Addition of dioxygen to an N4S(thiolate) iron(II) cysteine dioxygenase model gives a structurally characterized sulfinato-iron(II) complex

Article abstract of DOI:10.1021/ja302112y

The non-heme iron enzyme cysteine dioxygenase (CDO) catalyzes the S-oxygenation of cysteine by O₂ to give cysteine sulfinic acid. The synthesis of a new structural and functional model of the cysteine-bound CDO active site, [Fe^II(N3P

Full text of DOI:10.1021/ja302112y

Communication
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Addition of Dioxygen to an N4S(thiolate) Iron(II) Cysteine
Dioxygenase Model Gives a Structurally Characterized Sulfinato−
Iron(II) Complex
Alison C. McQuilken, Yunbo Jiang, Maxime A. Siegler, and David P. Goldberg*
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland, 21218, United States
S
* Supporting Information
Mononuclear non-heme iron model complexes have proven
ABSTRACT: The non-heme iron enzyme cysteine
dioxygenase (CDO) catalyzes the S-oxygenation of
cysteine by O2 to give cysteine sulfinic acid. The synthesis
of a new structural and functional model of the cysteine-
bound CDO active site, [FeII(N3PyS)(CH3CN)]BF4 (1)
is reported. This complex was prepared with a new facially
chelating 4N/1S(thiolate) pentadentate ligand. The
reaction of 1 with O2 resulted in oxygenation of the
thiolate donor to afford the doubly oxygenated sulfinate
product [FeII(N3PySO2)(NCS)] (2), which was crystallo-
graphically characterized. The thiolate donor provided by
the new N3PyS ligand has a dramatic influence on the
redox potential and O2 reactivity of this FeII model
complex.
beneficial for addressing questions regarding enzyme mecha-
nism, but the use of biologically relevant O2 as oxidant with
these models remains quite rare.4 We previously described the
syntheses of two N3S(thiolate)FeII model complexes of CDO
that react with O2 to yield sulfonato products.5 In one case, the
thiolate donor was covalently tethered to the N3 platform
(Figure 1a), whereas in the other, an exogenous arylthiolate
etalloenzymes containing mononuclear non-heme iron
M
centers have been implicated in a variety of biological
roles, including the activation of O2 for substrate oxidation.
These non-heme iron oxygenases form a class of enzymes that
utilize O2 for catalytic oxidations and typically consist of a 2-
His-1-carboxylate donor set bound to the iron center. Cysteine
dioxygenase (CDO), on the other hand, differs from the norm
in that its active site consists of a mononuclear high-spin (hs)
FeII center with three His residues bound in a “facial triad”
(Scheme 1).1 Upon substrate binding,2 CDO utilizes O2 to
Figure 1. (a) S-oxygenation of a previous CDO model containing a
covalently tethered phenylthiolate donor and (b) design of the N3PyS
ligand.
ligand was employed. In each of these models, an hs-FeII center
is ligated by three planar N donors from a bis(imino)pyridine
(BIP) framework in a distorted square-pyramidal geometry,
with a triflate anion occupying the axial position. One finding to
result from these studies was that there appears to be a
requirement for the S donor to be positioned cis to the
potential binding site for O2 to obtain S-oxygenation. These
analogues contain only three N donors bound to the FeII
center, unlike the four N donors ligated in the Cys-bound form
of the enzyme.1e The three N ligands are also constricted to
one plane by the BIP framework, leading to a meridional
configuration, unlike the facial arrangement of the three His
donors found in CDO. Furthermore, while there was some
indirect evidence that S-oxygenation gave sulfinato intermedi-
ates in these model systems, the final, isolated S-oxygenated
iron complexes were found to be triply oxygenated sulfonato
Scheme 1. Sulfur Oxygenation of Cysteine with O2 by CDO
catalyze the S-oxygenation of cysteine to cysteine sulfinic acid
(Cys-SO2H), which is essential for the biosynthesis of pyruvate
and taurine. This enzyme is also vital for the maintenance of
healthy levels of cysteine in the body. While information on the
mechanism of O2 activation in CDO is just emerging,3a,b several
key X-ray crystal structures of CDO have been determined,
including a substrate-bound form in which the Cys is
coordinated in a bidentate fashion through the S and NH2
donors.1e,3c
Received: March 2, 2012
Published: May 11, 2012
© 2012 American Chemical Society
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Communication
products, reaching a level of oxygenation beyond that observed
in the biochemical reaction.
