Enzymatic synthesis of novel quercetin sialyllactoside derivatives

Article abstract of DOI:10.1080/14786419.2018.1481842

Quercetin and its derivatives are important flavonols that show diverse biological activity, such as antioxidant, anticarcinogenic, anti-inflammatory, and antiviral activities. Adding different substituents to quercetin may change the biochemical activity

Full text of DOI:10.1080/14786419.2018.1481842

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: http://www.tandfonline.com/loi/gnpl20
Enzymatic synthesis of novel quercetin
sialyllactoside derivatives
Sumangala Darsandhari, Jae Yoon Bae, Biplav Shrestha, Tokutaro
Yamaguchi, Hye Jin Jung, Jang Mi Han, Chan-Su Rha, Ramesh Prasad Pandey
& Jae Kyung Sohng
To cite this article: Sumangala Darsandhari, Jae Yoon Bae, Biplav Shrestha, Tokutaro
Yamaguchi, Hye Jin Jung, Jang Mi Han, Chan-Su Rha, Ramesh Prasad Pandey & Jae Kyung
Sohng (2018): Enzymatic synthesis of novel quercetin sialyllactoside derivatives, Natural Product
Research, DOI: 10.1080/14786419.2018.1481842
To link to this article: https://doi.org/10.1080/14786419.2018.1481842
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Natural Product research, 2018
https://doi.org/10.1080/14786419.2018.1481842
Enzymatic synthesis of novel quercetin sialyllactoside
derivatives
Sumangala Darsandharia#, Jae Yoon Baea#, Biplav Shresthaa, Tokutaro Yamaguchia,
Hye Jin Junga, Jang Mi Hana, Chan-Su Rhac, Ramesh Prasad Pandeya,b and
Jae Kyung Sohnga,b
adepartment of life science and Biochemical engineering, sunMoon university, asan-si, Korea; bdepartment
c
of Bt-convergent Pharmaceutical engineering, sunMoon university, asan-si, Korea; department of Food
science and technology, Kyung hee university, Yongin, Korea
ABSTRACT
ARTICLE HISTORY
received 24 January 2018
accepted 23 May 2018
Quercetin and its derivatives are important flavonols that show diverse
biological activity, such as antioxidant, anticarcinogenic, anti-inflammatory,
and antiviral activities. Adding different substituents to quercetin may
change the biochemical activity and bioavailability of molecules, when
compared to the aglycone. Here, we have synthesised two novel derivatives
KEYWORDS
Quercetin derivatives;
galactosylation; sialylation
of quercetin, quercetin-3-O-β-d-glucopyranosyl, 4′′-O-d-galactopyranosyl
3′′′-O-α-N-acetyl neuraminic acid i.e. 3′-sialyllactosyl quercetin (3′SL-Q) and
quercetin-3-O-β-d-glucopyranosyl, 4′′-O-β-d-galactopyranosyl 6′′′-O-α-N-
acetyl neuraminic acid i.e. 6′-sialyllactosyl quercetin (6′SL-Q) with the use
of glycosyltransferases and sialyltransferases enzymes. These derivatives
of quercetin were characterised by high-resolution quadrupole-time-of-
flight electrospray ionisation mass spectrometry (HR-QTOF-ESI/MS) and
1H and 13C nuclear magnetic resonance (NMR) analyses.
1. Introduction
Quercetin is an important constituent of the flavonoid family and is found in many fruits
and vegetables, as well as red wine, and tea (Erlund 2004). Quercetin has been reported to
exhibit various pharmacological activities such as antioxidative (Murota and Terao 2003),
CONTACT Jae Kyung sohng
sohng@sunmoon.ac.kr; ramesh Prasad Pandey
pandey@sunmoon.ac.kr
#these authors contributed equally.
supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2018.1481842.
© 2018 Informa uK limited, trading as taylor & Francis Group

