GPER mediates estrogen cardioprotection against epinephrine-induced stress

2021 ◽  
Author(s):  
Lu Fu ◽  
Hongyuan Zhang ◽  
Jeremiah Ong’achwa Machuki ◽  
Tingting Zhang ◽  
Lin Han ◽  
...  
Keyword(s):  
Culture Supernatant ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Protective Role ◽  
Left Ventricular ◽  
Internal Diameter ◽  
High Dose ◽  
Adult Rats ◽  
Rat Cardiomyocytes ◽  
Lactate Dehydrogenase Release

Currently, there are no conventional treatments for stress-induced cardiomyopathy (SCM, also known as Takotsubo syndrome), and the existing therapies are not effective. The recently discovered G protein- coupled estrogen receptor (GPER) executes the rapid effects of estrogen (E2). In this study, we investigated the effects and mechanism of GPER on epinephrine (Epi)-induced cardiac stress. SCM was developed with a high dose of Epi in adult rats and human-induced pluripotent stem cells–derived cardiomyocytes(hiPSC-CMs). (1) GPER activation with agonist G1/ E2 prevented an increase in left ventricular internal diameter at end-systole, the decrease both in ejection fraction and cardiomyocyte shortening amplitude elicited by Epi. (2) G1/ E2 mitigated heart injury induced by Epi, as revealed by reduced plasma brain natriuretic peptide and lactate dehydrogenase release into culture supernatant. (3) G1/E2 prevented the raised phosphorylation and internalization of β2-adrenergic receptors（β2AR）. (4) Blocking Gαi abolished the cardiomyocyte contractile inhibition by Epi. G1/E2 downregulated Gαi activity of cardiomyocytes and further upregulated cyclic adenosine monophosphate concentration in culture supernatant treated with Epi. (5) G1/E2 rescued decreased Ca2+ amplitude and Ca2+ channel current (ICa-L) in rat cardiomyocytes. Notably, the above effects of E2 were blocked by the GPER antagonist, G15. In hiPSC-CM (which expressed GPER, β1AR and β2ARs), knockdown of GPER by siRNA abolished E2 effects on increasing ICa-L and action potential duration in the stress state. In conclusion, GPER played a protective role against SCM. Mechanistically, this effect was mediated by balancing the coupling of β2AR to the Gαs and Gαi signalling pathways.

scholarly journals Astragaloside IV Protects Rat Cardiomyocytes from Hypoxia-Induced Injury by Down-Regulation of miR-23a and miR-92a

2018 ◽  
Vol 49 (6) ◽  
pp. 2240-2253 ◽  
Author(s):  
Licheng Gong ◽  
Hong Chang ◽  
Jingze Zhang ◽  
Gongliang Guo ◽  
Jingwei Shi ◽  
...  
Keyword(s):  
Signaling Pathways ◽  
Target Gene ◽  
Infarct Volume ◽  
Left Ventricular ◽  
Erk Signaling ◽  
Internal Diameter ◽  
H9c2 Cells ◽  
Astragaloside Iv ◽  
Down Regulation ◽  
Rat Cardiomyocytes

Background/Aims: Astragaloside IV (AS-IV), a traditional Chinese medicine isolated from Astragalus membranaceus, has been shown to exert cardioprotective effect previously. This study aimed to reveal the effects of AS-IV on hypoxia-injured cardiomyocyte. Methods: H9c2 cells were treated with various doses of AS-IV for 24 h upon hypoxia. CCK-8 assay, flow cytometry/Western blot, and qRT-PCR were respectively conducted to measure the changes in cell viability, apoptosis, and the expression of miR-23a and miR-92a. Sprague–Dawley rats were received coronary ligation, and were administrated by various doses of AS-IV for 14 days. The infarct volume and outcome of rats followed by ligation were tested by ultrasound, arteriopuncture and nitrotetrazolium blue chloride (NBT) staining. Results: We found that 10 μg/ml of AS-IV exerted myocardioprotective effects against hypoxia-induced cell damage, as AS-IV significantly increased H9c2 cells viability and decreased apoptosis. Interestingly, the myocardioprotective effects of AS-IV were alleviated by miR-23a and/or miR-92a overexpression. Knockdown of miR-23a and miR-92a activated PI3K/AKT and MAPK/ ERK signaling pathways. Bcl-2 was a target gene for miR-23a, and BCL2L2 was a target gene for miR-92a. In the animal model of myocardial infarction (MI), AS-IV significantly reduced the infarct volume, ejection fraction (EF), shortening fraction (FS) and LV systolic pressure (LVSP), and significantly increased left ventricular end-diastolic internal diameter (LVEDd). And also, the elevated expression of miR-23a and miR-92a in MI rat was reduced by AS-IV. Conclusion: AS-IV protected cardiomyocytes against hypoxia-induced injury possibly via down-regulation of miR-23a and miR-92a, and via activation of PI3K/AKT and MAPK/ERK signaling pathways.

