a previously not
observed, ATP-independent reaction that involves the con-
version of CO2 to hydrocarbons by these unique metal
clusters in the isolated forms.
protons (H ) and electrons in these reactions, and it is
+
accompanied by the reduction of H to hydrogen (H2;
Table S1).
Interestingly, the activities of the three clusters seem to be
“normalized” upon isolation from their respective protein
environments. In addition to turning over each substrate with
comparable TONs, these clusters also generate the same
range of products at similar percentages from the same
It should be noted that the ATP-dependent reduction of
CO was reported both for a variant of Mo-nitrogenase and
2
[19–21]
for the wild-type V-nitrogenase;
however, CH4 was
substrate. All of them display a strong tendency toward the
ꢀ
detected as the sole hydrocarbon product in the case of the
formation of up to C products from CN (Figure 2A) and
2
[
20]
former, whereas CH , C H , and C H were detected only
CO (Figure 2B), with the C (CH ) and C (C H , C H )
4
2
4
2
6
1
4
2
2
4
2
6
[21]
upon substitution of D O for H O in the case of the latter.
products comprising a major portion (90.3–97.8%) of the
product profiles of these reactions. The tendency toward the
formation of small products is even more apparent in the
cases of CO2 reduction by these clusters, where the C1
2
2
In comparison, the isolated cofactors are “pushed” by SmI2
not only toward the formation of a CꢀC bond (i.e., hydro-
carbons larger than C1 products), but also toward the
formation of longer carbon chains (i.e., up to C products)
products (CO, CH ) constitute the predominant portion
3
4
from CO (Figure 1C). The C and C hydrocarbons do not
(97.1–97.5%) of the product profiles (Figure 2C). In all
2
2
3
originate from the coupling between the CO -derived CO in
these reactions, CH is the singularly dominant hydrocarbon
2
4
the SmI -driven reactions, as these products cannot be
product, making up 58.2–78.1% of the total amount of
2
1
220
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ꢀ
Figure 2. Product profiles of nitrogenase cofactors. Shown are the percentages of C , C , C , and C products formed in the reductions of A) CN ,
1
2
3
4
B) CO, and C) CO by M-, V-, and L-clusters. The turnover number (TON) was calculated based on the amount of carbon atoms (in nmol) that
2
appeared in the hydrocarbon products relative to the amount of isolated cluster (in nmol) used in the reaction.
products. Such a strong shift toward CH formation is not
also important because of the all-iron composition of the L-
cluster (see Figure S1), which may simplify the task of
synthesizing biomimetic nitrogenase “cofactors” by omitting
the need to incorporate heterometal and homocitrate.
Together with the M- and V-clusters, the L-cluster forms
a group of homologous, high-nuclearity metal–sulfur clusters
4
observed in the reduction of CO by the protein-bound M- or
[
2,3]
L-cluster,
where C H is produced as the major product
2 4
along with a more evenly distributed product profile toward
longer hydrocarbons. Moreover, the “normalization” of the
isolated M- or L-cluster in the reaction efficiency and product
distribution of CO reduction contrasts the approximately 700-
fold activity difference and a significant disparity in product
ꢀ
that are capable of catalyzing the unique conversion of CN ,
CO, and CO2 to hydrocarbon products. The success in
[3]
formation between their protein-bound counterparts, high-
lighting the impact of protein environment on the reactivities
of nitrogenase cofactors.
achieving the catalytic turnover of CO and CO by these
2
clusters in the presence of a stronger reductant, SmI , suggests
2
the possibility to develop nitrogenase-based electrocatalysts
for further improvement of catalytic efficiency and substrate
range. On the other hand, the differences between the
activities of the protein-bound and NMF-extracted clusters,
as well as the differences between the activities of the isolated
clusters, imply the potential to alter the product profiles of
these reactions by varying the compositions of the clusters
and attaching the clusters to artificial matrices for further
modulation of their catalytic properties. Perhaps most excit-
ingly, these studies have led to the identification of a room-
temperature, Fisher–Tropsch-type reaction with an atypical
Apart from the protein environment, variations of the
cofactor composition, particularly those at the “heterometal
end”, seem to play a role in modulating the catalytic
properties of these clusters. A good example in this regard
is the higher TONs of CO (Figure 2B) and CO (Figure 2C)
2
by the L-cluster, an all-iron form of the cofactor, than those by
the M- and V-clusters. Moreover, among the three clusters,
the L-cluster forms the highest percentage of CH from the
4
ꢀ
reduction of all three substrates and, in the reactions of CN
(
Figure 2A) and CO (Figure 2B) reduction, the increased
[
23]
formation of CH4 by the L-cluster is accompanied by
a decreased formation of C H , consistent with a preference
Fisher–Tropsch substrate, CO2. The formation of CO in this
reaction is likely analogous to the reaction of reverse water–
2
4
ꢀ
[24]
of this cluster to reduce CN and CO all the way to CH over
gas shift (i.e., CO + H !CO + H O). Only in this case, the
4
2
2
2
+
the CꢀC coupling of these substrates into C H . Strikingly, an
expensive syngas, H , is replaced by H (provided by Lut-H)
2
4
2
ꢀ
analogous reaction was shown to be enabled by iron sulfide
and e (supplied by SmI ), and it is further produced as an
abundant side product of H reduction (see Table S1). The
formation of hydrocarbons also utilizes H as a hydrogen
2
+
(
(
FeS), a simplest FeS unit; only in this case, methanethiol
CH SH) was generated as a product of CO reduction in the
+
3
2
[
22]
presence of FeS and hydrochloric acid (HCl).
The
source, and this reaction likely involves the direct CꢀC
increased formation of CH4 by the L-cluster is not only
coupling from CO or CO -derived intermediate(s) other than
2
2
interesting because of the value of CH as a fuel source, but
CO (see Figure S2). As such, the reduction of CO to CO and
4
2
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.
Angewandte
Communications
hydrocarbons by M-, V-, and L-clusters not only defines two
unique reactions that are related to two important industrial
processes, but also bears the potential to serve as a blueprint
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[
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Keywords: carbon dioxide · CꢀC coupling · enzyme catalysis ·
.
1
hydrocarbons · nitrogenase
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