Uncovering a latent ligation site for glycopeptide synthesis

Article abstract of DOI:10.1002/anie.200801097

Serine in site: A native chemical ligation concept at the serine site is described (see picture). This method was achieved by the CNBr conversion reaction from methylcysteine, which could be obtained by selective S methylation. This new method allowed a c

Full text of DOI:10.1002/anie.200801097

Communications
DOI: 10.1002/anie.200801097
Glycopeptide Ligation
Uncovering a Latent Ligation Site for Glycopeptide Synthesis**
Ryo Okamoto and Yasuhiro Kajihara*
Glycosylation, one of the most important posttranslational
modifications, plays an important role in a variety of
biological events.[1] Oligosaccharides on glycoproteins exhibit
structural heterogeneity, which makes it difficult to elucidate
the relationship between the oligosaccharide structure and
the function of the glycoprotein.
and attempted the conversion of a cysteine residue into a
serine residue after NCL. For such a technique, it was
necessary to explore concise reaction sequences. As a result,
we found the possibility of using a CNBr cleavage method at a
methylcysteine site, which could be obtained by specific
methylation of cysteine.[7] Herein, we report a new chemical
ligation approach at serine sites, which relies on the con-
version of a cysteine residue into a serine residue after NCL.
The strategy is shown in Scheme 1. After NCL (pro-
duct A), the conversion of cysteine into serine was performed
by the following reactions: S methylation of cysteine with
methyl 4-nitrobenzenesulfonate (product B) and intramolec-
ular rearrangement by activation with CNBr in 80%
HCOOH solution followed by an O- to N-acyl shift.
Activation of the S-methyl group by CNBr results in intra-
molecular attack by the neighboring carbonyl oxygen atom on
the b-carbon atom of the methylcysteine residue and gen-
erates an O-ester peptide intermediate (product C). This
intermediate can be converted into the desired peptide
(product D) through the O- to N-acyl shift under slightly
basic conditions (pH 7–8).
In order to examine this strategy, we first demonstrated
the utility of the reaction by means of a model tetrapeptide
with a cysteine residue (Table 1, entry 1). As shown in entry 1
in Table 1, in the case of tetrapeptide Ac-ACGL-OH, we
could achieve the conversion of cysteine into serine in
moderate yield. To confirm the optical purity of the peptide
thus prepared, we compared it with authentic peptide samples
containing d-amino acids, such as Ac-DASGL-OH, Ac-
ADSGL-OH, Ac-DADSGL-OH, and Ac-ASGL-OH, by
HPLC and NMR analysis (Figure 1 and the Supporting
Information). These results showed that product 2 is identical
to Ac-ASGL-OH and the conversion method did not cause
any epimerization in the peptide. We also examined this
method by using octa- and undecapeptides (Table 1, entries 2
and 3), and each of the conversion reactions was found to
afford the desired peptides in moderate yield.
It is known that CNBr has also been used for cleavage at
the methionine site in proteins. In order to distinguish
methionine from methylcysteine residues, we introduced
methionine in the sulfoxide form. Due to the fact that the
sulfoxide form of methionine is inactive for the CNBr
reaction,[8] we expected that an oxidation/reduction proto-
col[9] would enable us to use this new approach for the
synthesis of peptides with methionine residues. We examined
the strategy by using pentapeptide 7, which contained
cysteine and the sulfoxide form of methionine. S methylation
of this pentapeptide afforded 9. Conversion of S-methylcys-
teine to a serine residue and subsequent reduction of the
sulfoxide group by NH4I, SMe2, and trifluoroacetic acid
(TFA)[9] were performed as a one-pot reaction and afforded
the desired pentapeptide 8 in good yield (73% yield of
Chemical synthesis is one of the powerful approaches for
obtaining homogeneous glycoproteins.[2] We have already
reported the synthesis of a glycoprotein with a homogeneous
N-linked complex-type oligosaccharide.[3] This synthesis
employed native chemical ligation (NCL) to perform pep-
tide-segment coupling. NCL relies on the thiol-exchange
reaction between a peptide with an a-thioester group at the
C terminus and another peptide with a cysteine residue at the
N terminus and on the subsequent intramolecular acyl trans-
fer.[4] However, occasionally, the cysteine residue is not
properly located or does not exist in the target protein. To
take this potential difficulty into consideration, a long
glycopeptide sequence that is 30–50 amino acids from one
cysteine site to another cysteine site occasionally needs to be
synthesized for glycoprotein synthesis by the NCL method.
The synthesis of such a glycopeptide with an N-linked
glycopeptide is not easy to perform[2,5] and requires an
appropriate amount of N-linked complex-type oligosacchar-
ides; therefore, there is greater difficulty in glycoprotein
synthesis than in simple protein synthesis.
