Mugineic acid derivatives as molecular probes for the mechanistic elucidation of iron acquisition in barley

Article abstract of DOI:10.1002/anie.201004853

A light to show the way: A hydroxy group in mugineic acid (MA) that was not needed for Fe^III complexation was used for the introduction of various labeling groups through propargylation and a click reaction. Labeled MA was incorporated as a phy

Full text of DOI:10.1002/anie.201004853

Communications
DOI: 10.1002/anie.201004853
Iron Transport
Mugineic Acid Derivatives as Molecular Probes for the Mechanistic
Elucidation of Iron Acquisition in Barley**
Kosuke Namba,* Kaori Kobayashi, Yoshiko Murata, Hiroko Hirakawa, Tohru Yamagaki,
Takashi Iwashita, Mugio Nishizawa†, Shoichi Kusumoto,* and Keiji Tanino*
Iron, an essential element for plants, plays versatile and
significant roles in a variety of processes, including respira-
tion, photosynthesis, and nitrogen fixation. The element is
also indispensable for animals, for whom the source is usually
uptake from dietary plants.[1] Thus, iron uptake from the soil
by plants is crucial for all living creatures. However, despite
the high levels of iron on the surface of the earth,[2] most
plants have difficulty absorbing iron in alkaline environments
owing to the poor water solubility of its trivalent (Fe3+) salts.[3]
To overcome this problem, graminaceous plants have devel-
oped a specific strategy based on the secretion of phytosider-
Scheme 1. Structures of mugineic acid (1), 2’-deoxymugineic acid (2),
ophores as chelators to solubilize Fe3+ and the uptake of the
and 2’-epi-mugineic acid (3).
resulting iron complexes through selective transporters.[4]
Mugineic acid (MA, 1; Scheme 1) was first identified as a
phytosiderophore in barley,[5,6] and analogues of MA have
since been isolated from various graminaceous species and
cultivars.[7] MA and its analogues all form water-soluble 1:1
complexes with FeIII. In a previous study, we identified a gene
that specifically encodes an FeIII·MA transporter (HvYS1) in
barley;[8] the gene belongs to the YSL family.[9] The local-
ization and substrate specificity of HvYS1 indicate that it is a
specific transporter for the FeIII·MA complex in barley
roots.[8] We further revealed that the sixth outer-membrane
loop determines the FeIII–phytosiderophore specificity of
HvYS1.[10] Therefore, more detailed mechanisms, including
the elucidation of the 3D structural pattern of the transporter,
the recognition mechanism of FeIII complexes, and the fate of
the complexes inside the plants, have become our next focus.
To drive these functional studies forward, we had to
establish efficient preparative methods for MA derivatives to
be utilized as molecular probes. The introduction of function-
alities for labeling of the MA skeleton had so far been
unsuccessful, mainly because all previously prepared labeled
products lost their ability to form FeIII complexes as a result of
the structural modifications. We have now established an
efficient short-step synthesis of MA (1) and 2’-deoxymugineic
acid (DMA, 2),[11] which is a phytosiderophore for rice, wheat,
and maize[12] with a similar iron(III)-transport function.
Comparison of the activity of synthetic DMA (2) with the
activities of MA (1) and its diastereomer 2’-epi-mugineic acid
(2’-epi-MA, 3), which was synthesized in a similar manner,
clearly showed that these three phytosiderophores exhibit the
same level of iron-transport ability.[11] This result provided the
clue that the 2’-hydroxy group could be suitable for the
labeling of mugineic acid analogues for their functional study.
Thus, we introduced various labeling groups at the 2’-hydroxy
group of MA (1) and investigated the iron-transport activities
of the resulting probes.
[*] Dr. K. Namba, K. Kobayashi, Prof. Dr. K. Tanino
Division of Chemistry, Hokkaido University
Kita-ku, Sapporo 060-0810 (Japan)
Fax: (+81)11-706-4920
E-mail: namba@mail.sci.hokudai.ac.jp
ktanino@sci.hokudai.ac.jp
Homepage: http://barato.sci.hokudai.ac.jp/~oc2/
Dr. Y. Murata, Dr. T. Yamagaki, Dr. T. Iwashita, Prof. Dr. S. Kusumoto
Suntory Institute for Bioorganic Research
1-1-1 Wakayamadai, Shimamoto, Mishima
Osaka, 618-8503 (Japan)
E-mail: skus@sunbor.or.jp
H. Hirakawa, Prof. Dr. M. Nishizawa
Faculty of Pharmaceutical Science
Tokushima Bunri University (Japan)
Because of the multifunctional polar structures of unpro-
tected MA (1) or 2’-epi-MA (3), the preparation of labeled
probes by the selective introduction of any substituent at the
2’-hydroxy group is by no means advantageous, even though 1
and 3 can be readily prepared.[11,13] We therefore attempted to
synthesize protected MAs with a free 2’-hydroxy group as a
labeling precursor. We began the synthesis with
[†] Deceased May 1, 2010.
