Thiolate-bridged nickel-copper complexes: A binuclear model for the catalytic site of acetyl coenzyme A synthase?

Article abstract of DOI:10.1021/ja0346577

Reaction of the nickel metalloligands [EtN₂S₂]Ni (EtN₂S₂, N,N′-diethyl-3,7-diazanonane-1,9-dithiolate) or K₂[Ni(phmi)] (phmi, N,N′-1,2-phenylenebis(2-sulfanyl-2-methylpropionamide)) with [Cu(CH₃CN)₄]BF₄ yields polynuclear complexes in which two copper(I) ions are bridged by the nickel metalloligands. Alternatively, reaction with the Cu(I) source, [(PhTt^tBu)Cu] (PhTt^tBu, phenyltris((tert-butylthio)methyl)borate), generates discrete binuclear NiCu complexes that may serve as models of the acetyl coenzyme A synthase active site. The binuclear species react reversibly with CO via rupture of the thiolate bridges. Copyright

Full text of DOI:10.1021/ja0346577

Published on Web 03/19/2003
Thiolate-Bridged Nickel-Copper Complexes: A Binuclear Model for the
Catalytic Site of Acetyl Coenzyme A Synthase?
Rangan Krishnan, Janis K. Voo, Charles G. Riordan,* Lev Zahkarov, and Arnold L. Rheingold
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received February 13, 2003; E-mail: riordan@udel.edu
Acetyl coenzyme A synthase/CO dehydrogenase (ACS/COdH)
is a bifunctional enzyme found in acetogenic, methanogenic, and
sulfate-reducing archaea and bacteria which enables the organisms
to grow autotrophically on CO2 and H2.1-3 The ACS catalyzed
reaction entails the reversible assembly of acetyl CoA via the
coupling of a methyl substituent from enzyme-bound methyl-
cobamide, CO, and the thiolate, CoA.4 The methyl group and CO
fixed during the synthesis of the thioester are derived from CO2
Figure 1. Active site of acetyl coenzyme A synthase (A cluster). L is an
unknown, nonproteinaceous ligand.7
reduction, the latter at a different active site in the protein, the C
cluster. A molecular tunnel in the protein permits for shuttling of
CO from the C cluster to the A cluster, the site of acetyl CoA
synthesis.5,6 The enzyme from the acetogen, Moorella thermoace-
tica, has been examined extensively by enzymatic, biophysical, and,
most recently, crystallographic analysis.7 The last experiments
identified copper as an integral constituent of the A cluster
previously identified as a Ni-Fe4S4 cluster.8 The remarkable
structure consists of a cuboidal Fe4S4 unit linked via a cysteine to
a binuclear NiCu subcluster, Figure 1. In this respect, the active
site bears resemblance to the H cluster of the Fe-only hydrogenase,
which contains an Fe4S4 linked via a cysteine residue to an Fe2
subcluster.9 Further, amide backbone coordination at Ni was
unanticipated, although the N2S2 environment was predicted by
EXAFS spectroscopy.10 Our continued interest in functional models
for ACS11,12 prompted the investigation of thiolate-bridged NiCu
complexes as a new direction for understanding this fascinating
enzyme. Herein we report the first preparation of thiolate-bridged
nickel-copper binuclear complexes. Preliminary reactivity studies
confirm that the binuclear Ni(II)/Cu(I) complexes bind CO revers-
ibly, generating a Cu(CO) adduct resulting from rupture of the
thiolate bridges.
Scheme 1
Given the utility of metal complexes with cis-dithiolates to serve
as robust “metalloligands”,13-15 it is surprising that only a single
report of a thiolate-bridged NiCu(I) species preceded this study.16
Reactions of the two [N2S2]Ni species15,17 in Scheme 1 with [Cu-
(CH3CN)4]BF4 provided insight into the challenges in preparing
such species. The products are polynuclear complexes in which
two copper ions are bridged by [N2S2]Ni “ligands”. The structures
were authenticated by diffraction methods with details contained
in the Supporting Information. A key feature of the structures is
that the adjacent thiolate donors are coordinated to different Cu
ions, rather than serving as a chelate to a single metal as observed
for [N2S2]Ni ligated to Ni,18 Fe,14,15 and Cu(II).19 The different
stoichiometry of 1 versus 2 results in disparate reactivity, vide infra,
and may be attributed to the greater steric demands of the gem-
dimethyl substituents adjacent to the bridging sulfurs in 2.