FeII(BF4)2 to N3PySH in CH3CN in the presence of 1 equiv of
Et3N afforded the dark-red FeII complex [FeII(N3PyS)-
(CH3CN)]BF4 (1). X-ray quality crystals were grown by
vapor diffusion of Et2O into a solution of 1 in CH3CN. The X-
ray structure of 1 reveals that the pentadentate ligand is bound
in a pseudo-square-pyramidal geometry about the FeII center as
designed, with the phenylthiolate donor held cis to the sixth site
occupied by a labile CH3CN ligand (Figure 2). The four
neutral N plus one thiolate donor motif corresponds closely to
the cysteine-bound active site of CDO.
To improve our model design, we set out to prepare a
pentadentate N4S ligand that would be constrained to provide
three neutral N donors in a facial arrangement along with a
fourth N donor to mimic the amino coordination from Cys and
a single S donor held cis to the putative O2 binding site at the
metal center. The N4Py ligand established previously for non-
heme iron models (Figure 1b)6 appeared to be an ideal scaffold
for building the desired N4S system. The pentadentate N4Py
ligand has a rigid structure that binds FeII in a square-pyramidal
fashion, and it has allowed for the characterization and isolation
of several key Fe−O [e.g., FeO, Fe−OO(H)] intermediates.
These beneficial properties have provided motivation for the
synthesis of derivatives of the N4Py ligand by several groups,
but to date none of these efforts has led to the incorporation of
a thiolato donor.7 Herein we report the synthesis of a new
pentadentate ligand, N3PySH, and its corresponding FeII
complex [FeII(N3PyS)(CH3CN)]BF4 (1) (Scheme 2). We
Scheme 2. Synthesis of N3PySH and 1
Figure 2. Displacement ellipsoid plots (50% probability level) of the
cation of 1 and complex 2. H atoms have been omitted for clarity.
The Fe−N bond distances [av Fe−Npy = 1.9594(13) Å] in 1
are consistent with those in low-spin (ls) FeII complexes
utilizing pyridyl ligands.6a,9 Likewise, the Fe−S distance
[2.3018(4) Å] is consistent with those in other FeII−thiolate
complexes.5a,b,10 The diamagnetic 1H NMR spectrum in
CD3CN is characteristic of ls-FeII. In CH3CN, intense UV−
vis features at 325, 418, and 493 nm (ε = 4475, 4550, and 4820
M−1 cm−1, respectively) were observed (Figure S1a in the
Supporting Information). In CH3OH, however, 1 is converted
to an hs-FeII complex, as indicated by the solution-state
magnetic moment (μeff = 4.5 μB, Evans method) and less-
intense UV−vis bands (Figure S1b). We conclude that
CH3OH, a relatively weak-field ligand, likely displaces
CH3CN in the labile site of 1, providing a convenient switch
for accessing the biologically relevant hs-FeII state that mimics
resting CDO.
also show that 1 reacts with O2 via sulfur oxygenation to
generate the doubly oxygenated sulfinato complex,
[FeII(N3PySO2)]+ (Scheme 3). Thus, 1 is a close structural
Scheme 3. O2 Reactivity of [FeII(N3PyS)(CH3CN)]+ in
CH3OH
Addition of excess O2 to a methanolic solution of hs-1 results
in an immediate color change from the dark-brown of 1 (470
nm, 1000 M−1 cm−1) to a new deep-green (674 nm, 1300 M−1
cm−1) (Figure S3). Efforts to obtain X-ray quality crystals of
this green species for characterization have to date been
unsuccessful. However, preliminary experiments suggested that
this species may be an S-oxygenated intermediate (see below).
To isolate a crystalline product, SCN− was added as a
potentially good ligand for the labile site on the iron product.
Immediately upon addition of 1 equiv of KSCN, the green
solution turned brown. Vapor diffusion of iPr2O resulted in the
isolation of orange-red crystals of 2 in 6 days. Determination of
the structure of 2 by X-ray crystallography (Figure 2) revealed
an ls-FeII complex wherein the chelated S atom has been doubly
oxygenated to give the sulfinate product and a thiocyanate
anion has replaced the CH3CN ligand in 1.
and functional model of CDO and provides a rare example of
the reaction of an FeII−thiolate complex with O2 to give a
sulfinato complex. This product was characterized by X-ray
crystallography and is to our knowledge the first example of a
structurally characterized mononuclear FeII−sulfinate complex.
