Reversible photoresponsive activity of a carbonic anhydrase mimic

Article abstract of DOI:10.1039/c9cc00018f

The carbonic anhydrase (CA) enzyme reversibly transforms carbon dioxide and water to a carbonate ion and a proton. Photoresponsive enzyme mimics, where the CA-activity can be turned on and off reversibly with light, have not been reported so far. We have designed an active site mimic that offers reversible control of the catalytic activity using light. Moreover, in the presence of a cationic polymer, we have demonstrated that the CA-activity was further enhanced by stabilizing the transition state with the cis-form of the enzyme mimic which can catalyze the hydration of gaseous CO₂.

Full text of DOI:10.1039/c9cc00018f

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DOI: 10.1039/C9CC00018F
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ARTICLE
A reversible photoresponsive activity of a carbonic
anhydrase mimic
Received 00th January 20xx,
Accepted 00th January 20xx
a
a*
Monochura Saha and Subhajit Bandyopadhyay
Abstract: Carbonic anhydrase (CA) enzyme reversibly transforms carbondioxide and water to a carbonate ion and a
proton. Photoresponsive enzyme mimics, where the CA-activity can be turned on and off reversibly with light, has not
been reported so far. We have designed an active site mimic that offers reversible control on the catalytic activity using
light. Moreover, in the presence of a cationic polymer, we have demonstrated that the CA-activity was further enhanced
DOI: 10.1039/x0xx00000x
www.rsc.org/
2
by stabilzing the transition state with the cis-form of the enzyme mimic which can catalyze the hydration of gaseous CO .
Emulating biological activities with synthetic stimuli-responsive synthetic molecules18 and MOF19 that can perform the similar
1
2
20
catalysts is a long-standing interest of the chemists. Light function to that of natural enzyme.
provides a means of controlling a plethora of biological
3
processes in the natural systems.
Photoresponsive
compounds such as bacteriorhodopsins are common in
4
nature. Incorporation of an optically addressable unit to gain
control over biological processes has been in the frontiers of
5
research in modern chemical biology. Enzyme activity is often
modulated through feedback loops that can employ a variety
of external stimuli.6 Light stimulus is convenient as it offers
an easy spatio-temporal control and can operate without any
physical contact.
,7,8
2
1
Fig, 1 Active site of carbonic anhydrase (PDB: 1CA2); (b) Thecatalytically active
2+
2+
[
Zn . cis-1] and its less active [Zn . trans-1] photoisomer.
Photoisomerization of azobenzene compounds involves a large
9
10
geometrical change. Exploiting this, modulation of pK
a
and
In addition,
access of the active site and catalysis by photoswitchable
There exist a number of ingenious designs of CA mimics.22
1
1,12
catalytic activity by light have been acheived.
Recently, a remarkable photoregulated activity of the hCAII
15,23
enzyme has been demonstrated.
However, so far, there
1
3
substrates has been demonstrated. Carbonic anhydrase (CA)
is a class of enzyme that assists the rapid interconversion of
carbon dioxide into carbonic acid and bicarbonate ions. In
the human body, the enzyme hCAII controls a variety of
physiological roles ranging from neurological functions to
cancer.15 The hCAII blockers are prescribed for the control of
intraocular fluid pressure to treat glaucoma. The active site
of carbonic anhydrase consists of a Zn(II) metal ion,
coordinated to imidazole nitrogens of histidine residues and a
has not been any report of a carbonic anhydrase enzyme
mimic where the activity can be modulated reversibly with
light. This work demonstrates a photoregulated carbonic
anhydrase mimic with two imidazoles appended to the two
ends of an azobenzene system, coordinated to a Zn(II) ion. The
activity of the system can be modulated reversibly upon
photoisomerization of the azobenzene photochromic unit with
light. Furthermore, employing a carefully designed cationic
polymer, a rate enhancement of the catalytic activity was
observed.
