Enantioselectivity of haloalkane dehalogenases and its modulation by surface loop engineering

Article abstract of DOI:10.1002/anie.201001753

In the loop: Engineering of the surface loop in haloalkane dehalogenases affects their enantiodiscrimination behavior. The temperature dependence of the enantioselectivity (lnE versus 1/T) of β-bromoalkanes by haloalkane dehalogenases is reversed (red data points) by deletion of the surface loop; the selectivity switches back when an additional single-point mutation is made. This behavior is not observed for -bromoesters.

Full text of DOI:10.1002/anie.201001753

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Chemie
DOI: 10.1002/anie.201001753
Enantioselectivity
Enantioselectivity of Haloalkane Dehalogenases and its Modulation by
Surface Loop Engineering**
Zbynek Prokop, Yukari Sato, Jan Brezovsky, Tomas Mozga, Radka Chaloupkova,
Tana Koudelakova, Petr Jerabek, Veronika Stepankova, Ryo Natsume, Jan G. E. van Leeuwen,
Dick B. Janssen, Jan Florian, Yuji Nagata, Toshiya Senda, and Jiri Damborsky*
Dedicated to Dr. Alfred Bader on the occasion of his 85th birthday
group of enzymes that is explored for enantioselectivity is
Enzymes are widely used for the synthesis of pharmaceuticals,
agrochemicals, and food additives because they can catalyze
enantioselective transformations.[1] Understanding the molec-
ular basis of enzyme–substrate interactions that contribute to
enantioselectivity is important for constructing selective
enzymes by protein engineering.[2] Up to now, emphasis has
been on reactions such as lipase- or esterase-based kinetic
resolutions,[2d,3] as well as lyase-, aminotransferase- and
ketoreductase-mediated conversions.[1a,4] An emerging
dehalogenases. Haloalkane dehalogenases can convert a
broad range of halogenated aliphatic substrates to their
corresponding alcohols by an SN2 mechanism (Scheme 1),[5]
and because of the simplicity of the reaction represent a good
model system to study the structural basis of reactivity[6] and
enantioselectivity.
[*] Dr. Z. Prokop,[+] J. Brezovsky,[+] T. Mozga, Dr. R. Chaloupkova,
T. Koudelakova, P. Jerabek, V. Stepankova, Prof. J. Damborsky
Loschmidt Laboratories, Department of Experimental Biology and
Centre for Toxic Compounds in the Environment
Faculty of Science, Masaryk University
Kamenice 5/A13, 625 00 Brno (Czech Republic)
Fax. (+420)5-4949-2556
E-mail: jiri@chemi.muni.cz
Scheme 1. Reaction mechanism of haloalkane dehalogenases with a-
bromoesters and b-bromoalkanes. Enz-COOÀ: active site Asp.
Homepage: http://loschmidt.chemi.muni.cz/peg
Dr. Y. Sato,[+] Dr. Y. Nagata
Department of Life Sciences, Graduate School of Life Sciences,
Tohoku University, Sendai (Japan)
Dr. Y. Sato,[+] Dr. R. Natsume
However, only a weak enantioselectivity (enantiomeric
ratio, E value < 9)[7] has been reported with haloesters and
1,2- and 1,3-dihaloalkanes for the haloalkane dehalogenases
from Xanthobacter autotrophicus (DhlA)[8] and Rhodococcus
rhodochrous NCIMB13064 (DhaA).[9] To further understand
the enantioselectivity of these enzymes, we explored several
dehalogenases for which the X-ray structure is available. This
includes DhaA, LinB from Sphingobium japonicum UT26,[10]
and DbjA from Bradyrhizobium japonicum USDA110.[11]
Kinetic resolution of an expanded set of racemic substrates
was analyzed with recombinant proteins, and it revealed that
DhaA, LinB, and DbjA possess excellent enantioselectivity
for a-bromoesters (Table 1). Furthermore, DbjA showed high
enantioselectivity with two b-bromoalkanes.
Japan Biological Informatics Consortium, Tokyo (Japan)
J. G. E. van Leeuwen, Prof. D. B. Janssen
Department of Biochemistry, University of Groningen
(The Netherlands)
Dr. J. Florian
Department of Chemistry, Loyola University Chicago (USA)
Dr. T. Senda
Biomedicinal Information Research Center, National Institute of
Advanced Industrial Science and Technology, Tokyo (Japan)
[+] These authors contributed equally to this work.