2
S. DARSANDHARI ET AL.
anti-inflammatory (Kleemann et al. 2011), anticarcinogenic (Yang et al. 2001), antiangiogenic
(Tan et al. 2003), and anti-melanoma (Cao et al. 2014) effects. Different substituents attached
to quercetin change the biochemical activity and bioavailability of molecules when com-
pared to the aglycone (Materska 2008). The conjugation of quercetin with specific sugar
motifs is an effective strategy to improve the biological activity of quercetin in vivo (Makino
et al. 2013). There was 3- to 40-fold enhancement in the anticancer activity of geldanamycin
antibiotic, which was conjugated with galactose and lactose analogues when they were
incubated with β-galactosidase in the cells (Cheng et al. 2005). Sialic acids (SAs) are 9-carbon
monosaccharides directly involved in many important biological and pathological processes
(Chen and Varki 2010). Several naturally occurring SA forms have been found in sialic
acid-containing glycosphingolipids, namely gangliosides (GM1 and GM2). Since the biology
of SA is closely linked to human health, sialic acids have considerable significance in terms
of human health, disease prevention, drug discovery, and immunity modulation (Bagriaçik
and Miller 1999; Varki and Gagneux 2012; Bisel et al. 2014). SAs being exposed to the terminal
glycan chain have become a potential target for glycan remodeling for the synthesis and
design of novel medicines and vaccines (Byrne et al. 2007). It has been reported that querce-
tin 7-O-sialic acid show better anti-oxidation, anti-inflammation, cholesterol efflux promo-
tion, and protection of biomolecules from desialylation than quercetin (Tian et al. 2017).
Recently, two sialylated human milk oligosaccharides 3′-sialyllactose and 6′-sialyllactose
have shown an ability to neutralize different avian influenza viruses (Pandey et al. 2018).
In the current experiment, a regiospecific glycosyltransferases (UGT78K1) from Glycine
max and β-1,4-galactosyltransferase [β(1,4)GalT] were used for the synthesis of quercetin-
3-O-β-d-glucopyranosyl, 4′′-O-β-d-galactoside i.e. quercetin-3-O-β-d-lactoside (QL) from
quercetin. In addition, an α-N-acetyl neuraminic acid group was conjugated at 3′′′ and 6′′′-OH
group of galactose of quercetin-3-O-β-d-lactoside to synthesize quercetin-3-O-β-d-glu-
copyranosyl, 4′′-O-β- -galactopyranosyl 3′′′-O-α-N-acetyl neuraminic acid i.e. 3′-sialyllactosyl
d
quercetin (3′SL-Q) and quercetin-3-O-β-d-glucopyranosyl, 4′′-O-β-d-galactopyranosyl
6′′′-O-α-N-acetyl neuraminic acid i.e. 6′-sialyllactosyl quercetin (6′SL-Q) (Figure 1), respectively
by using two different sialyltransferases. Sialyllactose conjugated flavonoids have not been
reported yet.
2. Results and discussion
2.1. In vitro enzyme-catalysed quercetin-3-O-β-d-glucoside synthesis
Quercetin-3-O-β-d-glucoside (QG) was synthesised from the one-pot reaction as described
in materials and methods. A Glycine max glucosyltransferase UGT78K1 enzyme catalysed
more than 90% conversion of the 8 mM of quercetin to QG, which was purified and used as
a substrate for successive enzymatic reactions, to generate other derivatives. The high per-
formance liquid chromatography photodiode array (HPLC-PDA) of the reaction mixture
showed a prominent new single peak at retention time of 8.6 min, while the peak of quercetin
was observed at 11.9 min (Figure S1A). Since UGT78K1 has been characterised as a regio-
specific 3-O-glycosyltransferase and the retention time of this new peak matched exactly
with that of QG, it was confirmed to be QG product peak and thus was purified for further
reaction. By means of a cofactor recycling system, ~3.34 g/L of QG was obtained. In this
system, uridine monophosphate (UMP) is used to produce uridine triphosphate (UTP) via