1
4
1
6
molecule
of
water
in
a
distorted
tetrahedral
1
7
arrangement. (Fig. 1). The facile deprotonation of the Zn(II)–
bound water is crucial for the supply of an active hydroxyl unit
The azobenzene compound 1 containing the imidazole units
24
has been synthesized and it was fully chracterized by the all
-
that converts the CO
2
to a bicarbonate (HCO
3
) ion. Taking an
spectroscopic methods including the single crystal XRD (CCDC
inspiration from the nature, chemists have developed the
number 1846676). The trans isomer of compound 1 displayed
-1
-1
absorption bands at λmax = 230, 319 (ε = 11,800 M cm , π–π*
−1
−1
band), and 440 nm (ε = 110 M cm , n–π* band) (Fig. 2B).
Photoirradiation with UV-light of 366 nm at 273K converts the
trans-1 to the cis-1 isomer. Exposure of a solution of trans-1
[
a] Saha M, Bandyopadhyay S
Department of Chemical Sciences
Indian Institute of Science Education and Research (IISER) Kolkata.
Mohanpur ,Nadia, 741246, INDIA
E-mail: sb1@iiserkol.ac.in
-2
(30 M in methanol) to UV light (λ =366 nm, 2.3 mW cm )
generated the cis-rich photostationary state (PSS) within 30
min with cis:trans ratio of 85:15 as calculated from the UV-Vis
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Fig. 2B) and the H NMR spectra (Fig. S2, ESI). On the other titration plot revealed the pK
Journal Name
1
2+
a
value of 7.2 for the [Zn .cis-1]
View Article Online
(
2
+
DOI: 10.1039/C9CC00018F
hand, the reverse reaction of the cis-1 to the trans-1 was complex and for the [Zn .trans-1] it was 8.1. Thus to achieve a
achieved upon the exposure of the sample with blue light (λ = differential behavior, a pH of 7.6 which is also close to the
−2
4
66 nm) (blue LED light, 0.5 mW cm ). Upon continuous physiological pH was chosen for monitoring the activity of the
irradiation with the blue light the trans rich photostationary
state (PSS trans:cis 95:5) (Fig. S1B, ESI) was attained in 600 s.
Akin to the natural carbonic anhydrases,the Zn(II) ion was
selected for complexation with 1. A distinct blue shift from 318
nm to 310 nm of the π–π* band was observed upon addition
catalysts in the two photoisomeric forms. The catalytic activities
with Zn(II) bound trans-1 and cis-1 have been studied for the
carbonic anhydrase activity using the standard carbonic anhydrase
2+
assay with both the photoisomerized complexes [Zn . cis-1] (30
2
+

M) and the polymeric [Zn . trans-1] (30 M monomeric
of Zn(OAc)
2
(Fig.S3, ESI). The complexation of the metal ion
concentration) in the aqueous media with the standard
chromogenic CA substrate, p-nitrophenyl acetate (pNPA)27 in 1mM
PBS buffered at pH=7.6 following the growth of the p-nitrophenol
1
was also investigated by the H NMR spectroscopy. (Fig. 2C
and S4, ESI)
In the presence of one equivalent of the Zn(OAc)
2
, the change
o
production the absorbance spectra at 400 nm at 25 C (Fig. 3A and
in the chemical shifts ()of the imidazole and the –CH
2
S5. ESI). The catalytic activity with the cis-complex was found to be
far superior compared to the trans-complex (Figure S5A and 5B,
ESI).
protons with the trans-1(Fig. 2C) was more compared to the
cis-1 protons (details in ESI, also see Fig. S4).
Fig. 2 (A). Crystal structure (SCXRD) of trans-1 with 50% ellipsoid probability. (B).
2+
-5
Fig. 3(A). UV-vis spectra of pNPA in the presence of [Zn . cis-1], in PBS (pH=7.6)
UV–vis spectral changes upon exposure of trans-1 (3 × 10 M) in methanol with
buffer. The band at 271 nm gradually decreased and the one at 400 nm gradually
3
66 nm UV light (irradiation time: 0, 2, 5, 8, 10, 15, 30 min) for the transcis
photoisomerization; Inset: The zoomed-in version of the 450 nm bands of the
two isomers (C). NMR titration spectra of cis-1 (100 M) with gradual increase of
Zn(OAc) (0, 20, 40, 60, 80 and 100 M, 01 eqv.) in CD OD:D O (1:1 v/v). (D).