Reaction of the protected phenylthiolate 3-(2-
bromomethylphenylsulfanyl)propionitrile (BrCH2PhSEtCN)8
with N-[di(2-pyridinyl)methyl]-N-(2-pyridinylmethyl)amine7c
in the presence of K2CO3 at 60 °C in CH3CN afforded the
protected pentadentate ligand N3PySEtCN (Scheme 2).
The average Fe−N distance in 2 [1.971(3) Å] is consistent
with those in other ls-FeII complexes and only slightly larger
than that in 1. The Fe−S bond distance [2.1812(9) Å]
decreased upon S-oxygenation, consistent with the decrease in
other metal−sulfur (M−S) bond lengths observed upon S-
t
Addition of BuOK in THF yielded the deprotected ligand
N3PySH in moderate yield (43%) after workup. Addition of
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Communication
oxygenation.11a−e The reduction in the M−S bond length in
going from the thiolate to the sulfinate donor corresponds to
both the contraction in the size of the S atom in the sulfinate
due to the increase in the formal oxidation state and the
elimination of the repulsive interaction between the metal’s
filled d orbitals and the S lone pairs on the thiolate donor. The
S−O distances [S1−O1 = 1.488(2) Å, S1−O2 = 1.476(2) Å]
match other sulfinate bond lengths.11 Spectroscopic character-
ization of crystalline 2 in CH3CN showed two intense UV−vis
−1
bands at 380 and 451 nm (ε = 4600 and 4200 M
cm−1,
respectively; Figure S2), similar to the spectrum of 1 in
CH3CN. The attenuated total reflectance IR spectrum of neat 2
revealed two prominent peaks at 1129 and 1012 cm−1, which
fall in the observed range for metal−sulfinate complexes,11 and
we assign these bands to the asymmetric and symmetric S−O
stretching modes. Thus, addition of O2 to CDO model 1 led to
the isolation and characterization of an FeII−sulfinate complex
wherein the thiolate donor was doubly oxygenated in a manner
parallel with the enzymatic chemistry. Although there are a few
examples of structurally characterized FeIII−sulfinate complex-
es,11f,12 2 is to our knowledge the first example of a
crystallographically characterized mononuclear FeII−sulfinate
complex.
Figure 3. ESI-MS spectra after the reaction of 1 with (top left) O2,
(top right) 18O2, and (bottom) O2/H218O.
The influence of the thiolate donor on non-heme iron active
sites is of fundamental interest, and the N3PySH ligand
provided us with an ideal system to address this issue by
comparison with the all-nitrogen N4Py analogue. Complex 1
and [FeII(N4Py)]2+ show a dramatic difference in O2 reactivity:
1 reacts with O2 without the use of added reductants, whereas
[FeII(N4Py)]2+ reacts with O2 only in the presence of 1-benzyl-
1,4-dihydronicotinamide (BNAH) to give [(N4Py)-
FeIII(OOH)]2+.13 As we have reported, a prerequisite for O2
reactivity is E1/2(FeIII/II) < −0.1 V vs Fc+/Fc.5b Cyclic
voltammetry of 1 in CH3CN showed a single quasi-reversible
wave at −0.226 V vs Fc+/Fc (Figure S4a), which we assign to
the FeIII/II couple. This potential is shifted significantly more
negative relative to the same couple for the non-thiolate
analogue [FeII(N4Py)]2+ (E1/2 = +0.61 V vs Fc+/Fc in
CH3CN).14 These results show that replacement of a pyridyl
donor with a single thiolate ligand causes a dramatic cathodic
shift of over 800 mV relative to N4Py. The redox potential of 1
was also measured in CH3OH, where two quasi-reversible
waves at E1/2 = −233 and −583 mV vs Fc+/Fc were found. The
former is assigned to some CH3CN-bound 1 that remains in
equilibrium with the methanol complex, and the latter is
assigned to the FeIII/II couple of the hs CH3OH-bound 1, which
clearly falls well within the previously noted range for O2
reactivity. As expected, the redox potential for 2 (−0.001 V;
Figure S4b) is shifted more positive than 1 by 225 mV,
consistent with less electron donation from the S-bound
sulfinate group. However, the E1/2 value for S-bound 2 remains
significantly more negative than that for [FeII(N4Py)]2+.
