Chemical synthesis and biological activity of analogues of the lantibiotic epilancin 15X

Article abstract of DOI:10.1021/ja302435y

Lantibiotics are a large family of antibacterial peptide natural products containing multiple post-translational modifications, including the thioether structures lanthionine and methyllanthionine. Efforts to probe structure-activity relationships and engineer improved pharmacological properties have driven the development of new methods to produce non-natural analogues of these compounds. In this study, solid-supported chemical synthesis was used to produce analogues of the potent lantibiotic epilancin 15X, in order to assess the importance of several N-terminal post-translational modifications for biological activity. Surprisingly, substitution of these moieties, including the unusual N-terminal d-lactyl moiety, resulted in relatively small changes in the antimicrobial activity and pore-forming ability of the peptides.

Full text of DOI:10.1021/ja302435y

Communication
pubs.acs.org/JACS
Chemical Synthesis and Biological Activity of Analogues of the
Lantibiotic Epilancin 15X
Patrick J. Knerr and Wilfred A. van der Donk*
Howard Hughes Medical Institute and Roger Adams Laboratory, Department of Chemistry, University of Illinois at
Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States
S
* Supporting Information
ABSTRACT: Lantibiotics are a large family of antibacte-
rial peptide natural products containing multiple post-
translational modifications, including the thioether struc-
tures lanthionine and methyllanthionine. Efforts to probe
structure−activity relationships and engineer improved
pharmacological properties have driven the development
of new methods to produce non-natural analogues of these
compounds. In this study, solid-supported chemical
synthesis was used to produce analogues of the potent
lantibiotic epilancin 15X, in order to assess the importance
of several N-terminal post-translational modifications for
biological activity. Surprisingly, substitution of these
moieties, including the unusual N-terminal D-lactyl moiety,
resulted in relatively small changes in the antimicrobial
activity and pore-forming ability of the peptides.
antipeptides are a diverse and growing class of ribosomally
Lsynthesized, polycyclic natural products, classified by the
presence of the thioether-containing residues meso-lanthionine
(Lan) and (2S,3S,6R)-3-methyllanthionine (MeLan) (Figure
Figure 1. Sequences and ring topologies of nisin and epilancin 15X
1).1 These moieties are installed post-translationally into a
linear precursor peptide via enzymatic dehydration of serine
(1) and chemical structures of their post-translational modifications.
and threonine to the α,β-unsaturated residues 2,3-didehydroa-
Epilancin 15X (1, Figure 1), a cationic lantibiotic first
lanine (Dha) and (Z)-2,3-didehydrobutyrine (Dhb), respec-
isolated from the clinical strain Staphylococcus epidermidis
tively, followed by intramolecular Michael-type addition of
15X154, currently ranks among the most active lantibiotics
cysteine thiols to yield the thioether cross-links. Lantipeptides
against several clinically relevant bacterial pathogens, including
that have antimicrobial activities are called lantibiotics, and
methicillin-resistant Staphylococcus aureus and vancomycin-
resistant Enterococcus.12 Despite this potent activity, the mode
many of these exert their activity via binding to lipid II to
inhibit peptidoglycan biosynthesis and/or forming pores in the
of action of this compound is currently unknown. While its C-
cell membrane.2 As members of this family display substantial
terminal ring system resembles the pore-forming portion of the
activity against pathogenic bacteria,3 several lantibiotics are
lantibiotic nisin (Figure 1),2,6,13 1 lacks nisin’s A- and B-rings
currently in development for the treatment of bacterial
that target lipid II;14 indeed, 115 and the structurally related
infections.4 Fueled by this therapeutic potential, in vitro and
lantibiotic epilancin K716 do not bind to lipid II. The
in vivo engineering efforts have sought to improve the
combination of potent activity and lack of lipid II binding
pharmacological properties of these compounds using their
presents the intriguing possibility that 1 and related lantibiotics
natural biosynthetic machinery;5,6 such approaches include
may bind to a different biological target.16,17 The N-terminus of
recent methods to incorporate non-proteinogenic amino acids.7
1 is adorned with an unusual D-lactyl (DLac) cap, which is
installed by the oxidoreductase ElxO via stereospecific
As a complementary strategy to bioengineering, chemical
synthesis presents the opportunity to widen further the
reduction of a pyruvyl (Pyr) moiety generated from hydrolysis
chemical space accessible for the production of lantibiotics
of an N-terminal Dha.18 Additional post-translational mod-
ifications beyond Lan/MeLan are often critical for the
and other cyclic peptides,8 including the design of non-natural
cross-links.9 The successful construction of lactocin S10 and
both peptides of lacticin 314711 by solid-phase peptide
synthesis (SPPS) highlight the growing applicability of chemical
approaches to produce complex lantibiotics and analogues.