[**] This research was partially supported by Grants-in-Aid for Scientific
Research (Grant Nos. 21310148, 18710191, and 18510200) from the
Ministry of Education, Culture, Sports, Science and Technology
(Japan). We acknowledge Suntory Holdings Limited for their
financial support. K.N. is grateful to the Akiyama Foundation, the
Kaneko Narita Foundation, and the Naito Foundation for support
through a Research Fund for Young Scientists.
Cbz-protected 2-hydroxy-l-allylglycine tert-butyl ester
4
(Scheme 2).[11] A 4:1 diastereomeric mixture (in favor of the
diastereomer with the nonnatural a configuration of the
hydroxy group) was used as obtained by allylic oxidation of
Cbz-protected l-allylglycine tert-butyl ester. We knew that
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004853.
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Chemie
and propargyl bromide, proceeded faster than lactonization,
and the desired allyl- and propargyl-substituted products 11
and 12 were obtained in 64 and 74% yield, respectively. With
the first 2’-substituted MA analogues in hand, simple
2’-O-allylmugineic acid was used to confirm that the sub-
stituent on the 2’-hydroxy group does not inhibit complex-
ation with FeIII. Thus, acidic deprotection of 11 followed by
treatment with FeCl3·6H2O in H2O gave a yellow solution,
which suggested the formation of the FeIII complex 13. The
formation of a 1:1 complex 13 was confirmed unambiguously
by negative ESI Fourier transform ion cyclotron resonance
mass spectrometry (FTICRMS): peaks at the expected mass
numbers and the characteristic isotopic pattern of an FeIII
complex were observed in the mass spectra (see the Support-
ing Information).
The formation of complex 13 encouraged us to introduce
labeling groups by using the allyl group in 11 as a tether.
However, various reactions of 11, such as metathesis and the
Heck reaction, failed to incorporate the desired functional
groups, and only the starting compound 11 was recovered. As
MA is a strong metal chelator, we assume that 11 forms
complexes with various transition metals used as catalysts and
inhibits the desired reactions. Alternative transformations of
the allyl group, such as hydroboration, dihydroxylation, and
ozonolysis, were also prevented by other functional groups,
and the desired products were not obtained. We therefore
attempted to introduce the desired labeling groups by the use
of the propargyl group in 12 as a tether.
Scheme 2. Synthesis of the protected mugineic acid 8. Cbz=carboben-
zyloxy.
the configuration of the carbon atom bearing the hydroxy
group does not affect the iron-transport activity of the
product. After the ozonolysis of 4 in methanol at À788C,
the reaction mixture was treated directly with NaBH3CN and
l-azetidine-2-carboxylic acid tert-butyl ester acetic acid salt
(5) and warmed to 08C for 18 h to give 6 in 75% yield.
Hydrogenolytic deprotection of 6 and subsequent direct
reductive amination with aldehyde 7 after removal of the
palladium catalyst resulted in 8. Thus, the desired protected
MA derivative 8 was obtained in good yield in just three steps
from 4.
We next examined the introduction of various substituents
at the 2’-hydroxy group. Acylation of the hydroxy group gave
a mixture of O-acylated and N-acylated products. The
N-acylated product was generated by migration of the acyl
group to the neighboring secondary nitrogen atom. Alkyla-
tion of the 2’-hydroxy group was also attempted.[14] The
treatment of 8 with sodium hydride to generate an alkoxide
anion afforded carboxylic acid 10 through the unexpected
formation of lactone 9 and subsequent hydrolysis (Scheme 3).
This hydrolytic pathway was a serious problem: even the
sterically bulky tert-butyl ester did not block lactonization by
the alkoxide anion. Thus, the alkylation was conducted by
treatment with sodium hydride in the presence of an excess
amount of various halides. With almost all alkyl and allyl
halides, only carboxylic acid 10 was formed; however,
alkylation with the small and reactive reagents, allyl bromide
Since the C2’ diastereomers were separable at this stage,
the major isomer of 12 with the nonnatural a configuration
was used for further transformations after separation by
column chromatography.