NATURAL PRODUCT RESEARCH
3
Figure 1. schematic representation of overall strategy for enzymatic synthesis of quercetin sialoside
derivatives catalyzed by glycosyltransferase (uGt78K1, β(1,4)Galt), and sialyltransferases (2,3-siat and
2,6-siat).
uridine diphosphate (UDP) with the help of enzymes UMP kinase. An adenine 5′-triphosphate
(ATP) is consumed in the process and adenine 5′-diphosphate (ADP) formed is recycled by
acetate kinase with the consumption of an acetyl phosphate. Glucose 1-phosphate and UTP
is used for making UDP-α-d-glucose, which is used by UGT78K1 to attach d-glucose in querce-
tin to make QG. UDP released as a side product in the process is recycled in the system. This
system was developed in our laboratory for synthesising glycosides of curcumin (Gurung et
al. 2017), α-mangostin (Le et al. 2014), nargenicin A1 (Dhakal et al. 2015), and resvera-A (Shin
et al. 2016) in which YjiC enzyme was used. This method uses cheap starting materials and
a desired glycosylated natural product can be efficiently made with the help of regiospecific
and stereospecific glycosyltransferase. The pH of the reaction was maintained whenever a
decrease in pH was observed for the effective running of the reaction.

4
S. DARSANDHARI ET AL.
2.2. In vitro enzyme-catalyzed quercetin-3-O-β-d-lactoside synthesis
Similarly, the β-1,4-galactosyltransferase enzyme from Helicobacter pylori catalysed the reac-
tion mixture of UDP-α- -galactose as a sugar donor, and QG as the acceptor substrate. When
d
analysing the sample of this reaction mixture in HPLC-PDA, a peak was observed earlier than
the substrate peak at retention time of 8.3 min, while the substrate peak was at 8.6 min with
more than 95% conversion [Figure S1B(iii)]. After further analysing this peak by high-reso-
lution quadruple time-of-flight electrospray ionisation-mass spectrometry (HR-QTOF-ESI/
MS) in positive ion mode, the mass spectrum was found to be m/z 627.1568, which resembled
with that of quercetin conjugated with lactose i.e. [QL + H]+, which is m/z 627.1561 (Figure
S2A). 1.19 g/L of QL was formed in the process which was collected, purified, and further
used for sialylation reaction.
2.3. In vitro enzyme-catalysed sialyllactosyl quercetin derivatives synthesis
Two sialyltransferases, α2,3-SiaT and α2,6-SiaT were prepared as described in experimental
section and used to catalyse the independent reaction mixture containing quercetin-3-O-
β-d-lactoside as acceptor substrate and CMP-N-acetylneuraminic acid (CMP-NeuNAc) as a
sugar donor. Two sialosides of quercetin, 3′ and 6′-sialic acid conjugated products (3′SL-Q
and 6′SL-Q) were produced from α2,3-SiaT and α2,6-SiaT, respectively. Each reaction mixture
was first analysed by HPLC-PDA and then confirmed by LC-QTOF-ESI/MS in positive ion mode.
The HPLC-PDA analyses revealed product peaks before the retention time of QL at ~ 7.2 min
for 3′SL-Q, and 7.9 min for 6′SL-Q, at the conversion rate of ~95% for both products (data
not shown) and 4.35 g/L of both 3′SL-Q and 6′SL-Q were produced by the reaction. The
product peaks were collected and purified (Figure S1C). In the positive ion mode spectrum,
mass fragments of 3′SL-Q and 6′SL-Q showed m/z at 918.2540 and 918.2526 corresponding
to [3′SL-Q + H] + and [6′SL-Q + H]+, respectively (Figs. S2B and S2C). Mass spectra of attached
sugar units were clearly observed in positive ion mode (Figs. S2D and S2E). The possible
structures of different mass fragmentation of 3′SL-Q and 6′SL-Q observed are presented in
figures (Figure S2F).
2.4. Compounds purification and structural elucidation
For confirmation and determination of the structure of sialyllactose conjugated quercetin
derivatives, each product was isolated by preparative-HPLC, and analysed by NMR in
DMSO-d6 (Figure S3). The 1H-NMR spectrum of 3′SL-Q showed two anomeric protons with
two doublets at δ 4.05 (J = 7.7 Hz) and 5.38 (J = 6.5 Hz), representing beta (β) configuration
of two sugar moieties (d-glucose and d-galactose), whereas other proton signals for glucose,
galactose, and sialic acid moieties were observed in the region from δ (1.5–5.5) ppm. The
anomeric carbons of
d
-glucose and -galactose were obtained at 101 and 104 ppm, respec-
d
tively. The spectra for other carbons were also present in respective places as depicted in
13C-NMR (Figure S3A). Similar spectra were also observed in both 1H and 13C-NMR analyses
of 6′SL-Q. In 1H-NMR, the anomeric protons of glucose and galactose appeared at δ 4.11 (d,
J = 7.7 Hz) and 5.56 (d, J = 7.5 Hz), respectively. The anomeric spectra appeared exactly at
the same chemical shift in both sialosides. These data confirmed conjugation of d-glucose
and d-galactose in beta configuration. The 1H-NMR spectra for remaining protons were also