scholarly journals Interaction of Metabolic and Respiratory Acidosis with α and β-adrenoceptor Stimulation in Rat Myocardium

2012 ◽  
Vol 117 (6) ◽  
pp. 1212-1222 ◽  
Author(s):  
Matthieu Biais ◽  
Romain Jouffroy ◽  
Aude Carillion ◽  
Sarah Feldman ◽  
Aude Jobart-Malfait ◽  
...  
Keyword(s):  
Metabolic Acidosis ◽  
Intracellular Ph ◽  
Respiratory Acidosis ◽  
Adenosine Monophosphate ◽  
Left Ventricular ◽  
Rat Myocardium ◽  
Adrenergic Stimulation ◽  
Rat Cardiomyocytes ◽  
Inotropic Effects ◽  
Isolated Rat

Background The effects of acute respiratory versus metabolic acidosis on the myocardium and their consequences on adrenoceptor stimulation remain poorly described. We compared the effects of metabolic and respiratory acidosis on inotropy and lusitropy in rat myocardium and their effects on the responses to α- and β-adrenoceptor stimulations. Methods The effects of acute respiratory and metabolic acidosis (pH 7.10) and their interactions with α and β-adrenoceptor stimulations were studied in isolated rat left ventricular papillary muscle (n=8 per group). Intracellular pH was measured using confocal microscopy and a pH-sensitive fluorophore in isolated rat cardiomyocytes. Data are mean percentages of baseline±SD. Results Respiratory acidosis induced more pronounced negative inotropic effects than metabolic acidosis did both in isotonic (45±3 versus 63±6%, P<0.001) and isometric (44±5 versus 64±3%, P<0.001) conditions concomitant with a greater decrease in intracellular pH (6.85±0.07 versus 7.12±0.07, P<0.001). The response to α-adrenergic stimulation was not modified by respiratory or metabolic acidosis. The inotropic response to β-adrenergic stimulation was impaired only in metabolic acidosis (137±12 versus 200±33%, P<0.001), but this effect was not observed with administration of forskolin or dibutiryl-cyclic adenosine monophosphate. This effect might be explained by a change in transmembrane pH gradient only observed with metabolic acidosis. The lusitropic response to β-adrenergic stimulation was not modified by respiratory or metabolic acidosis. Conclusion Acute metabolic and respiratory acidosis induce different myocardial effects related to different decreases in intracellular pH. Only metabolic acidosis impairs the positive inotropic effect of β-adrenergic stimulation.

Role of phosphodiesterases in the development of takotsubo syndrome

2020 ◽  
Vol 41 (Supplement_2) ◽  
Author(s):  
D Huebscher ◽  
T Borchert ◽  
G Hasenfuss ◽  
V.O Nikolaev ◽  
K Streckfuss-Boemeke
Keyword(s):  
Resonance Energy ◽  
Induced Pluripotent Stem Cell ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Left Ventricular ◽  
Camp Level ◽  
Camp Signaling ◽  
Patient Specific ◽  
Funding Source ◽  
Takotsubo Syndrome