Probable binding mode of the cis-1 with Zn(OAc)
increased indicating the activity; inset: the reaction mixture before and after the
1
hydrolysis.(B).Hydrolysis of pNPA substrate.(C). H NMR traces of pNPA (30 mM)
2+
upon hydrolysis with the active catalyst [Zn . cis-1](0.3 mM)in CD
3
OD-D
2
O (1:1
2
3
2
v/v) (PBS buffered at pH 7.6).(D).The hydrolytic reaction monitored by UV-vis
2
.
2+
2+
spectroscopy at 400 nm with the [Zn . cis-1] (red), [Zn . trans-1] (green) catalyst
o
2
and only Zn(OAc) (blue) at pH 7.6, 25 C.
Experimental evidences obtained from the Job’s plot experiments
carried out at a total concentration of 60 µM) with Zn(OAc)
revealed a clear 1:1 binding with both the trans-1 and the cis-1 form
Fig. S10, ESI). The TOF-MS data displayed a clean peak for the 1:1
2
+
The hydrolytic reaction with the catalytically active [Zn . cis-1]
was also further monitored by FTIR and NMR spectroscopy
techniques. The IR spectra recorded with the dried aliquots
from the reaction at different time intervals clearly displayed a
gradual decrease of the ester carbonyl stretching band at 1765
(
2
(
complex involving a cis-1, an acetate ion and a water molecule with
the Zn(II) ion at m/z 482 (Fig. 1B and S19, ESI).The far infrared
spectra displays the vibrational modes of the Zn-ligand connectivity
which provides an insight to the coordination sphere of the Zn2+
ions in the complexes. The far-FTIR spectra of the cis- and the trans-
-1
2+
cm (Fig. S6, ESI).Upon addition of the substrate to the [Zn .
trans-1] complex, the rate of the hydrolysis did not increase
significantly as it happened with the [Zn . cis-1] form (see
2
+
Table 1 for the kinetic data).
-1
complexes were thus recorded in the range of 200-500 cm , as
1
The ester hydrolysis monitored by the H-NMR spectroscopy
displayed a gradual decrease of the signals at  2.29, 7.30 and
shown in Fig. S26. The peaks at 234 cm-1 (Zn-N(Im)), 254 cm-1 (Zn-
-
1
-1
N(Im)), 301 cm (Zn-OAc), 387 and 407 cm (both for the Zn-O(H)) were 8.31 for the aromatic protons of the ester substrate, and the
2
+
found for the [Zn . cis-1] complex. The corresponding peaks in the growth of the p-nitrophenolate signals at  7.90 and 6.29 for
far-IR complex for the [Zn . trans-1] species appeared at 240 cm , the phenolate product (Fig.3C). It is also noteworthy that in
60 cm , 308 cm , 376 and 389 cm respectively. These agree with the C NMR spectra of the extracted aliquot of the reaction in
2
+
-1
-1
-1
-1
13
2
the literature reported for the tetrahedral Zn-species25,30 and also
with the ESI-MS fragmentation patterns obtained (see ESI).
2
D O-MeOD (1:1 v/v), the ester carbonyl at  170 and the
methyl at  21 for the pNPA substrate progressively
diminished with time with the appearence of newsignals at 
Since the carbonic anhydrase activity is known to follow a general 175 ppm and  20.5 corresponding to the hydrolyzed acetate
2
6
2+
2+
base catalyzed mechanism, pH titrations of both the [Zn . cis-1] product (Fig. S7, ESI).The higher efficacy of the [Zn .cis-1] as
2
+
the catalyst compared to the polymeric [Zn2+.trans-1]at pH 7.6
and [Zn .trans-1] in aqueous media have been carried out. The
is likely a result of the lower pK of the cis-complex (pK = 7.2)
a
a
2
| J. Name., 2012, 00, 1-3
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Journal Name
than the pK
ARTICLE
2
+
a
of the polymeric Zn .-trans-complex (pKa 8.1). It proposed stabilization model, a negatively charged polymer
View Article Online
DOI: 10.1039/C9CC00018F
is likely that in the cis-complex the photochromic units brings was also tested. It was indeed found to decrease the k value (
the two imidazole units in a close proximity that facilitates the for details, see ESI) drastically to 1.3 x 10 min .
complex formation with the Zn(II) metal that is essential for
the generation of the active catalyst. The hydrated [Zn .cis-1]
then loses a proton to generate the active OH that acts as the
nucleophile (Fig. 4B).