Changing the source of Cu(I) to the less labile [PhTttBu]Cu(CH3-
CN)20 resulted in isolation of discrete binuclear complexes, 3 and
4, Scheme 1. The molecular structure of 3 determined by diffraction
methods is displayed in Figure 2. The N2S2 ligation at Ni is
unchanged as a consequence of coordination to the [PhTttBu]Cu
moiety. The Cu(I) is ligated in a roughly Td array of four sulfur
ligands, two thiolate and two thioether donors, the latter a
consequence of the κ2-coordination of the borato ligand. There is
one rather long Cu(I)-S(thiolate) bond distance at 2.515(1) Å, while
the remaining three Cu-S distances are in the range observed for
Figure 2. Thermal ellipsoid representation of 3. Selected distances (Å)
and angles (°), Cu(1)-S(1), 2.515(1); Cu(1)-S(2), 2.264(1); Cu(1)-S(3),
2.367(1); Cu(1)-S(4), 2.274(1); Ni(1)-Cu(1), 2.9173(8); S(1)-Ni(1)-S(2),
84.62(5); S(1)-Cu(1)-S(2), 75.66(4); S(3)-Cu(1)-S(4), 88.87(4).
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C O M M U N I C A T I O N S
other Cu(I) complexes, including the average Cu-S length in 1 of
2.284(1) Å. A similar asymmetry in the bridging cysteinates is
apparent in the A cluster structure.21 The Cu- - -Ni distance of 2.917-
(1) Å is only slightly longer than that determined for the A cluster,
Ni under native conditions. Such a low coordinate site should be
susceptible to demetalation by phen, accounting for the Ni lability,
whereas the diamidodithiolato [N2S2]Ni appears much less acces-
sible to chelating agents. Further, the former site possesses the
coordination flexibility to facilitate acetyl CoA dis/assembly. To
explore this hypothesis, we are preparing binuclear Ni2 complexes
akin to 3 and 4 in an attempt to elicit relevant reactivity to
understand the fundamental chemistry of this novel binuclear
system.11
2.79 Å. Proton NMR spectral data indicate that the κ2-[PhTttBu
]
coordination in 3 and 4 is maintained in solution as evidenced by
distinct signals for the ligated and free thioether substituents. The
cyclic voltammograms of 3 and 4 display processes assigned to
nickel-based redox events and oxidation of the [PhTttBu] ligand as
deduced by comparison with the starting reagents. Coordination
of the [PhTttBu]Cu moiety has only a modest effect on the [N2S2]-
Ni redox potentials, resulting in anodic shifts due to metalation of
the thiolates. No copper-based oxidation is available in these
complexes with sulfur-only coordination in accord with the finding
that N2O-treated ACS does not exhibit a Cu(II) ESR signal.8
An intriguing, yet unresolved, aspect of the enzymology of ACS
concerns the nature of the intermediate resulting from reductive
carbonylation of the A cluster. The so-called NiFeC state (alter-
natively, Ared-CO) results upon exposure of the isolated protein
with CO and exhibits ESR line-broadening upon isotopic perturba-
tion with 57Fe, 61Ni, or 13CO.22 This intermediate is characterized
further by a νCO band (1995 cm-1) consistent with a terminal
M-CO.23 In light of the recent structural evidence for Cu in the A
cluster, a Cu(I)-CO adduct has been proposed as an intermediate
on the pathway of acetate formation.7,8 To pursue this possibility
by establishing relevant small molecule transformations, solutions
of 1-4 were exposed to an atmosphere of CO. While 1 is unreactive
under these conditions, the other complexes are sensitive to CO,
yielding adducts characterized by terminal νCO modes, 2042 cm-1
Acknowledgment. We thank the National Institutes of Health
for financial support.
Note Added in Proof. Lindahl and Fontecilla-Camps have just
reported an X-ray structure of the COdH/ACS from M. thermoace-
tica that shows two different conformations of the R subunit, each
contains an A cluster of distinct metal composition: Ni2-[Fe4S4]
and NiZn-[Fe4S4]. The authors propose that only the former cluster
is active; Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.;
Vernede, X.; Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct.
Biol., published on March 10, 2003.
Supporting Information Available: Synthetic procedures and
characterization data (PDF); crystallographic information for compounds
1, 2, and 3 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) Ferry, J. G. Crit. ReV. Biochem. Mol. Biol. 1992, 27, 473-503.
(2) Ragsdale, S. W.; Kumar, M. Chem. ReV. 1996, 96, 2515-2540.
(3) Lindahl, P. A. Biochemistry 2002, 41, 2097-2105.
(4) Ragsdale, S. W.; Riordan, C. G. J. Biol. Inorg. Chem. 1996, 1, 489-493.
(5) Maynard, E. L.; Lindahl, P. A. J. Am. Chem. Soc. 1999, 121, 9221-
9222.