Received: March 12, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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Communication
biological activity of lantibiotics,19 but the importance of the
DLac group has not been assessed in this regard. Previous
attempts to reconstitute the biosynthesis of 1 in vitro or in a
heterologous host were unsuccessful,18 and no study has yet
demonstrated biosynthetic manipulation in the producing
organism. Hence, at present no analogues of this lantibiotic
have been reported. We therefore employed SPPS to construct
analogues of 1 to probe the importance of its N-terminal cluster
of post-translational modifications for bioactivity.
We envisioned the solid-supported total synthesis to utilize
orthogonally protected Lan/MeLan building blocks,10,11,20−22
which allow selective installation of each cross-link into the
growing peptide backbone via amide-forming cyclization
between the free carboxylate of the Lan/MeLan residue and
the N-terminus of the peptide. As 1 contains one Lan ring and
a pair of overlapping MeLan rings, three such building blocks
were necessary. Allyl-protected Lan building block 2 (Scheme
1) was accessed by the procedure of Pattabiraman et al.20
to investigate the importance of the unusual N-terminal
segment of 1 for antimicrobial activity. 9-Fluorenylmethox-
ycarbonyl (Fmoc)-based SPPS was commenced using
preloaded Wang resin (15, Scheme 2) with a substitution of
0.1 mmol/g to prevent intermolecular side reactions during
cyclization.20 N,N′-Diisopropylcarbodiimide (DIC) and 1-
hydroxybenzotriazole (HOBt) or 1-hydroxy-7-azabenzotriazole
(HOAt) were used for coupling reactions to minimize side
products observed with other activating agents. Fragment 13
(Figure 2), representing Phe27-Dhb28 of 1, and building blocks
2, 3, and 4 were incorporated successfully using this chemistry.
The thioether rings were installed by selective deprotection of
the appropriate Lan/MeLan residue followed by cyclization,
which was complete in all cases within 4 h. Nitrobenzyl-based
protecting groups were removed from the resin-bound
intermediate 16 (Scheme 2) by treatment with 6 M stannous
chloride and 5 mM HCl; allyl-based protecting groups were not
affected by these conditions. Subsequent cyclization promoted
by (7-azabenzotriazol-1-yloxy)tripyrrolidinophosphonium hex-
afluorophosphate (PyAOP), HOAt, and 2,4,6-collidine yielded
ring C (17). During the installation of ring B (18) and later
ring A (20), allyl-based protecting groups were removed with
Pd(PPh3)4 and phenylsilane. After successful construction of
ring A, the desired absolute stereochemistry of the thioether
rings was confirmed by peptide hydrolysis, derivatization of the
resulting amino acids, and analysis by gas chromatography−
mass spectrometry (GC-MS) with a chiral stationary phase (see
Supporting Information).
Scheme 1. Structure of Lan Building Block 2 and Synthesis
a
of MeLan Building Blocks 3 and 4
Following synthesis of resin-bound intermediate 20,
analogues of 1 were prepared in a divergent fashion. In the
first analogue (21, Scheme 2), the importance of the three N-
terminal α,β-unsaturated residues (Dha3, Dhb7, Dhb8) was
probed by replacement with their reduced counterparts L-
alanine and L-2-aminobutyric acid (Abu), respectively,
(Dha3Ala/Dhb7Abu/Dhb8Abu). To probe the importance of
the last step in epilancin biosynthesis, the reduction of an N-
terminal pyruvyl moiety to DLac,18 a second analogue (22) was
prepared that also contained Pyr in place of DLac. To construct
this analogue, fragment 14 (Figure 2) was coupled to the resin-
bound 29-mer; the Boc-Dha moiety spontaneously hydrolyzed
to the desired ketone upon tert-butyloxycarbonyl (Boc)
removal. Based on the biological activities of these peptides
(see below), a third analogue (23, Scheme 2) was prepared in
which the eight N-terminal residues were truncated and an
acetyl cap was added. All peptides were cleaved from the resin
and globally deprotected with trifluoroacetic acid using
triisopropylsilane (for 21 and 23) or anisole (for 22) as the
primary scavenging agent, as triisopropylsilane is known to
(partially) reduce peptide ketones.23 After reversed-phase
HPLC purification, these peptides were isolated in overall
yields of 1.6−1.9%, with an average yield of 92−93% per step.