[**] Z.P. acknowledges EMBO for financial support of his stay at the
University of Groningen. Financial support is gratefully acknowl-
edged from: the Ministry of Education, Youth, and Sports of the
Czech Republic (grants LC06010 to J.D. and MSM0021622412 to
Z.P.); the Grant Agency of the Czech Academy of Sciences (grant
no. IAA401630901 to J.B.); the Grants-in-Aid from Ministry of
Education, Culture, Sports, Science, and Technology, Japan and the
Ministry of Agriculture, Forestry, and Fisheries, Japan (Y.N.); and
the New Energy and Industrial Technology Development Organ-
ization (NEDO) of Japan (T.S.). Access to the METACentrum
supercomputing facilities is highly appreciated (MSM6383917201).
We thank Prof. Uwe Bornscheuer from the University of Greifswald
for critical reading of this manuscript.
The steady-state kinetics of DbjA determined with (R)-
and (S)-2-bromopentane showed
a large difference in
Michaelis constants Km (24 and 570 mm, respectively) and
similar catalytic constants kcat (0.36 and 0.27 sÀ1), which
indicates that enantioselectivity in this case is mainly the
result of substrate binding. The high enantioselectivity of
DbjA allowed use of the enzyme for kinetic resolution of 2-
bromopentane on a preparative scale. Incubation of racemic
substrate (7 g) in a 4:1 mixture of Tris buffer (24 L, 50 mm,
pH 8.2) and dimethyl sulfoxide with DbjA enzyme (240 mg as
extract of Escherichia coli cells) at room temperature gave
complete conversion of the R enantiomer (> 99% ee) after
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001753.
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Table 1: Enantioselectivity of haloalkane dehalogenases DhaA, LinB, and DbjA towards b-bromoalkanes and a-bromoesters.[a]
DhaA
LinB
DbjA
E value
>200
ee [%]
c [%]
E value
ee [%]
c [%]
E value
>200
ee [%]
c [%]
>99
97
54
52
92
95
51
>99
>99
97
51
85
n.d.
50
n.d.
51
97
28
51
51
51
>200
>200
>200
50
54
50
a-bromoesters
n.d.
>99
88
>200
>200
>99
>99
2
7
4
3
24
79
43
41
57
61
60
48
2
16
12
3
16
80
53
39
56
53
55
51
1
145
68
7
>99
90
50
53
50
50
b-bromoalkanes
28
80
[a] n.d.=no activity detectable (below the detection limit of 0.5 nmolminÀ1 mgÀ1 of enzyme). The enantiomeric ratio (E value) is a quantitative
measure of enzyme stereospecificity and its relationship with enantiomeric excess (ee) and degree of conversion (c) has been described.[12] See the
Supporting Information for a complete list of tested substrates.
150 minutes.[13] Subsequent extraction and purification
yielded the pure S enantiomer (21% yield, purity 86%,
> 99% ee).
To dissect the molecular basis of DbjA enantioselectivity,
we undertook a detailed analysis using kinetic measurements,
mutagenesis, protein crystallography, thermodynamic analy-
sis, and molecular modeling. We selected 2-bromopentane
and methyl 2-bromobutyrate as representative substrates for
b-bromoalkanes and a-bromoesters, respectively.
A sequence alignment of haloalkane dehalogenases
suggested that the high enantioselectivity of DbjA arises
from an additional segment between the core a/b-hydrolase
and cap domains (Figure 1a). The crystal structure of DbjA
revealed that this segment is located on the protein surface
and does not directly take part in shaping the active-site
pocket (Figure 1b). The effect of loops on enzyme enantio-
selectivity has been reported,[2h] which let us construct a
deletion mutant (DbjAD) lacking the fragment 140-His-Thr-
Glu-Val-Ala-Glu-Glu-146 (hereafter termed the EB frag-
ment, Extra region of B. japonicum, Figure 1a) in this region.
Indeed, the deletion of the fragment changed the enantiose-
lectivity with both substrates (Figure 2a,b). Surprisingly, an
inverse effect of the deletion was observed with two
representative substrates: decreased enantioselectivity with
2-bromopentane and increased enantioselectivity with methyl
2-bromobutyrate (Figure 2a,b).