(for 2) and 2078 cm-1 (3 and 4), that exhibited the expected shifts
13
12
13
following incubation with CO (ν CO/ν CO, 1.023; calcd, 1.023).
Spectral analysis confirmed that the carbonylation of 3 and 4 results
in rupture of the thiolate bridges generating the respective mono-
nuclear species, [N2S2]Ni and [PhTttBu]Cu(CO), the latter also
available via the carbonylation of [PhTttBu]Cu(CH3CN) under
comparable conditions. Interestingly, the carbonylations of 3 and
4 are reversible as purging solutions with N2 results in the
disappearance of the 2078 cm-1 IR band that reappears upon
subsequent cycling with CO. At 25 °C and 1 atm of CO, 3 is fully
carbonylated, while 4 shows only partial product formation.
As the identification of Cu within the A cluster was unanticipated,
its role in ACS catalysis remains to be elucidated. The ligation
environment of the copper site is consistent with the Cu(I) oxidation
level as is the postulated CO binding.7,8 However, the isotope
sensitivity of the NiFeC ESR signal is difficult to interpret in the
context of the X-ray structure. The direct (or through bond) distance
between Ni and the Fe4S4 cluster appears too great to account for
the spin delocalization required to account for the hyperfine line-
broadening attributed to both 57Fe and 61Ni, although this clearly
warrants a detailed spectroscopic and computational investigation.
Furthermore, the structure does not appear to account for the
observation that phen removes some of the Ni in the protein (“labile
Ni”).24 Alternatively, it is possible the native enzyme functions with
Ni in the “Cu site” as has been suggested for the methanogenic
protein.25 Specifically, the A cluster may be of Ni2-[Fe4S4]
composition with the trigonal Cys3 metal binding site coordinating
(6) Seravalli, J.; Ragsdale, S. W. Biochemistry 2000, 39, 1274-1277.
(7) Doukov, T. I.; Iverson, T. M.; Seravalli, J.; Ragsdale, S. W.; Drennan, C.
L. Science 2002, 298, 567-572.
(8) Seravalli, J.; Gu, W.; Tam, A.; Strauss, E.; Begley, T. P.; Cramer, S. P.;
Ragsdale, S. W. Proc. Natl. Acad. Sci. U.S.A., published on Feb. 14, 2003.
(9) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853-1858.
(10) Russell, W. K.; Sta˚lhandske, C. M. V.; Xia, J. Q.; Scott, R. A.; Lindahl,
P. A. J. Am. Chem. Soc. 1998, 120, 7502-7510.
(11) Ram, M. S.; Riordan, C. G.; Yap, G. P. A.; Liable-Sands, L.; Rheingold,
A. L.; Marchaj, A.; Norton, J. R. J. Am. Chem. Soc. 1997, 119, 1648-
1655.
(12) Craft, J. L.; Mandimutsira, B. S.; Fujita, K.; Riordan, C. G.; Brunold, T.
C. Inorg. Chem. 2003, 42, 859-867.
(13) Lai, C.-H.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2390-2393.
(14) Colpas, G. J.; Day, R. O.; Maroney, M. J. Inorg. Chem. 1992, 31, 5053-
5055.
(15) Osterloh, F.; Saak, W.; Pohl, S. J. Am. Chem. Soc. 1997, 119, 5648-
5656.
(16) Jicha, D. C.; Busch, D. H. Inorg. Chem. 1962, 1, 878-883.
(17) Kruger, H. J.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30, 734-742.
(18) Farmer, P. J.; Solouki, T.; Mills, D. K.; Soma, T.; Russell, D. H.;
Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1992, 114,
4601-4605.
(19) Kaur, P. Indian J. Chem., Sect. A 1997, 36, 605-607.
(20) Ohrenberg, C.; Riordan, C. G.; Liable-Sands, L. M.; Rheingold, A. L.
Inorg. Chem. 2001, 40, 4276-4283.
(21) Protein Data Bank code 1MJG; there are four A clusters in the asymmetric
unit, two per R2â2 heterotetramer. Consequently, the values reported are
the distances averaged over the four unique sites.
(22) Gorst, C. M.; Ragsdale, S. W. J. Biol. Chem. 1991, 266, 20687-20693.
(23) Kumar, M.; Ragsdale, S. W. J. Am. Chem. Soc. 1992, 114, 8713-8715.
(24) Shin, W.; Lindahl, P. A. J. Am. Chem. Soc. 1992, 114, 9718-9719.
(25) Gencic, S.; Grahame, D. A. J. Biol. Chem. 2003, 278, 6101-6110.
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