NATURAL PRODUCT RESEARCH
5
present in the region of 3.0–5.0 ppm. Similarly, in 13C-NMR analysis, the anomeric carbons
of d-glucose and d-galactose appeared at δ 101 ppm and 104 ppm, respectively. Moreover,
the carbon spectra of both d-glucose and d-galactose were between 60 and 80 ppm (Figure
S3). Because the CMP-NeuNAc (CMP-sialic acid) does not have an anomeric proton, we con-
firmed its conjugation by the presence of specific groups on them. The sialic acid contains
NHCOCH3 moiety and CH2 at C-3″′′, which were distinct at δ (1.87 and 1.70) ppm and δ (1.88
and 1.67) ppm in 3′SL-Q and 6′SL-Q, respectively. Similarly, while analysing 13C NMR of both
compounds, COCH3 was seen at δ 172.1 and 172.18 ppm, and CH3 group was distinct at δ
23.15 and 23.16 ppm in 3′SL-Q and 6′SL-Q respectively (Table S1; Figs. S3A and S3B). These
data confirmed the enzymatic synthesis of derivatives of quercetin conjugated with 3′-sialyl-
lactose and 6′-sialyllactose. The spectra for other carbons and hydrogen were also present
in respective places as shown in Table S1 and Figs. S3. In the previous studies similar groups
were confirmed to be conjugated in vancomycin and epothilone A using same glycosyl-
transferase and sialyltransferase enzymes (Oh et al. 2011; Parajuli et al. 2016).
2.5. Biological activity
The assessment of cytotoxicity effect of Q, QG, QL, 3′SL-Q and 6′SL-Q in in vitro indicated
that different quercetin derivatives inhibited the growth of various cancer cells with different
sensitivity (Figure S4). The inhibitory effects of 3′SL-Q, 6′SL-Q, and QL were better than QG
among all the tested cancer cells. Among all the four quercetin derivatives, quercetin
3-O-lactoside (QL) exhibited the greatest anticancer activity. Although the synthesized com-
pounds could not show much improvement in anti-cancer activities over gastric cancer cells
(AGS), liver cancer cells (HepG2) and uterine cervical cancer cells (HeLa) in compared to the
aglycone, the anticancer activity was still conserved in all of them. The results were analysed
using taxol treated cells as positive control against AGS, HepG2 and Hela cells (Figure S4).
All the compounds tested showed very less or no cytotoxicity at 6.25 μM which is 125-fold
higher than that of the highest concentration of taxol used (0.05 μM). Since taxol is used as
drug to treat various cancers, it is effective to inhibit cancer cells growth at relatively low
concentration ranging from 0.005 μM to 0.05 μM (Yeung et al. 1999). It exhibits complete
cell cytotoxicity at the concentration between 6.25 and 200 μM at which other quercetin
glycoside derivatives were tested. The inhibitory concentration (IC50) value of all of the tested
quercetin derivatives against HeLa cells was above 200 μM. Similarly, it was above 200 μM
in the case of 6′SL-Q, 3′SL-Q and QG against HepG2 cells. But the values were 197.14 μM and
70.84 μM with QL and Q, respectively. However, IC50 values were lower than 200 μM in most
of the derivatives against AGS cells except QG. The activity of quercetin derivative was sig-
nificantly improved with the addition of a disaccharide sugar (lactose) moiety when com-
paring with the addition of monosaccharide sugar (glucose) in case of all the cancer cells
tested.
3. Experimental
3.1. Chemicals and reagents
Standard UDP-α-d-glucose, d-glucose-1-phosphate, UDP-α-d-galactose, isopropyl-β-d-thi-
ogalactopyranoside (IPTG), adenine 5′-triphosphate (ATP), uridine monophosphate (UMP),