(Figure 1b). This change mainly arises from modulation of
the conformational behavior of His139, located next to the
deleted fragment. His139 adopts two different conformations,
inclined and deflected, in DbjA, whereas only the inclined
conformation can be seen in DbjAD (Figure 1c). Deletion of
the loop region resulted in a reduction of the volume of the
space that accommodates the side chain of His139 (Fig-
ure 1d).
Thermodynamic analysis of the reactions revealed that
enantiodiscrimination of methyl 2-bromobutyrate arises from
nearly identical enthalpy–entropy contributions in both the
wild type and deletion mutant (Figure 2a,b). In both cases,
the difference in transition-state enthalpy (DRÀSDH°) sup-
ports preferential conversion of the R enantiomer, while
entropy promotes conversion of the S enantiomer.
The thermodynamic characteristics of the reaction with 2-
bromopentane were significantly changed by the mutation.
The preferential conversion of the R enantiomer found with
wild-type DbjA is because of a stronger enthalpic contribu-
tion to transition-state stabilization, which is partially sup-
pressed by entropy favoring the S enantiomer (Figure 2a,b).
Strikingly, in DbjAD, the enthalpic contribution to enantio-
selectivity is opposite to that for the wild type, thus resulting
in a reversed temperature dependence of the E value that has
rarely been observed (Figure 2a,b).[16] Enantiopreference was
preserved in the deletion mutant since the entropic contribu-
tion is also opposite and becomes dominant, now strongly
favoring the enantiopreference of the R enantiomer. The
The crystal structure of DbjAD shows that deletion of the
EB fragment alters the shape and size of the active-site pocket
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hydrophobic wall, and by two
hydrogen bonds between the
bromine atom and the side
chains of the halide-stabiliz-
ing residues Trp104 and
Asn38. During the molecular
dynamics simulation, the
R enantiomer was sampled
exclusively in a reactive bind-
ing mode, while (S)-2-bromo-
pentane adopted a mixture of
reactive and nonreactive
binding
modes.
His139
appeared to modulate the
distribution of reactive con-
figurations[17] among these
binding modes by its interac-
tion with the substrate mole-
cule (Figure 2c, Supporting
Information Figure 1). The
simulations showed that the
inclined conformation of
His139 in DbjAD decreases
reactivity with the R enan-
tiomer and increases reactiv-
ity with the S enantiomer
(Supporting
Information
Figure 3 and Table 3), which
results in reduced enantiose-
lectivity of this mutant with 2-
bromopentane. The effect of
the mutation is opposite for
the two enantiomers because
of the different location of
their chiral centers (Fig-
ure 2c).
Figure 1. Structural comparison of haloalkane dehalogenases. a) The amino acid sequence alignment of
haloalkane dehalogenases LinB,[10] DhaA,[9] DbjA,[11] DbjAD, DbjAD+His139Ala (this study, PDB ID 3A2M
and 3A2L), DbeA (Ikeda-Ohtsubo et al., unpublished), DmlA,[11] DmsA, DmbA,[14] DrbA, and DmbC.[15]
Halide-stabilizing Asn and Trp residues are shown in green, the nucleophile in orange, the EB fragment
deleted in DbjAD in red, and the residue 139 in black (left panel). The right-hand panel shows the overall
structure of DbjA with the deleted residues within the EB loop in red. b) Active-site structures of haloalkane
dehalogenases. The nucleophile (Asp103) is depicted as an orange dot. His139 can adopt inclined and
declined conformations, which affects the size and hydrophobicity of the active-site pocket. c) Fo (DbjA)ÀFo
(DbjAD) difference Fourier maps, contoured at 3.0s (red) and À3.0s (blue), around His139 of DbjA
(orange) and DbjAD (cyan). Red density indicated by red arrows suggests that the deflected conformation
of His139 (His139-II) in DbjA has higher occupancy than in DbjAD. Blue density indicated by blue arrows
suggests that the inclined conformation of His139 (His139-I) in DbjAD has higher occupancy than in DbjA.
Densities indicated by green arrows indicate the difference in the polypeptide chain conformation between
DbjA and DbjAD. d) Deletion of the EB fragment affects the conformation of His139 (orange). The model
structures of two alternative conformations of His139 in DbjAD were prepared on the basis of the crystal
structure of DbjA. Deflected His139 in DbjAD makes close contacts with the N atom of Gln147 and the
Cb atom of Ala150 (red). See the Supporting Information for details of structural analysis.