Abstract Background/Purpose Takotsubo syndrome (TTS) is characterized by acute transient left ventricular dysfunction in the absence of obstructive coronary lesions. We identified a higher sensitivity to catecholamine-induced stress toxicity as mechanism associated with the TTS phenotype in our former study, but the pathogenesis of TTS is still not completely understood. In this study our aim was to prove the hypothesis of an altered phosphodiesterase (PDE)-dependent 3',5'-cyclic adenosine monophosphate (cAMP)-signaling in TTS in patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Methods and results We generated functional TTS-iPSC-CMs and treated them with catecholamines to mimic a TTS-phenotype. To directly address the hypothesis that local cAMP dynamics might be altered in TTS, we used Förster resonance energy transfer (FRET) based cAMP sensors, which are specifically located in the cytosol or at the sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA) micro domain. We demonstrated that β-adrenergic receptor (β-AR) stimulations resulted in stronger cytosolic FRET responses in TTS-CMs compared to controls. In contrast, no differences of cAMP level were observed in the SERCA-PLN micro domain between TTS- and control-iPSC-CMs. To analyze the interplay of β-AR signaling and specific PDE contribution to the cAMP signaling in TTS, specific PDE-inhibitors were used. We were able to show in the cytosol that after β-AR stimulation, the strong effects of the PDE4 family of control cells were significantly decreased in diseased TTS CMs, which is in line with previously described reduced PDE4 activity in failing mouse hearts. In contrast, the contribution of PDE3 to cytoplasmic cAMP degradation was increased in TTS (Figure 1 A). This is in line with increased PDE3A and down-regulated PDE4D protein expression in TTS-iPSC-CMs compared to control cells. Analysis of PDE-dependent cAMP level in the SERCA micro domain show also a significantly reduced PDE4 activity. But the dynamic cytosolic PDE contribution of PDE2 and PDE3 after catecholamine treatment in TTS is lost in SERCA micro domain (Figure1B). Conclusion Our data showed for the first time alterations of local cAMP signaling in healthy and diseased TTS-iPSC-CMs. We demonstrated an isozym shift from PDE4 in control to PDE3 and PDE2 in TTS and identified PDE4 as an important player in the β-adrenergic cAMP signaling in TTS. Therefore, PDE4 activators may be a possible new therapeutic target option in the treatment of TTS. Figure 1 Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): DZHK

230 EVALUATION OF APOPTOSIS IN IN VITRO-PRODUCED BOVINE EMBRYOS MATURED WITH FORSKOLIN

2013 ◽  
Vol 25 (1) ◽  
pp. 263
Author(s):  
D. M. Paschoal ◽  
M. J. Sudano ◽  
R. R. D. Maziero ◽  
M. D. Guastali ◽  
L. F. Crocomo ◽  
...  
Keyword(s):  
Linear Models ◽  
Germinal Vesicle ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
High Dose ◽  
Bovine Embryos ◽  
Nuclear Maturation ◽  
Tunel Reaction ◽  
Level Of Significance

The maintenance of oocytes in the germinal vesicle stage for a few hours could result in more competent oocytes for use in biotechnology. Related to this, forskolin (Sigma-Aldrich, St. Louis, MO, USA) is an efficient inhibitor of nuclear maturation because of its ability to increase the levels of intracellular cyclic adenosine monophosphate. This study aimed to show whether the use of forskolin would be able to inhibit maturation in bovine oocytes, producing a higher rate of in vitro embryos. Nellore oocytes from a slaughterhouse (n = 960) were matured in TCM-199 with Earle’s salt + 10% FCS, FSH, and LH, in a 5% CO2 atmosphere. To delay meiosis, the oocytes were maintained for 6 h in medium with forskolin at 3 different concentrations, 0. 1 mM (n = 240), 0.05 mM (n = 240), and 0.025 mM (n = 240), whereas untreated oocytes acted as controls (n = 240). The oocytes were then cultured for 18 h in agent-free medium to resume meiosis, completing 24 h of maturation. After 24 h (Day 0) of maturation, oocytes were fertilized in human tubal fluid (HTF, Irvine, New Zealand) under the same conditions as described above. Semen was selected through Percoll gradient, and the concentration was adjusted to 2 × 106 sperm mL–1. The presumed zygotes were cultured in 90-µL droplets of SOFaa + 0.6% BSA + 2.5% FCS in a 5% CO2, 5% O2, and 90% N2 atmosphere until Day 7, when blastocysts were evaluated. Apoptosis in blastocysts was accessed through TUNEL reaction. Data were analysed by ANOVA, followed by Tukey’s test using the general linear models procedure (PROC GLM) of SAS (SAS Institute Inc., Cary, NC, USA). The level of significance adopted was 5%. No statistical differences were observed in blastocyst production rate (n = 297): control (n = 88): 36.7% ± 3.7; 0.1 mM forskolin (n = 61): 25.1% ± 3.7; 0.05 mM forskolin (n = 70): 29.2% ± 3.7; 0.025 mM forskolin (n = 78): 32.6% ± 3.7 (P > 0.05). However, when we analysed the apoptosis rates, differences were found among groups: control: 6.0% ± 6.3a; 0.1 mM forskolin: 33.4% ± 6.3b; 0.05 mM forskolin: 27.2% ± 6.3ab; 0.025 mM forskolin: 10.0% ± 6.3ab (P < 0.05). Although there was no difference in blastocyst rate, the TUNEL technique allowed us to identify that a high dose of forskolin was detrimental for in vitro-produced bovine embryos. FAPESP: 10/50410-2.

