Biomedical applications of prokaryotic carbonic anhydrases

Phosphonamidates are the first phosphorus-based zinc binding motif to show inhibition of β-class carbonic anhydrases from bacteria, fungi, and protozoa

A primary strategy to combat antimicrobial resistance is the identification of novel therapeutic targets and anti-infectives with alternative mechanisms of action. The inhibition of the metalloenzymes carbonic anhydrases (CAs, EC 4.2.1.1) from pathogens (bacteria, fungi, and protozoa) was shown to produce an impairment of the microorganism growth and virulence. As phosphonamidates have been recently validated as human α-CA inhibitors (CAIs) and no phosphorus-based zinc-binding group have been assessed to date against β-class CAs, herein we report an inhibition study with this class of compounds against β-CAs from pathogenic bacteria, fungi, and protozoa. Our data suggest that phosphonamidates are among the CAIs with the best selectivity for β-class over human isozymes, making them interesting leads for the development of new anti-infectives.

Benzoxaboroles: New Potent Inhibitors of the Carbonic Anhydrases of the Pathogenic Bacterium Vibrio cholerae

A series of urea/thiourea substituted benzoxaboroles was investigated for the inhibition of the three carbonic anhydrases encoded by Vibrio cholerae (VchCAα, VchCAβ, and VchCAγ). In particular, benzoxaborole derivatives were here first assayed for the inhibition of a γ-class CA, extending the panel of CA classes that benzoxaboroles efficiently target beyond α and β. Inhibition profiles demonstrated that VchCAα was significantly more inhibited compared to VchCAγ and, in turn, more efficiently modulated than VchCAβ. Among the many selective benzoxaborole ligands detected against VchCAα over the off-target hCA II, compound 18, a p-NO₂-phenylthiourea derivative, even exhibited a fully selective inhibition profile against the three VchCAs over hCA II. A comprehensive ligand/target interaction study was performed in silico for all three VchCA isoforms providing the first molecular modeling investigation with inhibitors of a γ-class CA to the best of our knowledge. The present study reinforces the rationale behind the use of benzoxaboroles as innovative antibacterial agents with a new mechanism of action, furnishing suggestions for the rational design of new potent and selective inhibitors targeting V. cholerae CAs over human off-target ones.

Inhibition of the newly discovered β‑carbonic anhydrase from the protozoan pathogen Trichomonas vaginalis with inorganic anions and small molecules

The protozoan pathogen Trichomonas vaginalis encodes two carbonic anhydrases (CAs, EC 4.2.1.1) belonging to the β-class. One of these enzymes, T. vaginalis carbonic anhydrase 1 (TvaCA1), was recently cloned and characterized by our group, and its X-ray crystal structure reported. No inhibitors of this enzyme were reported up until now. Here we investigated the inhibition of TvaCA1 with inorganic anions and small molecules and observed that thiocyanate, cyanide, selenite, selenocyanate and divanadate are sub-millimolar inhibitors, whereas sulfamide, sulfate, phenylboronic acid and phenylarsonic acid are micromolar inhibitors. Finding effective TvaCA1 inhibitors may be useful for developing new antiprotozoan drugs.

Synthesis, Computational Studies and Assessment of in Vitro Activity of Squalene Derivatives as Carbonic Anhydrase Inhibitors

We report novel molecules incorporating the nontoxic squalene scaffold and different carbonic anhydrase inhibitors (CAIs). Potent inhibitory action, in the low-nanomolar range, was detected against isoforms hCA II for sulfonamide derivatives, which proved to be selective against this isoform over the tumor-associate hCA IX and XII isoforms. On the other hand, coumarin derivatives showed weak potency but high selectivity against the tumor-associated isoform CA IX. These compounds are interesting candidates for preclinical evaluation in glaucoma or various tumors in which the two enzymes are involved. In addition, an in silico study of inhibitor-bound hCA II revealed extensive interactions with the hydrophobic pocket of the active site and provided molecular insights into the binding properties of these new inhibitors.

Development of oxathiino[6,5-b]pyridine 2,2-dioxide derivatives as selective inhibitors of tumor-related carbonic anhydrases IX and XII

Oxathiino[6,5-b]pyridine 2,2-dioxides are identified as a new class of isoform-selective nanomolar inhibitors of tumor associated human carbonic anhydrases (hCA) IX and XII. At the same time they do not inhibit or poorly inhibit cytosolic isoforms hCA I and II. Oxathiino[6,5-b]pyridine 2,2-dioxides exhibited good antiproliferative properties on tumor cell lines MCF-7 (Human breast adenocarcinoma), A549 (human lung (alveolar) adenocarcinoma) and HeLa (epithelioid cervix carcinoma).