The transition state of the chemical transformation in a natural
enzyme is stabilized by the perfect complementarity of the
steric and electronic factors in suitable micro environments
which are the key to the remarkable rate acceleration with the
-2
-1
Table 1. Rate constants for the catalysis in the presence of the cationic
polymer A.
2
+
-
[a]
k (min-1)
Catalytic conditions
2
+
-2
[Zn . cis-1]
8.2 × 10
2.0 × 10
2
+
-4
-
n
[Zn . trans-1] -
o
[
[
a] Buffered at pH 7.6 with PBS at 25 C.
b] See Table S2 for the rates of the controls without the polymer A and without
enzymes. Chemists have used polymers and dendrimers to the Zn(II) ion.
emulate such sophisticated systems with judicious choice of
the suitable functionality.
Kinetic experiments were carried out at different substrate
concentrations and the kinetic parameters were determined in
the presence of the cationic polymer. The kinetic data
2
8,29
displayed
a MM behavior (Fig 4C). The parameters
2
+
summarized in Table 2 demonstrate that the [Zn .cis-1] in the
presence of the polymer shows an enzyme-like activity. With
2
+
the [Zn .trans-1] and the polymer, the MM behavior was not
observed.
Table 2. Kinetic parameters for the [Zn2+.cis-1] with the cationic polymer A in PBS
o
buffered at pH 7.6 at 25 C.
-1
-1
-
k
cat
k
noncat (min )
k
cat/knoncat
K
M
(mM)
k
cat/K
M
(mM min )
-
1
(
min )
0
.24
1.1 × 10-4
2.2 ×103
1.02
0.24
Fig.4(A). The cationic polymeric host for the catalysis. (B).Probable mechanism
for the hydrolysis of pNPA in the presence of the cationic polymer.
(
C).Dependence of the initial rate on the 4-nitrophenyl acetate (pNPA) for the
2+
hydrolysis reaction promoted by the active catalyst [Zn . cis-1] at pH = 7.6, 25
o
C; inset: the corresponding Lineweaver-Burk plot. (D).Alternate exposure to UV
o
o
(
366 nm, 15 min, 0 C) and visible light (.466 nm, 10 min, 0 C) in a reaction
mixture containing pNPA and catalyst at pH 7.6.
2
+
The [Zn .cis-1] catalyst in the presence of the polymer was
2
+
converted to the [Zn .trans-1] catalyst upon exposure to
visible light for 10 min under ice-cold conditions (orange bar,
Fig.4D). The relative rate of the hydrolysis, obtained by fitting
the absorbancedata, with the trans-catalyst dropped
drastically. The high activity was restored upon the reversal of
2
+
Encouraged by the efficient catalysis of the [Zn . cis-1]
complex, a cationic amphiphilic co-polymer A (Fig. 4A,
synthesis in Scheme S3; characterization: Fig. S27 & S28, ESI)
was employed as a supramolecular host to encapsulate the
catalyst and also perhaps as a phase transfer catalyst for the
2
+
the trans-catalyst to the [Zn .cis-1] form upon exposure to the
66 nm light under ice-cold condition for 15 min (pink bar,
Fig.4D).
+
substrate-bound-catalyst. The polymer with cationic –N (CH
)
3 3
3
residues was capable of stabilizing the tetrahedral oxyanionic
transition state of the reaction and can form a suitable micro
environment with the phenyl rings required to stabilize the 4-
2
+
Switching the catalyst back to the [Zn .cis-1] form revived the
catalytic efficiency. The photostability of compound 1 was
checked using UV-vis spectroscopy over ten switching cycles in
a methanol solution purged with dry nitrogen less than 5 %
decomposition of the sample was observed. (Fig. S11, ESI).