scholarly journals Citrulline Improves Early Post-Ischemic Recovery or Rat Hearts In Vitro by Shifting Arginine Metabolism From Polyamine to Nitric Oxide Formation

2018 ◽  
Vol 12 ◽  
pp. 117954681877190 ◽  
Author(s):  
Marc Heidorn ◽  
Tim Frodermann ◽  
Andreas Böning ◽  
Rolf Schreckenberg ◽  
Klaus-Dieter Schlüter
Keyword(s):  
Nitric Oxide ◽  
Functional Recovery ◽  
Heart Function ◽  
Ischemia Reperfusion ◽  
Left Ventricular ◽  
Adult Rats ◽  
Rat Cardiomyocytes ◽  
No Formation ◽  
Arginase 1 ◽  
Early Functional Recovery

Background: Reperfusion or reopening of occluded vessels is the gold standard to terminate ischemia. However, early functional recovery after reperfusion is often low requiring inotropic intervention. Although catecholamines increase inotropy and chronotropy, they are not the best choice because they increase myocardial oxygen and substrate demand. As nitric oxide (NO) contributes to cardiac function, we tested the hypothesis that addition of citrulline during the onset of reperfusion improves post-ischemic recovery because citrulline can reenter arginine consumption of NO synthases (NOS) but not of arginases. Methods: Hearts from adult rats were used in this study, exposed to 45-minute global ischemia and subsequently reperfused for 180 minutes. Citrulline (100 µM) or arginine (100 µM) was added with reperfusion and remained in the perfusion buffer for 180 minutes. Nω-nitro-l-arginine methyl ester (l-NAME) was used to antagonize NOS activity. Results: Citrulline increased load-free cell shortening of isolated adult rat cardiomyocytes and improved left ventricular developed pressure (LVDP) under normoxic conditions, indicating that citrulline can affect heart function. Ischemia/reperfusion caused a constitutive loss of function during 3 hours of reperfusion, whereas citrulline, but not arginine, improved the functional recovery during reperfusion. This effect was attenuated by co-administration of l-NAME. Although citrulline increased the formation of nitrite, l-NAME attenuated this effect indicating again a positive effect of citrulline on NO formation. Citrulline, but not arginine, increased the expression of arginase-1 (protein and mRNA) but l-NAME attenuated this effect again. Collectively, citrulline improved the post-ischemic recovery in an NO-dependent way. Conclusions: Citrulline, known to block arginase and to support NO formation, improves the early functional recovery of post-ischemic hearts and may be an alternative to catecholamines to improve early post-ischemic recovery.