A click reaction of 12 with
p-bromobenzyl azide in the presence of CuI (1 mol%)
afforded 14a in quantitative yield (Scheme 4). This result
was a clear contrast to our previous unsuccessful attempts, in
which the mugineic acid derivatives inhibited many transi-
tion-metal-catalyzed reactions. Although the mechanistic
details are not yet understood, we found that a 1:1 complex
of 14a with copper(I) is an excellent catalyst for the click
reaction.[15] Finally, the deprotection of 14a gave the
2’-p-bromobenzyltriazole mugineic acid derivative 15a in
quantitative yield. We also prepared the benzophenone
derivative 15b as a photoaffinity probe and the coumarin
derivative 15c as a fluorescent probe in a similar manner. The
complexation of 15a–15c with FeIII was confirmed as
described for 13 (see the Supporting Information). With the
aid of the efficient click reaction, further labeled derivatives
were also synthesized readily: the benzyl derivative 15d was
synthesized as an HPLC probe and the acridine derivative
15e as another fluorescent probe.
We next examined the iron-transport activity of the
2’-O-modified derivatives by measuring the currents associ-
ated with electrophysiological transport in Xenopus laevis
oocytes by modification of a previously reported protocol (see
the Supporting Information).[8,16] Oocytes injected with
HvYS1 cRNA exhibited iron-transport ability, whereas
those injected with water (negative control) hardly responded
to the iron complex 13 or iron complexes of the functionally
labeled MA derivatives 15a–15c (Figure 1). The difference in
Scheme 3. Synthesis of 2’-O-substituted mugineic acid analogues and
the FeIII complex 13. TFA=trifluoroacetic acid.
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Communications
Figure 2. Observation of fluorescence-labeled MA as its FeIII complex
in Xenopus oocytes. Oocytes injected with a,b) water as a control or
c–f) HvYS1 cRNA were incubated in ND96 buffer containing 15c–FeIII
(50 mm) for 15 min at 168C. Uptake of fluorescence-labeled MA–FeIII
was monitored by using a laser scanning confocal imaging system
(Olympus Fluoview 1000) in a differential interference contrast (DIC)
mode (a, c, and e) or in a laser fluorescence mode (b, d, and f). Scale
bar: 200 mm (a–d); 50 mm (e and f).
through the HvYS1 transporter. To our knowledge, this is the
first direct experimental evidence of the transporter-mediated
internalization of mugineic acid into cells.
Scheme 4. Synthesis of labeled mugineic acid derivatives 15.
In this study, an efficient and versatile method was
established for the preparation of labeled mugineic acid
(MA) derivatives, which form water-soluble FeIII complexes
similar to that of natural MA. The complexes were incorpo-
rated into cells through the HvYS1 transporter, as observed
directly by fluorescence microscopy. We are now investigating
the conversion of synthetic 2’-O-allylmugineic acid into a
tritium-labeled MA derivative through hydrogenation in
tritium gas. This compound and the functionally labeled
MA derivatives described herein will enable further, more
detailed investigation of the molecular mechanism of iron
acquisition from roots and the fate of the FeIII complexes of
MA and its analogues inside plants.
Figure 1. Iron(III)-transport activities of synthetic labeled mugineic
acid derivatives as measured by two-electrode voltage-clamp analysis
in Xenopus oocytes. Currents induced by FeIII complexes of the MA
derivatives 15a, 15b, and 15c and the FeIII complex 13 (each 50 mm) in
oocytes injected with HvYS1 cRNA (black bars, n=5–11) or water
(white bars, n=3–10). Significant differences between injection with
HvYS1 cRNA and water, as determined by the Tukey test, are indicated
with an asterisk (*, p<0.01).
Received: August 4, 2010
Revised: October 4, 2010
Published online: November 24, 2010
the current intensities between the oocytes injected with
HvYS1 cRNA and those injected with water was remarkable
for all FeIII complexes of the 2’-O-modified MA derivatives
tested. We therefore conclude that the HvYS1 transporter
responded to all of these complexes. Thus, the 2’-OH group
appeared to be an appropriate site for the labeling of MAwith
various groups without the loss of its functions as a
phytosiderophore.[17]
We expected to be able to detect the incorporation of the
functionally active fluorescence-labeled MA derivative 15c
into Xenopus oocytes through the HvYS1 transporter by
microscopic observation (see the Supporting Information).
The inside of HvYS1-expressed oocyte cells was clearly
visualized in the fluorescence mode (Figure 2d,f) with a
confocal microscope, whereas the inside of the control
oocytes was not (Figure 2b). This experiment indicated that
the FeIII complex of the MA analogue 15c was incorporated
Keywords: cellular uptake · click chemistry · fluorescent probes ·
iron transport · phytosiderophores
.
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