Sulfonamide Inhibitors of Human Carbonic Anhydrases Designed through a Three-Tails Approach: Improving Ligand/Isoform Matching and Selectivity of Action

The “tail approach” has become a milestone in human carbonic anhydrase inhibitor (hCAI) design for various therapeutics, including antiglaucoma agents. Besides the classical hydrophobic/hydrophilic division of hCAs active site, several subpockets have been identified at the middle/outer active sites rim, which could be targeted to increase the CAI isoform selectivity. This postulate is explored here by three-tailed benzenesulfonamide CAIs (TTI) to fully exploit such amino acid differences among hCAs. In this proof-of-concept study, an extensive structure–activity relationship (SAR) study was carried out with 32 such benzenesulfonamides differing in tails combination that were assayed for hCAs I, II, IV, and XII inhibition. A structural study was undertaken by X-ray crystallography and in silico tools to assess the ligand/target interaction mode. The most active and selective inhibitors against isoforms implicated in glaucoma were assessed in a rabbit model of the disease achieving an intraocular pressure-lowering action comparable to the clinically used dorzolamide.

The Effect of Substituted Benzene-Sulfonamides and Clinically Licensed Drugs on the Catalytic Activity of CynT2, a Carbonic Anhydrase Crucial for Escherichia coli Life Cycle

Proteins are relevant antimicrobial drug targets, and among them, enzymes represent a significant group, since most of them catalyze reactions essential for supporting the central metabolism, or are necessary for the pathogen vitality. Genomic exploration of pathogenic and non-pathogenic microorganisms has revealed genes encoding for a superfamily of metalloenzymes, known as carbonic anhydrases (CAs, EC 4.2.1.1). CAs catalyze the physiologically crucial reversible reaction of the carbon dioxide hydration to bicarbonate and protons. Herein, we investigated the sulfonamide inhibition profile of the recombinant β-CA (CynT2) identified in the genome of the Gram-negative bacterium Escherichia coli. This biocatalyst is indispensable for the growth of the microbe at atmospheric pCO₂. Surprisingly, this enzyme has not been investigated for its inhibition with any class of CA inhibitors. Here, we show that CynT2 was strongly inhibited by some substituted benzene-sulfonamides and the clinically used inhibitor sulpiride (K_Is in the range of 82–97 nM). This study may be relevant for identifying novel CA inhibitors, as well as for another essential part of the drug discovery pipeline, such as the structure–activity relationship for this class of enzyme inhibitors.

Crystal Structure of a Tetrameric Type II β-Carbonic Anhydrase from the Pathogenic Bacterium Burkholderia pseudomallei

Carbonic anhydrase (CA) is a zinc enzyme that catalyzes the reversible conversion of carbon dioxide to bicarbonate and proton. Currently, CA inhibitors are widely used as antiglaucoma, anticancer, and anti-obesity drugs and for the treatment of neurological disorders. Recently, the potential use of CA inhibitors to fight infections caused by protozoa, fungi, and bacteria has emerged as a new research line. In this article, the X-ray crystal structure of β-CA from Burkholderia pseudomallei was reported. The X-ray crystal structure of this new enzyme was solved at 2.7 Å resolution, revealing a tetrameric type II β-CA with a “closed” active site in which the zinc is tetrahedrally coordinated to Cys46, Asp48, His102, and Cys105. B. pseudomallei is known to encode at least two CAs, a β-CA, and a γ-CA. These proteins, playing a pivotal role in its life cycle and pathogenicity, offer a novel therapeutic opportunity to obtain antibiotics with a different mechanism of action. Furthermore, the new structure can provide a clear view of the β-CA mechanism of action and the possibility to find selective inhibitors for this class of CAs.

Exploring the multiple binding modes of inhibitors to carbonic anhydrases for novel drug discovery

INTRODUCTION

The spacious active site cavity of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1) shows a great versatility for a variety of binding modes for modulators of activity, inhibitors, and activators, some of which are clinically used drugs.