Overexpression of Cyclic Adenosine Monophosphate Effluent Protein MRP4 Induces an Altered Response to β-Adrenergic Stimulation in the Senescent Rat Heart

2015 ◽  
Vol 122 (2) ◽  
pp. 334-342 ◽  
Author(s):  
Aude Carillion ◽  
Sarah Feldman ◽  
Cheng Jiang ◽  
Fabrice Atassi ◽  
Na Na ◽  
...  
Keyword(s):  
Positive Inotropic Effect ◽  
Cyclic Adenosine Monophosphate ◽  
Calcium Transient ◽  
Adenosine Monophosphate ◽  
Left Ventricular ◽  
Inotropic Effect ◽  
Inotropic Response ◽  
Adrenergic Stimulation

Abstract Background: In the senescent heart, the positive inotropic response to β-adrenoceptor stimulation is reduced, partly by dysregulation of β1- and β3-adrenoceptors. The multidrug resistance protein 4 (MRP4) takes part in the control of intracellular cyclic adenosine monophosphate concentration by controlling its efflux but the role of MRP4 in the β-adrenergic dysfunction of the senescent heart remains unknown. Methods: The β-adrenergic responses to isoproterenol were investigated in vivo (stress echocardiography) and in vitro (isolated cardiomyocyte by Ionoptix® with sarcomere shortening and calcium transient) in young (3 months old) and senescent (24 months old) rats pretreated or not with MK571, a specific MRP4 inhibitor. MRP4 was quantified in left ventricular homogenates by Western blotting. Data are mean ± SD expressed as percent of baseline value. Results: The positive inotropic effect of isoproterenol was reduced in senescent rats in vivo (left ventricular shortening fraction 120 ± 16% vs. 158 ± 20%, P &lt; 0.001, n = 16 rats) and in vitro (sarcomere shortening 129 ± 37% vs. 148 ± 35%, P = 0.004, n = 41 or 43 cells) as compared to young rats. MRP4 expression increased 3.6-fold in senescent compared to young rat myocardium (P = 0.012, n = 8 rats per group). In senescent rats, inhibition of MRP4 by MK571 restored the positive inotropic effect of isoproterenol in vivo (143 ± 11%, n = 8 rats). In vitro in senescent cardiomyocytes pretreated with MK571, both sarcomere shortening (161 ± 45% vs. 129 ± 37%, P = 0.007, n = 41 cells per group) and calcium transient amplitude (132 ± 25% vs. 113 ± 27%, P = 0.007) increased significantly. Conclusion: MRP4 overexpression contributes to the reduction of the positive inotropic response to β-adrenoceptor stimulation in the senescent heart.

scholarly journals Effect of parathyroid hormone-related protein on intracellular calcium ion and cyclic adenosine monophosphate concentrations in cardiac fibroblasts

2020 ◽  
Vol 48 (9) ◽  
pp. 030006052093124
Author(s):  
Ping Zhou ◽  
Qiong Xiao ◽  
Zhao-Ting Su ◽  
Lin Zhu ◽  
Fang-Xia Jin ◽  
...  
Keyword(s):  
Calcium Ion ◽  
Wistar Rats ◽  
Low Dose ◽  
Cardiac Fibroblasts ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Control Group ◽  
High Dose ◽  
Related Protein ◽  
Parathyroid Hormone Related Protein

Objective This study aimed to determine the effect of parathyroid hormone-related protein (PTHrP) on proliferation of cardiac fibroblasts (CFs) in primary cultures of neonatal Wistar rats. Methods Different PTHrP concentrations were added to CFs of neonatal Wistar rats and the cells were grouped according to the concentrations added. A verapamil (VPL) group and a calcitriol (CAL) group were also established. Changes in cell proliferation and in cyclic adenosine monophosphate and calcium ion levels were identified and recorded. Results We found that as the concentration of PTHrP increased, 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT, a tetrazolium salt) colorimetric absorbance values (A values) decreased. These values in the PTHrP groups were significantly lower than those in the control group. MTT colorimetric A values and 3H-thymidine deoxyribose intake were lower in the VPL group, low-dose CAL group, and the PTHrP 10−7 mol/L group compared with the control group. However, MTT colorimetric A values and 3H-thymidine deoxyribose intake were higher in the high-dose CAL group than in the PTHrP 10−7 mol/L group. As PTHrP concentrations increased, intracellular cyclic adenosine monophosphate concentrations also increased. Conclusion PTHrp, VPL, and low-dose CAL inhibit proliferation of CFs, while high-dose CAL promotes proliferation of CFs.

scholarly journals Treadmill exercise does not change gene expression of adrenal catecholamine biosynthetic enzymes in chronically stressed rats