6
S. DARSANDHARI ET AL.
and acetyl phosphate were purchased from GeneChem (Daejeon, Korea). High-performance
liquid chromatography (HPLC)-grade acetonitrile and water were purchased from
Mallinckrodt Baker (Phillipsburg, NJ, USA). Quercetin (Q), standard quercetin 3-O-β-d-
glucoside (QG) (CAS Number 482-35-9), taxol (paclitaxel), dimethyl sulfoxide-d6 (DMSO-d6),
and cytidine monophosphate (CMP)-N-acetylneuraminic acid (CMP-NeuNAc) were pur-
chased from Sigma-Aldrich (St. Louis, MO, USA). All the other chemicals used were of high
analytical grade and were commercially available.
3.2. Plasmids, protein expression and culture condition
Escherichia coli BL21 (DE3) (Stratagene, USA) was used as the host for the expression of
proteins acetate kinase (ACK-pET24ma), UMP kinase (UMK-pET15b), UDP-α-d-glucose syn-
thase (GalU-pET24ma), glycosyltransferase (UGT78K1-pET28a), and β-1,4-galactosyltrans-
ferase [β(1,4)GalT -pET24a], as in the previous researches (Malla et al. 2013; Le et al. 2014; Oh
et al. 2011). Likewise, previously cloned α2,3-SiaT in pET32a and α2,6-SiaT in pET15b (Oh et
al. 2011) in E. coli (DE3) were used for protein production. All the recombinant strains were
grown using respective seed culture with the supplementation of antibiotic (50 μμg/mL
kanamycin or 100 μg/mL ampicillin) in Luria–Bertani (LB) media, and allowed to grow at
37 °C, until the optical density at 600 nm (OD600) reached ~0.8. After that, the final concen-
tration of 0.5 mM of IPTG was added for induction, and the culture was further allowed to
grow at 20 °C for 20 h. The cells were harvested by centrifugation at 842 × g at 4 °C and
washed with 50 mM Tris-HCl buffer of pH 7.5. The cells were sonicated, and the clear lysate
was collected by high-speed centrifugation at 13,475 × g for 30 min at 4 °C. The clear lysate
thus obtained was used for in vitro reactions.
3.3. Enzymatic synthesis of quercetin derivatives
3.3.1. Synthesis of quercetin 3-O-β-d-glucoside
The one-pot reaction mixture for the synthesis of quercetin 3-O-β-d-glucosides (QG) con-
tained 100 mM Tris buffer, 20 mM MgCl2, 150 mM acetyl phosphate, 50 mM glucose 1-phos-
phate, 1 mM ATP, 2 mM UMP, 8 mM quercetin, and ~50 μg/mL of each crude enzyme (UMK,
ACK, GalU, and UGT78K1). The reactions were incubated at 37 °C for an appropriate time,
and 10 μL reaction sample was diluted in 500 μL methanol at different times, to check for
conversion of the product.
3.3.2. Synthesis of quercetin 3-O-β-d-lactoside
Quercetin 3-O-β-d-lactoside (QL) was prepared by enzymatic reaction, where 2 mM of QG
was used as a substrate in the reaction consisting of 50 mM Tris-Cl buffer (pH 7), 10 mM
MnCl2, 10 mM UDP-α- -galactose, and 20% crude β(1,4)GalT. The reaction mixture was incu-
d
bated at 37 °C for 2 h, and terminated by adding a double volume of methanol.
3.3.3. Synthesis of 3′ sialyllactosyl quercetin and 6′ sialyllactosyl quercetin
CMP-NeuNAc was used as sugar donors with sialyltransferase enzymes (α2,3-SiaT and
α2,6-SiaT) for the synthesis of 3′SL-Q and 6′SL-Q, respectively. The reaction condition con-
sisted of 25 mM Tris·HCl buffer (pH 7.5), 20 mM MgCl2·6H2O, 5 mM QL dissolved in dimethyl
sulfoxide (DMSO), 10 mM of CMP-NeuNAc, and ~50 μg/mL purified sialyltransferase enzymes.