2
+
nitrophenolic unit of the substrate (Fig. 4B). The [Zn .cis-1]
complex buffered at pH 7.6 was incubated with the cationic
polymer A for 15 min in the darkness prior to the addition of
the substrate at room temperature. The catalytic activity of
To assess the system further towards a realistic goal of CO
hydration, gaseous CO was bubbled through a solution of the
Zn .cis-1] and the [Zn .trans-1] catalyst (30 M, 20 mL) in water
at pH 7.6, 298 K, and the pH of the mixture was monitored. Control
experiments were also carried out without the catalysts and the
Zn2+ only with the cis-ligand (Fig. S28). Upon increasing the bubbling
time, the drop in the pH attained a steady value within the 30
minutes. The hydrated CO was sequestered upon addition of a
CaCl2 solution (10 mL, 10 mm) with the catalyst-treated CO2-
2
2
+
the polymer-[Zn .cis-1] composite was determined using the
by monitoring the absorbance at 400 nm. The growth of the
product peaks appeared to be visibly faster during the
experiments in the presence of the polymer. The hydrolysis
2
2
+
2+
[
2
+
reaction catalyzed by [Zn . cis-1] in the presence of the
polymer A displayed a significant rate enhancement compared
2
+
to the [Zn . trans-1]. The pseudo first-order rate constants (k)
2
+
with the [Zn . trans-1], (30 uM, 1 mM PBS buffer, 0.2 mM of
2
o
-4
-1
polymer A at pH 7.6, 25 C) of was found to be 2.0 × 10 min ,
2
+
3
whereas with [Zn . cis-1] under identical conditions the k was saturated water (20 mL) to precipitate CaCO where voluminous
-2
-1
2+
found to be 8.2 x 10 min . (Table 1) Thus, in the presence of precipitate was obtained for the [Zn .cis-1] and the other controls
the polymer A, the reaction was 410 times faster with the cis- produced small turbidity.30
complex than the trans-complex. To test the validity of our
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In summary, inspired by the CA enzyme, a model active site
Journal Name
1
1
1 (a) F. Wurthner and J. Rebek,J. Chem. ViSewocA.rticlPe eOrnklinine
Trans.,1995, 2 ,1727; (b) D. Sud, T. B. Norsten and N. R.
DOI: 10.1039/C9CC00018F
of the enzyme was designed. The active site mimic consisted of
imidazole-appended azobenzene system that offered a light
mediated modulation of the distance between the two
imidazole units. Zn(II) ions with the cis-form formed a 1:1
Branda,Angew. Chem., Int. Ed. 2005, 44, 2019.
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3
4, 446–448;(b) M. Samanta, V. S. Rama Krishna and S.
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Commun., 2013, 49, 255–257.
II
complex whereas the trans-isomer afforded a polymeric -(Zn -
2
+
trans-1)- network.The [Zn .cis-1] worked efficiently as an
active site mimic of the CA enzyme. In contrast, the efficiency
II
with the –-(Zn -trans-1)- formwere far less. The catalytically
1
3 (a) S. Neri, SG. Martin, C. Pezzato and J. Prins., J. Am. Chem.
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efficient cis-complex was used akin to a cofactor in conjunction
with a cationic polymer in order to stabilize the oxyanionic
transition state of the reaction for achieving further rate
enhancement. Indeed, in the presence of the cationic polymer,
a dramatic rate enhancement was observed that served as
substantiation of our strategy. The cis-catalyst could also
convert gaseous CO to carbonic acid. This work demonstrates
2
an effective means of controlling a model carbonic anhydrase
activity with a photoresponsive enzyme mimic.
(
c) O. B. Berryman, A. C. Sather, A. Lledo´ and J. Rebek,
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Conflicts of interest
There are no conflicts to declare.
5
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Acknowledgements
The authors thank DST-SERB for financial support (Grant
number EMR/2017/003720). MS is supported by an INSPIRE 19 C. Jin, S. Zhang, Z. Zhang and Y. Chen,Inorg. Chem.,2018, 57,
doctoral fellowship by DST-India. We thank Prof. Priyadarshi
De for his help with the preparation of the polymer.
2
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A reversible photoresponsive activity of a carbonic
anhydrase mimic
Monochura Saha and Subhajit Bandyopadhyay*
Carbonic anhydrase activity of an enzyme mimic can be turned off and on reversibly with light.