AREAS COVERED

There are at least four well-documented CA inhibition mechanisms and the same number of binding modes for CA inhibitors (CAIs), one of which superposes with the binding of activators (CAAs). They include (i) coordination to the catalytic metal ion; (ii) anchoring to the water molecule coordinated to the metal ion; (iii) occlusion of the active site entrance; and (iv) binding outside the active site. A large number of chemical classes of CAIs show these binding modes explored in detail by kinetic, crystallographic, and other techniques. The tail approach was applied to all of them and allowed many classes of highly isoform-selective inhibitors. This is the subject of our review.

EXPERT OPINION

All active site regions of CAs accommodate inhibitors to bind, which is reflected in very different inhibition profiles for such compounds and the possibility to design drugs with effective action and new applications, such as for the management of hypoxic tumors, neuropathic pain, cerebral ischemia, arthritis, and degenerative disorders.

A class of carbonic anhydrase IX/XII - selective carboxylate inhibitors

A small series of 2,4-dioxothiazolidinyl acetic acids was prepared from thiourea, chloroacetic acid, aromatic aldehydes, and ethyl-2-bromoacetate. They were assayed for the inhibition of four physiologically relevant carbonic anhydrase (CA, EC 4.2.1.1) isoforms of human (h) origin, the cytosolic hCA I and II, and the transmembrane hCA IX and XII, involved among others in tumorigenesis (hCA IX and XII) and glaucoma (hCA II and XII). The two cytosolic isoforms were not inhibited by these carboxylates, which were also rather ineffective as hCA IX inhibitors. On the other hand, they showed submicromolar hCA XII inhibition, with K_Is in the range of 0.30–0.93 µM, making them highly CA XII-selective inhibitors.

Anion Inhibition Studies of the β-Class Carbonic Anhydrase CAS3 from the Filamentous Ascomycete Sordaria macrospora

CAS3 is a newly cloned cytosolic β-class carbonic anhydrase (CA, EC 4.2.1.1) from the filamentous ascomycete Sordaria macrospora. This enzyme has a high catalytic activity for the physiological CO₂ hydration reaction and herein, we report the inhibition profile of CAS3 with anions and small molecules. The most effective CAS3 anions/small molecule inhibitors were diethyl-dithiocarbamate, sulfamide, sulfamate, phenyl boronic and phenyl arsonic acids, with K_Is in the range of 0.89 mM–97 µM. Anions such as iodide, the pseudohalides, bicarbonate, carbonate, nitrate, nitrite, hydrogensulfide, stannate, selenate, tellurate, tetraborate, perrhenate, perruthenate, selenocyanide and trithiocarbonate were low millimolar CAS3 inhibitors. The light halides, sulfate, hydrogensulfite, peroxydisulfate, diphosphate, divanadate, perchlorate, tetrafluoroborate, fluorosulfonate and iminodisulfonate did not significantly inhibit this enzyme. These data may be useful for developing antifungals based on CA inhibition, considering the fact that many of the inhibitors reported here may be used as lead molecules and, by incorporating the appropriate organic scaffolds, potent nanomolar inhibitors could be developed.

Sulfonamide Inhibition Studies of the β-Class Carbonic Anhydrase CAS3 from the Filamentous Ascomycete Sordaria macrospora

A new β-class carbonic anhydrase was cloned and purified from the filamentous ascomycete Sordaria macrospora, CAS3. This enzyme has a higher catalytic activity compared to the other two such enzymes from this fungus, CAS1 and CAS2, which were reported earlier, with the following kinetic parameters: k_cat of (7.9 ± 0.2) × 10⁵ s⁻¹, and k_cat/K_m of (9.5 ± 0.12) × 10⁷ M⁻¹∙s⁻¹. An inhibition study with a panel of sulfonamides and one sulfamate was also performed. The most effective CAS3 inhibitors were benzolamide, brinzolamide, dichlorophnamide, methazolamide, acetazolamide, ethoxzolamide, sulfanilamide, methanilamide, and benzene-1,3-disulfonamide, with K_Is in the range of 54–95 nM. CAS3 generally shows a higher affinity for this class of inhibitors compared to CAS1 and CAS2. As S. macrospora is a model organism for the study of fruiting body development in fungi, these data may be useful for developing antifungal compounds based on CA inhibition.