Liquid culture antimicrobial assays using a standard two-fold
dilution protocol were performed with authentic 1 and 21−23
against the indicator strain Staphylococcus carnosus TM300 to
assess quantitatively the effect of each set of alterations on
antimicrobial activity. Interestingly, both full-length analogues
21 and 22 largely maintained the nanomolar activity of parent
peptide 1 (Figure 3). Substitution of the three N-terminal α,β-
unsaturated residues of 1 with the corresponding L-amino acids
in 21 gave only a modest increase in the IC50 value from 95
9.6 to 270 23 nM (minimum inhibitory concentration (MIC)
values of 250 and 625 nM, respectively). This increase may
potentially be due to loss of the planarity of these unsaturated
a
Reagents and conditions: (a) tert-butyl 2,2,2-trichloroacetimidate,
EtOAc, cyclohexane, 93%; (b) PPh3, CBr4, CH2Cl2, 79%; (c)
AlocOSu, iPr2NEt, CH2Cl2, 97%; (d) Hg(OAc)2, PhOMe,
CF3CO2H, then dithiothreitol; (e) 6, NaHCO3, Bu4NBr, Bu3P,
EtOAc, H2O, 64% (two steps); (f) CF3CO2H, PhSiH3, CH2Cl2, 96%;
i
(g) pNzCl, Pr2NEt, CH2Cl2, 96%; (h) Pd(PPh3)4, PhNHMe, THF;
(i) pNbBr, NaHCO3, DMF, 93% (two steps); (j) 6, NaHCO3,
Bu4NBr, Bu3P, EtOAc, H2O, 85% (two steps); (k) CF3CO2H, PhSiH3,
CH2Cl2, 95%. Aloc, allyloxycarbonyl; pNz, p-nitrobenzyloxycarbonyl;
pNb, p-nitrobenzyl.
However, attempting to apply a similar scheme to MeLan
building blocks 3 and 4, featuring allyl and p-nitrobenzyl
protection, respectively, resulted in low yields and partial loss of
stereochemical integrity. Therefore, an alternative scheme was
developed using protected β-methyl-D-cysteine 8, which was
prepared from D-threonine (7) in five steps (Scheme 1).21 We
envisioned that 8 might be utilized as a common intermediate
in the syntheses of both 3 and 4, thereby streamlining the
overall approach. Following protecting group manipulation of
8, the corresponding p-methoxybenzyl (Mob)-protected thiols
9 and 11 were deprotected and condensed with L-bromoalanine
6 under phase-transfer conditions, resulting in 10 and 12 with
full preservation of stereochemistry. tert-Butyl ester removal
yielded MeLan building blocks 3 and 4, which were used
subsequently for SPPS.
With these building blocks in hand, syntheses of epilancin
15X analogues were performed. Given the structural similarities
between nisin and 1 in the C-terminal ring system, we decided
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Communication
a
Scheme 2. Synthesis of Epilancin Analogues 21, 22, and 23
a
Reagents and conditions: (a) SnCl2, HCl, DMF; (b) piperidine, DMF; (c) PyAOP, HOAt, 2,4,6-collidine, DMF; (d) Fmoc-Leu-OH, DIC, HOBt,
DMF; (e) Pd(PPh3)4, PhSiH3, DMF, CH2Cl2. Prior to cleavage from resin, all residues contained appropriate side-chain protecting groups for Fmoc-
SPPS: tert-butyloxycarbonyl (Boc) for Lys, 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl (Pbf) for Arg, trityl (Trt) for His. Green residues are
those modified from the natural product.