To examine NCL without a cysteine residue in a long
target peptide, reduction methods changing the sulfhydryl
group of cysteine to a hydrogen atom after NCL and utilizing
an auxiliary group have been developed.[6] In the latter
method, the amino acid sequence at the ligation site is limited
for performance. For the development of a widely usable
method in glycopeptide synthesis, we have also explored
suitable NCL approaches; this endeavor enabled us to find a
new ligation position at the serine site in the consensus
sequence NXS (X: any amino acid except for proline), by
which an asparagine residue is generally incorporated in an N-
linked oligosaccharide. This sequence is found in glycopro-
teins along with the NXT sequence.[1] In order to use the
serine site for a new NCL, we have examined the new concept
[*] R. Okamoto, Prof. Dr. Y. Kajihara
International Graduate School of Arts and Sciences
Yokohama City University
22-2, Seto, Kanazawa-ku, Yokohama, 236-0027 (Japan)
Fax: (+81)45-787-2413
E-mail: kajihara@yokohama-cu.ac.jp
[**] Financial support from the Japan Society for the Promotion of
Science (Grant-in-Aid for Creative Scientific Research
no. 17GS0420) is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801097.
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Scheme 1. Reaction mechanism of the conversion of cysteine into serine. For details see text.
Table 1: Conversion of cysteine into serine in model peptides.[a]
(residues 79–98; Scheme 3). The NCL, between a glycosyl
hexapeptide thioester with a complex-type N-linked asialo-
No. Cys peptide
Cys
CNBr
O- to
oligosaccharide, 12, prepared by a reported method,[10] and a
tetradecapeptide with a cysteine residue, 13, was performed
by the conventional method. In this case, many of the hydroxy
groups of the oligosaccharide were free (see the Supporting
Information). The glycosyl icosapeptide 14 thus obtained was
subjected to S-selective methylation (Figure 2a–c) followed
by activation of the S-methylcysteine residue by CNBr
(Figure 2d). The product was observed with broad peaks by
reversed-phase HPLC (RP-HPLC; Figure 2e). We concluded
that this was because of random formylation of the sugar
hydroxy groups during the CNBr activation reaction under
the formic acid and CNBr conditions. After lyophilization, the
residue was dissolved in 5% hydrazine hydrate solution (or a
slightly basic solution: < pH 10) for 10 min (Figure 2f) in
order for the peptide to undergo the O- to N-acyl shift and to
remove the formyl groups from the sugar hydroxy groups. As
expected, this treatment afforded the target glycosyl icosa-
peptide with the serine residue, 16 (Figure 2g). The structure
and purity of this glycopeptide was confirmed by comparison
with an authentic sample synthesized by SPPS (see the
Supporting Information).
!Ser peptide
methylation conversion N-acyl
[%]
[%]
shift [%]
1
2
3
4
Ac-ACGL-OH (1)
!Ac-ASGL-OH (2)
quant (>90) 75 (62)
quant (70)
VDKAVCGL-OH (3)
!VDKAVSGL-OH (4)
LFRVYCNFLRG-OH (5)
!LFRVYSNFLRG-OH (6)
Ac-GCGM(O)A-OH (7)
! Ac-GSGMA-OH (8)
97 (90)
85 (63)
51 (41)
90 (82)
90 (81)
quant (77)
quant (80)
82 (73)[b]
[a] Yields were estimated from HPLC peak areas, withyields after
isolation given in parenthesis. [b] One-pot reaction.
This finding encouraged us to undertake the synthesis of
an O-linked glycopeptide, the MUC1 repeat segment of the
variable-number tandem-repeat region,[11] a segment in which
there are abundant proline, threonine, and serine residues.
The NCL between two O-linked glycosyl icosapeptides with
mono N-acetylgalactosamine (GalNAc) segments, 17 and 18,
afforded the desired glycosyl tetracontapeptide 19 with a
cysteine residue (Scheme 4 and the Supporting Information).
The cysteine residue that was the ligation position in
glycopeptide 19 was converted into a serine residue through
the approach described above. This conversion afforded the
desired glycopeptide 21 in good yield (43% overall conver-
sion yield estimated by HPLC peak area, 16% yield of
isolated product; Scheme 4 and the Supporting Information).
Figure 1. Analytical HPLC profiles (absorbance at 220 nm) of mixtures
of synthetic tetrapeptide 2 and authentic tetrapeptides with d-amino
acids. The ratios of the injected amounts of synthetic sample and
authentic sample were about 1:2.
isolated product; Scheme 2and data shown in the Supporting
Information).
We then applied the new NCL method for the synthesis of
an N-linked glycopeptide that is a fragment of erythropoietin
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Scheme 2. Conversion of a cysteine into a serine residue in a pentapeptide with a methionine residue. S methylation was performed from 7, which was
synthesized by solid-phase peptide synthesis (SPPS).