Isotope labeling studies with 18O2 were conducted to gain
insight into the mechanism of the reaction of 1 with O2.
Analysis of reaction mixtures with natural-abundance O2 by
electrospray ionization mass spectrometry (ESI-MS) revealed a
prominent isotopic cluster centered at m/z 581.7 that can be
assigned to [(K)(FeII(N3PyS16O2)(NCS))]+ ([M + K]+;
Figure 3). When the reaction was run with 18O2 (98%
isotope-enriched), a shift in the isotopic pattern was observed.
Simulations of this pattern indicated that 14% of the FeII−
sulfinate complex contained one labeled O atom (S16O18O),
while ∼2% of the product was labeled in both O positions
(S18O2). These results suggest that O2 is a source of O atoms
for the S-oxygenation but that significant exchange with
exogenous H2O during the course of the reaction is likely.
To test this idea, the reaction was carried out using H218O (50
μL). Subsequent ESI-MS analysis revealed 70% S18O2 and 28%
S16O18O, suggesting that the O atoms in the sulfinate complex
are susceptible to exchange during S-oxygenation. However,
independent experiments with pure 2 and H218O revealed that
the O atoms of the ls FeII−sulfinate complex are not
exchangeable with H218O under the same conditions (data not
shown). Thus, the isotope labeling results implicate one or
more intermediates in the S-oxygenation pathway that undergo
facile O exchange with H2O. One such intermediate could be a
monooxygenated sulfenato−iron species. Sulfenato−metal
[RS(O)−M] complexes are rare but have been shown to
undergo facile O-atom exchange with H2O.15
Given these results, we speculated that the green
intermediate (674 nm) may contain a singly oxygenated
sulfenate donor. In previous studies, it has been shown that
dimedone reacts selectively with sulfenic acids but not thiols or
sulfinic acids.16 Addition of dimedone (50 equiv) caused the
complete decay of the band at 674 nm over 2 h. MS analysis
revealed a major peak at m/z 591.4, corresponding to [Fe−
N3PyS−dimedone − H]+, the product expected from the
reaction of dimedone with a sulfenic acid group (Scheme 4).
Metal sulfenates are also known to react with PR3.17 Anaerobic
addition of PPh3 (2 equiv) to the green intermediate resulted in
a color change to brown and the production of OPPh3 (50%,
Scheme 4. Reaction of Dimedone with the Green Species
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31P NMR, based on total Fe). Addition of H218O to the green
intermediate followed by reaction with PPh3 led to 49% 18O-
labeled OPPh3. In contrast, sulfinato complex 2 did not react
with either dimedone or PPh3 under the same conditions.
These data are consistent with the presence of a sulfenato−iron
intermediate containing an exchangeable O atom, although the
presence of other intermediates at this stage, including doubly
oxygenated sulfinato species, cannot be ruled out.
In summary, we have reported the synthesis of a novel
N4S(thiolate) ligand that functions as designed to give
[FeII(N3PyS)]+, a structural analogue of substrate-bound
CDO. The incorporation of a single thiolate donor into the
coordination sphere to mimic the Cys sulfur ligation in the
enzyme was critical for the reactivity of the FeII complex with
O2. In contrast, the all-N analogue N4Py is completely inert to
O2 in the absence of coreductants. The present findings, in
comparison with those for our previous CDO model
complexes, suggest that a facial arrangement of the N donors
about the FeII center biases the O2 reactivity toward production
of the biologically relevant sulfinato product. Although more
work is needed to understand the mechanism of S-oxygenation
of 1, the preliminary data suggest that the O2 addition proceeds
through monooxygenase-like steps, as indicated by calculations
for the CDO mechanism.3b
(d) Thibon, A.; England, J.; Martinho, M.; Young, V. G., Jr.; Frisch, J.
R.; Guillot, R.; Girerd, J.-J.; Munck, E.; Que, L., Jr.; Banse, F. Angew.
̈
Chem., Int. Ed. 2008, 47, 7064. (e) A CDO model study employing O2
was recently described. See: Sallmann, M.; Siewert, I.; Fohlmeister, L.;
Limberg, C.; Knispel, C. Angew. Chem., Int. Ed. 2012, 51, 2234.