7-Acylamino-3H-1,2-benzoxathiepine 2,2-dioxides as new isoform-selective carbonic anhydrase IX and XII inhibitors

A series of 3H-1,2-benzoxathiepine 2,2-dioxides incorporating 7-acylamino moieties were obtained by an original procedure starting from 5-nitrosalicylaldehyde, which was treated with propenylsulfonyl chloride followed by Wittig reaction of the bis-olefin intermediate. The new derivatives, belonging to the homosulfocoumarin chemotype, were assayed as inhibitors of the zinc metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). Four pharmacologically relevant human (h) isoforms were investigated, the cytosolic hCA I and II and the transmembrane, tumour-associated hCA IX and XII. No relevant inhibition of hCA I and II was observed, whereas some of the new derivatives were effective, low nanomolar hCA IX/XII inhibitors, making them of interest for investigations in situations in which the activity of these isoforms is overexpressed, such as hypoxic tumours, arthritis or cerebral ischaemia.

Synthesis and biological evaluation of novel 3-(quinolin-4-ylamino)benzenesulfonamidesAQ3 as carbonic anhydrase isoforms I and II inhibitors

Carbonic anhydrases (CAs, EC 4.2.1.1) are crucial metalloenzymes that are involved in diverse bioprocesses. We report the synthesis and biological evaluation of novel series of benzenesulfonamides incorporating un/substituted ethyl quinoline-3-carboxylate moieties. The newly synthesised compounds were in vitro evaluated as inhibitors of the cytosolic human (h) isoforms hCA I and II. Both isoforms hCA I and II were inhibited by the quinolines reported here in variable degrees: hCA I was inhibited with KIs in the range of 0.966-9.091 μM, whereas hCA II in the range of 0.083-3.594 μM. The primary 7-chloro-6-flouro substituted sulphfonamide derivative 6e (KI = 0.083 μM) proved to be the most active quinoline in inhibiting hCA II, whereas, its secondary sulfonamide analog failed to inhibit the hCA II up to 10 μM, confirming the crucial role of the primary sulphfonamide group, as a zinc-binding group for CA inhibitory activity.

Identification and characterization of the α-CA in the outer membrane vesicles produced by Helicobacter pylori

The genome of Helicobacter pylori encodes for carbonic anhydrases (CAs, EC 4.2.1.1) belonging to the α- and β-CA classes, which together with urease, have a pivotal role in the acid acclimation of the microorganism within the human stomach. Recently, in the exoproteome of H. pylori, a CA with no indication of the corresponding class was identified. Here, using the protonography and the mass spectrometry, a CA belonging to the α-class was detected in the outer membrane vesicles (OMVs) generated by planktonic and biofilm phenotypes of four H. pylori strains. The amount of this metalloenzyme was higher in the planktonic OMVs (pOMVs) than in the biofilm OMVs (bOMVs). Furthermore, the content of α-CA increases over time in the pOMVs. The identification of the α-CA in pOMVs and bOMVs might shed new light on the role of this enzyme in the colonization, survival, persistence, and pathogenesis of H. pylori.

Design, synthesis and biological activity of selective hCAs inhibitors based on 2-(benzylsulfinyl)benzoic acid scaffold

A large library of derivatives based on the scaffold of 2-(benzylsulfinyl)benzoic acid were synthesised and tested as atypical inhibitors against four different isoforms of human carbonic anhydrase (hCA I, II, IX and XII, EC 4.2.1.1). The exploration of the chemical space around the main functional groups led to the discovery of selective hCA IX inhibitors in the micromolar/nanomolar range, thus establishing robust structure-activity relationships within this versatile scaffold. HPLC separation of some selected chiral compounds and biological evaluation of the corresponding enantiomers was performed along with molecular modelling studies on the most active derivatives.

Thermostability enhancement of the α-carbonic anhydrase from Sulfurihydrogenibium yellowstonense by using the anchoring-and-self-labelling-protein-tag system (ASLtag)

Carbonic anhydrases (CAs, EC 4.2.1.1) are a superfamily of ubiquitous metalloenzymes present in all living organisms on the planet. They are classified into seven genetically distinct families and catalyse the hydration reaction of carbon dioxide to bicarbonate and protons, as well as the opposite reaction. CAs were proposed to be used for biotechnological applications, such as the post-combustion carbon capture processes. In this context, there is a great interest in searching CAs with robust chemical and physical properties. Here, we describe the enhancement of thermostability of the α-CA from Sulfurihydrogenibium yellowstonense (SspCA) by using the anchoring-and-self-labelling-protein-tag system (ASL^tag). The anchored chimeric H⁵-SspCA was active for the CO₂ hydration reaction and its thermostability increased when the cells were heated for a prolonged period at high temperatures (e.g. 70 °C). The ASL^tag can be considered as a useful method for enhancing the thermostability of a protein useful for biotechnological applications, which often need harsh operating conditions.