In contrast to what was
observed with 2-bromopen-
tane, the enantiomers of
methyl
2-bromobutyrate
appeared to bind in different
orientations with their chiral
centers aligned and the two
substituting alkyl groups
pointing towards different
sides of the active site (Fig-
ure 2c). These orientations
are stabilized by three hydro-
results of thermodynamic and mutagenesis analysis indicate
that the enantioselective reactions with 2-bromopentane and
methyl 2-bromobutyrate are controlled by different molec-
ular bases.
Next, we tried to link these molecular bases to three-
dimensional structures of wild-type and mutant DbjA by
molecular modeling. Both enantiomers of 2-bromopentane
bind along the same wall of the active-site pocket, and adopt a
mirror-image orientation with displaced chiral centers (Fig-
ure 2c). This binding is characterized by hydrophobic inter-
actions between the alkyl chain of the substrate and the
gen bonds: two between bromine and the side chains of
halide-stabilizing residues, and one between the substrate
carbonyl group and the side chain of Asn38 or Trp104 for the
R and S enantiomer, respectively (Figure 2c, Supporting
Information Figure 1). Hydrophobic interactions with the
wall of the active-site pocket are less important for methyl 2-
bromobutyrate than for 2-bromopentane. The binding free
energies calculated for these binding modes favor binding of
the R enantiomer over the S enantiomer, irrespective of the
protein variant, because of better conformity of the R enan-
tiomer with the active site. The discrimination against the
Angew. Chem. Int. Ed. 2010, 49, 6111 –6115
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S enantiomer is further reinforced in the chem-
ical step of the reaction (Supporting Information
Table 3). The inclined conformation of His139 in
DbjAD decreases reactivity with both enantio-
mers since their chiral centers are spatially
aligned. The magnitude of this effect is larger
for the S enantiomer, thus resulting in increased
enantioselectivity with methyl 2-bromobutyrate
(Supporting Information Figure 2 and Table 3).
Our analysis of DbjA enantioselectivity dem-
onstrates that different molecular bases underlie
the enantioselective conversion of methyl 2-
bromobutyrate and 2-bromopentane (Figure 2c).
Furthermore, the enantioselectivity of DbjA can
be modulated by mutation at the surface loop
region. Assuming that the inclined conformation
of His139 in DbjAD significantly reduces the
volume of the active-site pocket, substitution of
His139 by Ala should restore the original enzyme
enantioselectivity. Indeed, the enantioselectivity
of the DbjAD + His139Ala mutant was reconsti-
tuted for both substrates (Figure 2a,b). The
effects of the mutations were stronger for 2-
bromopentane than for methyl 2-bromobutyrate
because of different binding orientations and the
distinct nature of the interactions involved in
their enantiodiscrimination.
In conclusion, we have shown that haloalkane
dehalogenases: 1) can kinetically discriminate
between enantiomers of two distinct groups of
substrates, a-bromoesters and b-bromoalkanes;
2) have enantioselectivity based on distinct
molecular interactions, which can be modified
separately by engineering of a surface loop; and
3) can adopt an inverse temperature dependence
of enantioselectivity for b-bromoalkanes, but not
a-bromoesters, by mutating this surface loop and
a flanking residue. Our study contributes towards
understanding of the molecular basis and ther-
modynamics of the enantioselectivity of
enzymes,[18] and opens up new possibilities for
constructing enantioselective biocatalysts by pro-
tein engineering.
Received: March 24, 2010
Published online: July 19, 2010
Keywords: enantioselectivity · enzymes ·
.
haloalkane dehalogenases · protein engineering ·
thermodynamics
Figure 2. Two molecular bases of enantioselectivity of haloalkane dehalogenases.
a) Thermodynamic analysis of the reactions catalyzed by DbjA (green), DbjAD (red),
and DbjAD+His139Ala (black) with methyl 2-bromobutyrate (left) and 2-bromopen-
tane (right) illustrates the temperature dependence of their enantiomeric ratios and
the enthalpy–entropy compensation (in the insets). b) Thermodynamic components
of the enantioselectivity of DbjA, DbjAD, and DbjAD+His139Ala: enantioselectivity
(E value), differential transition state free energy (DRÀSDG° =DRÀSDH°ÀTDRÀSDS°)
and its enthalpic (DRÀSDH°) and entropic (TDRÀSDS°) contributions at T=298 K.
c) Mutations have distinct effects on the active-site pocket, binding orientations, and
reactivity of R enantiomers (black) and S enantiomers (red).