2013 ◽  
Vol 85 (3) ◽  
pp. 999-1012 ◽  
Author(s):  
LJUBICA GAVRILOVIC ◽  
VESNA STOJILJKOVIC ◽  
JELENA KASAPOVIC ◽  
NATASA POPOVIC ◽  
SNEZANA B. PAJOVIC ◽  
...  
Keyword(s):  
Gene Expression ◽  
Adrenal Medulla ◽  
Immobilization Stress ◽  
Treadmill Exercise ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Treadmill Running ◽  
Biosynthetic Enzymes ◽  
Adult Rats ◽  
Stress Changes

ABSTRACT Chronic isolation of adult animals represents a form of psychological stress that produces sympatho-adrenomedullar activation. Exercise training acts as an important modulator of sympatho-adrenomedullary system. This study aimed to investigate physical exercise-related changes in gene expression of catecholamine biosynthetic enzymes (tyrosine hydroxylase, dopamine-ß-hydroxylase and phenylethanolamine N-methyltransferase) and cyclic adenosine monophosphate response element-binding (CREB) in the adrenal medulla, concentrations of catecholamines and corticosterone (CORT) in the plasma and the weight of adrenal glands of chronically psychosocially stressed adult rats exposed daily to 20 min treadmill running for 12 weeks. Also, we examined how additional acute immobilization stress changes the mentioned parameters. Treadmill running did not result in modulation of gene expression of catecholamine synthesizing enzymes and it decreased the level of CREB mRNA in the adrenal medulla of chronically psychosocially stressed adult rats. The potentially negative physiological adaptations after treadmill running were recorded as increased concentrations of catecholamines and decreased morning CORT concentration in the plasma, as well as the adrenal gland hypertrophy of chronically psychosocially stressed rats. The additional acute immobilization stress increases gene expression of catecholamine biosynthetic enzymes in the adrenal medulla, as well as catecholamines and CORT levels in the plasma. Treadmill exercise does not change the activity of sympatho-adrenomedullary system of chronically psychosocially stressed rats.

Mechanisms of ATP to cAMP Conversion Catalyzed by the Mammalian Adenylyl Cyclase: A Role of Magnesium Coordination Shells and Proton Wires

2019 ◽  
Author(s):  
Bella Grigorenko ◽  
Igor Polyakov ◽  
Alexander Nemukhin
Keyword(s):  
Adenylyl Cyclase ◽  
Bond Cleavage ◽  
Cyclic Adenosine Monophosphate ◽  
Adenosine Monophosphate ◽  
Md Simulations ◽  
Coordination Shell ◽  
Energy Profile ◽  
One Step ◽  
Type V

<p>We report a mechanism of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) conversion by the mammalian type V adenylyl cyclase revealed in molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) simulations. We characterize a set of computationally derived enzyme-substrate (ES) structures showing an important role of coordination shells of magnesium ions in the solvent accessible active site. Several stable six-fold coordination shells of Mg<sub>A</sub><sup>2+ </sup>are observed in MD simulations of ES complexes. In the lowest energy ES conformation, the coordination shell of Mg<sub>A</sub><sup>2+ </sup>does not include the O<sub>δ1</sub> atom of the conserved Asp440 residue. Starting from this conformation, a one-step reaction mechanism is characterized which includes proton transfer from the ribose O<sup>3'</sup>H<sup>3' </sup>group in ATP to Asp440 via a shuttling water molecule and P<sup>A</sup>-O<sup>3A</sup> bond cleavage and O<sup>3'</sup>-P<sup>A</sup> bond formation. The energy profile of this route is consistent with the observed reaction kinetics. In a higher energy ES conformation, Mg<sub>A</sub><sup>2+</sup> is bound to the O<sub>δ1</sub>(Asp440) atom as suggested in the relevant crystal structure of the protein with a substrate analog. The computed energy profile initiated by this ES is characterized by higher energy expenses to complete the reaction. Consistently with experimental data, we show that the Asp440Ala mutant of the enzyme should exhibit a reduced but retained activity. All considered reaction pathways include proton wires from the O<sup>3'</sup>H<sup>3' </sup>group via shuttling water molecules. </p>

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