To test the latter possibility, we used flow cytometry to
examine the ability of analogues 21 and 23 to alter the
membrane potential and permeability of S. carnosus cells.
Despite removal of the N-terminal segment, 23 still depolarized
Figure 2. Structures of Dha/Dhb-containing fragments prepared by
solution-phase chemistry (see Supporting Information).
the membrane (Figure 4a) and disrupted membrane integrity
Figure 4. Flow cytometry analysis of the pore-forming abilities of 21
and 23 against S. carnosus TM300. (a) Membrane depolarization
activity measured by DiOC2(3) mean fluorescence intensity (MFI).24
Figure 3. Growth inhibition activity of authentic epilancin 15X (1)
and analogues 21−23 against Staphylococcus carnosus TM300.
(b) Membrane permeability activity measured by propidium iodide
(PI) uptake.25
(Figure 4b). Higher concentrations of 23 were needed than for
full-length analogue 21 to elicit these responses, and these
observations correlated well to the relative differences in
antibiotic activity (Figure 3). These findings are consistent with
the importance of the nisin-like C-terminal portion of 1 for
pore-forming activity and suggest that the N-terminal segment
may be involved in binding to a putative non-lipid II biological
target to increase potency. The pore-forming activity of nisin is
enhanced about 103-fold by binding to lipid II.26 In
comparison, the effect of the N-terminus of epilancin 15X is
more modest, with its MIC values increasing only 2 orders of
magnitude upon removal of the eight N-terminal residues.
The work presented here extends the application of solid-
supported chemical synthesis for the production of lantibiotic
peptides. Analogues of epilancin 15X, a lantibiotic for which
variants have not been made using biosynthetic machinery
reconstituted in vitro or in bacterial hosts, were successfully
produced. Interestingly, analogues 21 and 22 largely main-
tained the potent activity of the natural product. In addition,
structures, but a covalent mechanism of action involving these
residues can be ruled out. Replacement of the DLac moiety of
21 with Pyr in 22 yielded a slight additional increase in IC50 to
354 8 nM (MIC 1.25 μM). This observation suggests that
the N-terminal acyl group plays only a minor role in activity
and more likely serves to improve the stability of the peptide.
While both Pyr and Lac moieties are known to protect
lantibiotics from aminopeptidases,18 enzymatic formation of
DLac from Pyr by ElxO may further stabilize 1 by removing the
ketone, a potential site of nucleophilic attack in vivo. Analogue
23, in which the eight N-terminal residues of 1 were removed,
still possessed activity against S. carnosus (MIC 12.5 μM) but
diminished by nearly 2 orders of magnitude compared to 1.
This N-terminal tail region, which contains primarily nonpolar
residues, may therefore be important for proper binding
interactions between epilancin 15X and its putative biological
target, or alternatively for the molecule to span the membrane
in pore-forming activity.
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ASSOCIATED CONTENT
* Supporting Information
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Experimental procedures, characterization of new molecules,
and GC-MS results. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
vddonk@illinois.edu
■
(15) Oman, T. J.; Lupoli, T. J.; Wang, T.-S. A.; Kahne, D.; Walker, S.;
van der Donk, W. A. J. Am. Chem. Soc. 2011, 133, 17544−17547.
Notes
(16) Brotz, H.; Josten, M.; Wiedemann, I.; Schneider, U.; Gotz, F.;
̈
̈
The authors declare no competing financial interest.
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(17) Hoffmann, A.; Schneider, T.; Pag, U.; Sahl, H.-G. Appl. Environ.
Microbiol. 2004, 70, 3263−3271.
ACKNOWLEDGMENTS
We thank Weixin Tang (University of Illinois) for providing
Lan/MeLan standards for GC-MS analysis and Dr. Juan
■
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epilancin 15X. This work was supported by the National
Institutes of Health (GM58822 to W.A.V.), the Robert C. and
Carolyn J. Springborn Endowment (to P.J.K.), and an
American Heart Association Midwest Affiliate predoctoral
fellowship (11PRE7620039 to P.J.K.).
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