NATURAL PRODUCT RESEARCH
7
The reaction mixtures were incubated at 37 °C for 24 h, and terminated by boiling for 5 min.
Denatured proteins from the reaction mixtures were separated by centrifugation at 13,475 × g
for 30 min.
3.4. Analytical procedures
High-performance liquid chromatography-photodiode array (HPLC-PDA) analysis was per-
formed using a reversed phase column (Mightysil RP-18 GP 250-4.6 (5 μm)) at a UV absorb-
ance of 312 nm. The binary mobile phases were composed of solvent A (0.025% trifluoro
acetic acid (TFA) in HPLC-grade water) and solvent B (100% acetonitrile, ACN). The total flow
rate was maintained at 1 mL/min for the 25-min program. The program used was as follows;
ACN 10% (0 min), 30% (0–5 min), 50% (5–10 min), 90% (10–15 min), 70% (15–18 min), and
10% (18–25 min). The high-resolution quadruple time-of-flight electrospray ionisation-mass
spectrometry (HR-QTOF ESI/MS) analysis was performed in positive ion mode using an
ACQUITY (UPLC, Waters Corp., Billerica, MA, USA) column coupled with a SYNAPT G2-S (Water
Corp.) column. The purification of compounds was performed by preparative HPLC equipped
with a C18 column (YMC-Pack ODS-AQ (250 × 20 mm I.D., 10 μm) connected to a UV detector
(312 nm) using a 40-min binary program with 100% ACN and water in following condition:
20% (0–5 min), 40% (5–15 min), 90% (15–30 min), and 20% (30–40 min) at a flow rate of
10 mL/min. The purified products were dried, lyophilised, dissolved in DMSO-d6, and
subjected to 700 MHz Bruker, BioSpin nuclear magnetic resonance (NMR) analysis, for
one-dimensional 1H NMR and 13C NMR (176 MHz).
3.5. Biological activity
The effect of Q, QG, QL, 3′SL-Q and 6′SL-Q on the proliferation and viability of different cancer
cell lines were evaluated. Taxol was used as a positive control for all the cancer cell lines. AGS
gastric carcinoma, HepG2 liver carcinoma, and HeLa cervical carcinoma cells were grown in
Dulbecco’s modified Eagle’s medium (DMEM, Invitrogen, Grand Island, NY, USA) supple-
mented with 10% fetal bovine serum (FBS, Invitrogen). All cells were maintained at 37 °C in
a humidified 5% CO2 incubator. For cell growth assay, the cells seeded at 2000 cells/well in
96-well plates (SPL Life Sciences, Korea) were treated with each compound at various con-
centrations for 72 h. Cell growth was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphe-
nyltetrazolium bromide (MTT) colorimetric assay.
4. Conclusion
In conclusion, the present study successfully synthesised two novel sialyllactose conjugated
derivatives of quercetin. The structures of those derivatives were elucidated by various chro-
matographic and spectroscopic analyses. These new molecules were tested for their ability
to inhibit various cancer cells. Although they did not show much anticancer activity, they
may offer as potential compounds for other biological studies in future research.
Disclosure statement
No potential conflict of interest was reported by the authors.

8
S. DARSANDHARI ET AL.
Funding
This work was supported by a grant from the Next-Generation BioGreen 21 Program (SSAC, grant#:
PJ01111901), Rural Development Administration, Republic of Korea.
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