[1] a) K. Faber, Biotransformations in Organic
Chemistry, Springer, Heidelberg, 2000; b) H. E.
Schoemaker, D. Mink, M. G. Wubbolts, Science
2003, 299, 1694 – 1697.
[2] a) R. J. Kazlauskas, Trends Biotechnol. 1994, 12,
464 – 472; b) W. V. Tuomi, R. J. Kazlauskas, J. Org.
Chem. 1999, 64, 2638 – 2647; c) D. Rotticci, J. C.
Rotticci-Mulder, S. Denman, T. Norin, K. Hult,
6114
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6111 –6115

Angewandte
Chemie
ChemBioChem 2001, 2, 766 – 770; d) U. T. Bornscheuer, Curr.
Opin. Biotechnol. 2002, 13, 543 – 547; e) E. Henke, U. T.
Bornscheuer, R. D. Schmid, J. Pleiss, ChemBioChem 2003, 4,
485 – 493; f) K. Hult, P. Berglund, Curr. Opin. Biotechnol. 2003,
14, 395 – 400; g) T. Ema, T. Fujii, M. Ozaki, T. Korenaga, T.
Sakai, Chem. Commun. 2005, 4650 – 4651; h) Y. L. Boersma, T.
Pijning, M. S. Bosma, A. M. van der Sloot, L. F. Godinho, M. J.
Drꢀge, R. T. Winter, G. van Pouderoyen, B. W. Dijkstra, W. J.
Quax, Chem. Biol. 2008, 15, 782 – 789; i) M. T. Reetz, M. Bocola,
L. W. Wang, J. Sanchis, A. Cronin, M. Arand, J. Zou, A.
Archelas, A. L. Bottalla, A. Naworyta, S. L. Mowbray, J. Am.
Chem. Soc. 2009, 131, 7334 – 7343.
[10] Y. Nagata, K. Miyauchi, J. Damborsky, K. Manova, A. Ansor-
gova, M. Takagi, Appl. Environ. Microbiol. 1997, 63, 3707 – 3710.
[11] Y. Sato, M. Monincova, R. Chaloupkova, Z. Prokop, Y. Ohtsubo,
K. Minamisawa, M. Tsuda, J. Damborsky, Y. Nagata, Appl.
Environ. Microbiol. 2005, 71, 4372 – 4379.
[12] C. S. Chen, Y. Fujimoto, G. Girdaukas, C. J. Sih, J. Am. Chem.
Soc. 1982, 104, 7294 – 7299.
[13] Z. Prokop, J. Damborsky, D. B. Janssen, Y. Nagata, US7,632,666,
2009.
[14] A. Jesenska, M. Pavlova, M. Strouhal, R. Chaloupkova, I.
Tesinska, M. Monincova, Z. Prokop, M. Bartos, I. Pavlik, I.
Rychlik, P. Mobius, Y. Nagata, J. Damborsky, Appl. Environ.
Microbiol. 2005, 71, 6736 – 6745.
[15] A. Jesenska, J. Sykora, A. Olzynska, J. Brezovsky, Z. Zdrahal, J.
Damborsky, M. Hof, J. Am. Chem. Soc. 2009, 131, 494 – 501.
[16] a) V. T. Pham, R. S. Phillips, J. Am. Chem. Soc. 1990, 112, 3629 –
3632; b) R. S. Phillips, J. Mol. Catal. B 2002, 19–20, 103 – 107;
c) A. O. Magnusson, M. Takwa, A. Hamberg, K. Hult, Angew.
Chem. 2005, 117, 4658 – 4661; Angew. Chem. Int. Ed. 2005, 44,
4582 – 4585.
[3] M. T. Reetz, Curr. Opin. Chem. Biol. 2002, 6, 145 – 150.
[4] H. Iding, P. Siegert, K. Mesch, M. Pohl, Biochim. Biophys. Acta
Protein Struct. Mol. Enzymol. 1998, 1385, 307 – 322.