(5) (a) Jiang, Y. B.; Widger, L. R.; Kasper, G. D.; Siegler, M. A.;
Goldberg, D. P. J. Am. Chem. Soc. 2010, 132, 12214. (b) Badiei, Y. M.;
Siegler, M. A.; Goldberg, D. P. J. Am. Chem. Soc. 2011, 133, 1274.
(c) Kumar, D.; Sastry, G. N.; Goldberg, D. P.; de Visser, S. P. J. Phys.
Chem. A 2012, 116, 582.
(6) (a) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B.; Que,
L., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 1512. (b) Ho, R. Y. N.;
Roelfes, G.; Hermant, R.; Hage, R.; Feringa, B. L.; Que, L., Jr. Chem.
Commun. 1999, 2161. (c) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.;
Woodrum, N. L.; Cramer, C. J.; Que, L., Jr. Angew. Chem., Int. Ed.
2005, 44, 3690.
(7) (a) Roelfes, G.; Vrajmasu, V.; Chen, K.; Ho, R. Y. N.; Rohde, J.-
U.; Zondervan, C.; la Crois, R. M.; Schudde, E. P.; Lutz, M.; Spek, A.
L.; Hage, R.; Feringa, B. L.; Munck, E.; Que, L., Jr. Inorg. Chem. 2003,
̈
42, 2639. (b) Ligtenbarg, A. G. J.; Oosting, P.; Roelfes, G.; La Crois, R.
M.; Lutz, M.; Spek, A. L.; Hage, R.; Feringa, B. L. Chem. Commun.
2001, 385. (c) Roelfes, G.; Branum, M. E.; Wang, L.; Que, L., Jr.;
Feringa, B. L. J. Am. Chem. Soc. 2000, 122, 11517. (d) Sen Soo, H.;
Komor, A. C.; Iavarone, A. T.; Chang, C. J. Inorg. Chem. 2009, 48,
10024. (e) van den Heuvel, M.; van den Berg, T. A.; Kellogg, R. M.;
Choma, C. T.; Feringa, B. L. J. Org. Chem. 2004, 69, 250.
(8) Thapper, A.; Behrens, A.; Fryxelius, J.; Johansson, M. H.;
Prestopino, F.; Czaun, M.; Rehder, D.; Nordlander, E. Dalton Trans.
2005, 3566.
ASSOCIATED CONTENT
* Supporting Information
■
S
(9) Zang, Y.; Kim, J.; Dong, Y. H.; Wilkinson, E. C.; Appelman, E.
H.; Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197.
(10) (a) Namuswe, F.; Kasper, G. D.; Sarjeant, A. A. N.; Hayashi, T.;
Experimental details, spectra, electrochemical data, and
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Krest, C. M.; Green, M. T.; Moenne-Loccoz, P.; Goldberg, D. P. J. Am.
̈
Chem. Soc. 2008, 130, 14189. (b) Kitagawa, T.; Dey, A.; Lugo-Mas, P.;
Benedict, J. B.; Kaminsky, W.; Solomon, E.; Kovacs, J. A. J. Am. Chem.
Soc. 2006, 128, 14448.
AUTHOR INFORMATION
Corresponding Author
dpg@jhu.edu
■
(11) (a) Buonomo, R. M.; Font, I.; Maguire, M. J.; Reibenspies, J. H.;
Tuntulani, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 1995, 117, 963.
(b) Mullins, C. S.; Grapperhaus, C. A.; Frye, B. C.; Wood, L. H.; Hay,
A. J.; Buchanan, R. M.; Mashuta, M. S. Inorg. Chem. 2009, 48, 9974.
(c) Grapperhaus, C. A.; Darensbourg, M. Y. Acc. Chem. Res. 1998, 31,
451. (d) Masitas, C. A.; Kumar, M.; Mashuta, M. S.; Kozlowski, P. M.;
Grapperhaus, C. A. Inorg. Chem. 2010, 49, 10875. (e) Masitas, C. A.;
Mashuta, M. S.; Grapperhaus, C. A. Inorg. Chem. 2010, 49, 5344.
(f) Galardon, E.; Giorgi, M.; Artaud, I. Chem. Commun. 2004, 286.
(g) Lee, C.-M.; Hsieh, C.-H.; Dutta, A.; Lee, G.-H.; Liaw, W.-F. J. Am.
Chem. Soc. 2003, 125, 11492.