Scheme 3. Synthesis of an N-linked glycosyl icosapeptide through ligation at the serine site. a) 6m Guanidine hydrochloride, 0.1m sodium
phosphate buffer (pH 7.1) containing thiophenol (0.5% v/v) and phenylmethanthiol (0.5% v/v), 70% yield after isolation; b) 6m guanidine
hydrochloride, 0.25m tris(hydroxymethyl)aminomethane/HCl (Tris-HCl), 3.3 mm ethylenediaminetetraacetate sodium salt (EDTA-2Na) buffer
(pH 8.6), CH3CN, methyl 4-nitrobenzenesulfonate, 85% conversion yield estimated by HPLC peak area (67% yield after isolation); c) 1. CNBr,
80% HCOOH, 2. 5% hydrazine hydrate solution, 80% conversion yield estimated by HPLC peak area (70% yield after isolation).
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This method was successfully used for the synthesis of both N-
linked and O-linked glycopeptides. Although the synthesis of
large glycopeptides has been troublesome, NCL at a serine
residue between a short glycopeptide thioester segment and a
nonglycosylated long peptide segment could solve this prob-
lem, even if there is no cysteine in the target glycopeptide. We
also demonstrated that this methodology could be used for
peptides containing methionine, histidine, or tryptophan
residues, which are potentially problematic residues during
the CNBr conversion or methylation step. The conversion
yield by this procedure may be dependent on the peptide
sequence and number of amino acids in the peptide backbone.
As far as we have monitored the reactions, we could not find
b-elimination products or peptide–hydrazide derivatives.
Research is underway to utilize this approach for the
threonine site in peptides, in addition to the serine site, and
to synthesize a number of glycoproteins.
Figure 2. HPLC profiles
(absorbance at 220 nm) of the
conversion of a cysteine into a
serine residue in the N-linked
glycosyl icosapeptide 14. S Me-
thylation: a) at the start
(t<1 min), b) after 20 min,
and c) after purification. Con-
version of the S-methylcysteine
into a serine residue: d) at the
start (t<1 min), e) after 38 h,
f) after hydrazine treatment,
and g) after purification.
In order to confirm the
structure of the final glyco-
peptide, we examined the
peptide digestion of com-
pound 21 by actinase E
(Streptomyces
griseus,
Kaken pharma, Tokyo),
and this nonspecific diges-
tion afforded
a glycosyl
octadecapeptide involving
a ligation site (SAPDTR-
PAPGST(GalNAc)APPA-
HG). Comparison of this
peptide fragment with the
authentic glycosyl octade-
capeptide, synthesized by
SPPS, proved that the con-
version reaction success-
fully afforded the desired
serine residue from the cys-
teine residue (see the Sup-
porting Information).
In summary, this report
Scheme 4. Conversion of a cysteine into a serine residue for the synthesis of the MUC1 repeat segment. a) 6m
Guanidine hydrochloride, 0.1m sodium phosphate, 60 mm 4-mercaptophenylacetic acid, 20 mm tris(2-
carboxyethyl)phosphine buffer (pH 7.2); b) methyl 4-nitrobenzenesulfsonate, 6m guanidine hydrochloride,
0.25m Tris-HCl 3.3 mm EDTA-2Na buffer (pH 8.6), CH3CN; c) 1. CNBr, 80% HCOOH, 2. TFA, NH4I, Me2S,
presents
strategy that uses
method of converting cys-
a new ligation
a
teine into serine residues. 3. 5% hydrazine hydrate solution.
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Experimental Section
S Methylation of 14: Compound 14 (1 equiv) was dissolved in the
0.25m Tris-HCl buffer (pH 8.6) containing 6m guanidine hydrochlo-
ride and 3.3 mm EDTA-2Na (peptide concentration was adjusted to
1 mm). Methyl 4-nitrobenzenesulfonate (20 equiv) in acetonitrile
(peptide concentration was adjusted to 3 mm) was added, and the
whole mixture was stirred for 20 min. The mixture was then
neutralized by 10% TFA solution, and the mixture was washed
with Et2O to remove the excess methylation reagent. The solution
was concentrated in vacuo. The residue was purified by RP-HPLC or
chromatography on a short ODS (octadecylsilyl) column to afford the
desired S-methylated peptide 15.
Conversion of S-methylcysteine in 15 to a serine residue by
CNBr: Peptide 15 (1 equiv) was dissolved in 80% HCOOH solution
(peptide concentration was adjusted to 1 mm). CNBr (100 equiv) was
added to the solution, and the mixture was stirred for 38 h in the dark
and under an Ar atmosphere at 378C. The mixture was concentrated
in vacuo or lyophilized. The residue was dissolved in 5% hydrazine
hydrate solution and left for 10 min before the solution was
neutralized with AcOH. The mixture was purified by RP-HPLC to
afford the desired peptide with the serine residue, 16.
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Received: March 6, 2008
Published online: June 11, 2008
Keywords: chemical ligation · glycopeptides · glycoproteins ·
.
oligosaccharides
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