[5] J. Damborsky, E. Rorije, A. Jesenska, Y. Nagata, G. Klopman,
W. J. G. M. Peijnenburg, Environ. Toxicol. Chem. 2001, 20,
2681 – 2689.
[6] a) J. Damborsky, M. Kuty, M. Nemec, J. Koca, J. Chem. Inf.
Comput. Sci. 1997, 37, 562 – 568; b) F. C. Lightstone, Y. Zheng,
T. C. Bruice, J. Am. Chem. Soc. 1998, 120, 5611 – 5621; c) M.
[17] S. Hur, K. Kahn, T. C. Bruice, Proc. Natl. Acad. Sci. USA 2003,
100, 2215 – 2219.
´
Otyepka, J. Damborsky, Protein Sci. 2002, 11, 1206 – 1217; d) A.
[18] a) G. Gartler, C. Kratky, K. Gruber, J. Biotechnol. 2007, 129, 87 –
97; b) R. M. de Jong, J. J. Tiesinga, A. Villa, L. Tang, D. B.
Janssen, B. W. Dijkstra, J. Am. Chem. Soc. 2005, 127, 13338 –
13343; c) I. Ivanov, S. Romanov, C. Ozdoba, H. G. Holzhutter,
G. Myagkova, H. Kuhn, Biochemistry 2004, 43, 15720 – 15728;
d) S. V. Singh, V. Varma, P. Zimniak, S. K. Srivastava, S. W.
Marynowski, D. Desai, S. Amin, X. Ji, Biochemistry 2004, 43,
9708 – 9715; e) H. J. Yang, T. Matsui, S. Ozaki, S. Kato, T. Ueno,
G. N. Phillips, Jr., S. Fukuzumi, Y. Watanabe, Biochemistry 2003,
42, 10174 – 10181; f) I. G. Muꢂoz, S. L. Mowbray, J. Stahlberg,
Acta Crystallogr. Sect. D 2003, 59, 637 – 643; g) J. Stꢄhlberg, H.
Henriksson, C. Divne, R. Isaksson, G. Pettersson, G. Johansson,
T. A. Jones, J. Mol. Biol. 2001, 305, 79 – 93; h) M. Sundaramoor-
thy, J. Terner, T. L. Poulos, Chem. Biol. 1998, 5, 461 – 473; i) D. A.
Lang, M. L. Mannesse, G. H. de Haas, H. M. Verheij, B. W.
Dijkstra, Eur. J. Biochem. 1998, 254, 333 – 340; j) M. Holmquist,
F. Haeffner, T. Norin, K. Hult, Protein Sci. 1996, 5, 83 – 88.
Shurki, M. Strajbl, J. Villꢁ, A. Warshel, J. Am. Chem. Soc. 2002,
124, 4097 – 4107; e) K. Nam, X. Prat-Resina, M. Garcia-Viloca,
L. S. Devi-Kesavan, J. Gao, J. Am. Chem. Soc. 2004, 126, 1369 –
1376; f) A. Soriano, E. Silla, I. Tuꢂꢃn, M. F. Ruiz-Lꢃpez, J. Am.
Chem. Soc. 2005, 127, 1946 – 1957; g) A. Negri, E. Marco, J.
Damborsky, F. Gago, J. Mol. Graphics Modell. 2007, 26, 643 –
651; h) M. Klvana, M. Pavlova, T. Koudelakova, R. Chaloup-
kova, P. Dvorak, Z. Prokop, A. Stsiapanava, M. Kuty, I. Kuta-
Smatanova, J. Dohnalek, P. Kulhanek, R. C. Wade, J. Dambor-
sky, J. Mol. Biol. 2009, 392, 1339 – 1356.
[7] R. J. Pieters, J. H. L. Spelberg, R. M. Kellogg, D. B. Janssen,
Tetrahedron Lett. 2001, 42, 469 – 471.
[8] S. Keuning, D. B. Janssen, B. Witholt, J. Bacteriol. 1985, 163,
635 – 639.
[9] A. N. Kulakova, M. J. Larkin, L. A. Kulakov, Microbiology 1997,
143, 109 – 115.
Angew. Chem. Int. Ed. 2010, 49, 6111 –6115
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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