(12) (a) Heinrich, L.; Li, Y.; Vaissermann, J.; Chottard, G.; Chottard,
J.-C. Angew. Chem., Int. Ed. 1999, 38, 3526. (b) Lugo-Mas, P.; Taylor,
W.; Schweitzer, D.; Theisen, R. M.; Xu, L.; Shearer, J.; Swartz, R. D.;
Gleaves, M. C.; DiPasquale, A.; Kaminsky, W.; Kovacs, J. A. Inorg.
Chem. 2008, 47, 11228. (c) Noveron, J. C.; Olmstead, M. M.;
Mascharak, P. K. J. Am. Chem. Soc. 2001, 123, 3247. (d) Tyler, L. A.;
Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 1999,
38, 616.
(13) Hong, S.; Lee, Y.-M.; Shin, W.; Fukuzumi, S.; Nam, W. J. Am.
Chem. Soc. 2009, 131, 13910.
(14) Collins, M. J.; Ray, K.; Que, L., Jr. Inorg. Chem. 2006, 45, 8009.
(15) Heinrich, L.; Mary-Verla, A.; Li, Y.; Vaissermann, J.; Chottard,
J.-C. Eur. J. Inorg. Chem. 2001, 2203.
(16) (a) Heinecke, J.; Ford, P. C. J. Am. Chem. Soc. 2010, 132, 9240.
(b) Seo, Y. H.; Carroll, K. S. Angew. Chem., Int. Ed. 2011, 50, 1342.
(17) (a) Farmer, P. J.; Verpeaux, J.-N.; Amatore, C.; Darensbourg, M.
Y.; Musie, G. J. Am. Chem. Soc. 1994, 116, 9355. (b) Lugo-Mas, P.;
Dey, A.; Xu, L.; Davin, S. D.; Benedict, J.; Kaminsky, W.; Hodgson, K.
O.; Hedman, B.; Solomon, E. I.; Kovacs, J. A. J. Am. Chem. Soc. 2006,
128, 11211.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The NIH (GM62309) is gratefully acknowledged for financial
support, and Y.J. is grateful for the Ernest M. Marks Fellowship.
■
REFERENCES
■
(1) (a) Joseph, C. A.; Maroney, M. J. Chem. Commun. 2007, 3338.
(b) McCoy, J. G.; Bailey, L. J.; Bitto, E.; Bingman, C. A.; Aceti, D. J.;
Fox, B. G.; Phillips, G. N., Jr. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
3084. (c) Simmons, C. R.; Liu, Q.; Huang, Q. Q.; Hao, Q.; Begley, T.
P.; Karplus, P. A.; Stipanuk, M. H. J. Biol. Chem. 2006, 281, 18723.
(d) Stipanuk, M. H.; Ueki, I.; Dominy, J. E., Jr.; Simmons, C. R.;
Hirschberger, L. L. Amino Acids 2009, 37, 55. (e) Ye, S.; Wu, X.; Wei,
L.; Tang, D. M.; Sun, P.; Bartlam, M.; Rao, Z. H. J. Biol. Chem. 2007,
282, 3391.
(2) (a) Gardner, J. D.; Pierce, B. S.; Fox, B. G.; Brunold, T. C.
Biochemistry 2010, 49, 6033. (b) Tchesnokov, E. P.; Wilbanks, S. M.;
Jameson, G. N. L. Biochemistry 2012, 51, 257.
(3) (a) Crawford, J. A.; Li, W.; Pierce, B. S. Biochemistry 2011, 50,
10241. (b) Kumar, D.; Thiel, W.; de Visser, S. P. J. Am. Chem. Soc.
2011, 133, 3869. (c) Simmons, C. R.; Krishnamoorthy, K.; Granett, S.
L.; Schuller, D. J.; Dominy, J. E., Jr.; Begley, T. P.; Stipanuk, M. H.;
Karplus, P. A. Biochemistry 2008, 47, 11390.
(4) (a) Kim, S. O.; Sastri, C. V.; Seo, M. S.; Kim, J.; Nam, W. J. Am.
Chem. Soc. 2005, 127, 4178. (b) Lee, Y.-M.; Hong, S.; Morimoto, Y.;
Shin, W.; Fukuzumi, S.; Nam, W. J. Am. Chem. Soc. 2010, 132, 10668.
(c) MacBeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.;
Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938.
8761
dx.doi.org/10.1021/ja302112y | J. Am. Chem. Soc. 2012, 134, 8758−8761