Cloning, Purification, and Characterization of a β-Carbonic Anhydrase from Malassezia restricta, an Opportunistic Pathogen Involved in Dandruff and Seborrheic Dermatitis

The cloning, purification, and initial characterization of the β-carbonic anhydrase (CA, EC 4.2.1.1) from the genome of the opportunistic pathogen Malassezia restricta (MreCA), which a fungus involved in dandruff and seborrheic dermatitis (SD), is reported. MreCA is a protein consisting of 230 amino acid residues and shows high catalytic activity for the hydration of CO₂ into bicarbonate and protons, with the following kinetic parameters: k_cat of 1.06 × 10⁶ s⁻¹ and k_cat/K_M of 1.07 × 10⁸ M⁻¹ s⁻¹. It is also sensitive to inhibition by the sulfonamide acetazolamide (K_I of 50.7 nM). Phylogenetically, MreCA and other CAs from various Malassezia species seem to be on a different branch, distinct from that of other β-CAs found in fungi, such as Candida spp., Saccharomyces cerevisiae, Aspergillus fumigatus, and Sordaria macrospora, with only Cryptococcus neoformans and Ustilago maydis enzymes clustering near MreCA. The further characterization of this enzyme and the identification of inhibitors that may interfere with its life cycle might constitute new strategies for fighting dandruff and SD.

Click-tailed benzenesulfonamides as potent bacterial carbonic anhydrase inhibitors for targeting Mycobacterium tuberculosis and Vibrio cholerae

A series of 1,2,3-triazole-bearing benzenesulfonamides was assessed for the inhibition of carbonic anhydrases (CA, EC 4.2.1.1) from bacteria Vibrio cholerae (VchCAα and VchCAβ) and Mycobacterium tuberculosis (β-mtCA3). Growing resistance phenomena against existing antimicrobial drugs are globally spreading and highlight a urgent need of agents endowed with alternative mechanisms of action. Two global WHO strategies aim to reduce cholera deaths by 90% and eradicate the tuberculosis epidemic by 2030. The derivatives here reported represent interesting leads towards the optimization of new antibiotic agents showing excellent inhibitory efficiency and selectivity for the target CAs over the human (h) off-target isoform hCA I. In detail, the first subset of derivatives potently inhibits VchCAα in a low nanomolar range (KIs between 0.72 and 22.6 nM). Compounds of a second subset, differing from the first one for the position of the spacer between benzenesulfonamide and triazole, preferentially inhibit VchCAβ (KIs in the range 54.8-102.4 nM) and β-mtCA3 (KIs in the range 28.2-192.5 nM) even more than the clinically used AAZ, used as the standard.

Activation Studies of the γ-Carbonic Anhydrases from the Antarctic Marine Bacteria Pseudoalteromonas haloplanktis and Colwellia psychrerythraea with Amino Acids and Amines

The γ-carbonic anhydrases (CAs, EC 4.2.1.1) present in the Antarctic marine bacteria Pseudoalteromonas haloplanktis and Colwellia psychrerythraea, herein referred to as PhaCA and CpsCA, respectively, were investigated for their activation with a panel of 24 amino acids and amines. Both bacteria are considered Antarctic models for the investigation of photosynthetic and metabolic pathways in organisms adapted to live in cold seawater. PhaCA was much more sensitive to activation by these compounds compared to the genetically related enzyme CpsCA. The most effective PhaCA activators were d-Phe, l-/d-DOPA, l-Tyr and 2-pyridyl-methylamine, with the activation constant K_A values of 0.72–3.27 µM. d-His, l-Trp, d-Tyr, histamine, dopamine, serotonin anddicarboxylic amino acids were also effective activators of PhaCA, with K_A values of 6.48–9.85 µM. CpsCA was activated by d-Phe, d-DOPA, l-Trp, l-/d-Tyr, 4-amino-l-Phe, histamine, 2-pyridyl-methylamine and l-/d-Glu with K_A values of 11.2–24.4 µM. The most effective CpsCA activator was l-DOPA (K_A of 4.79 µM). Given that modulators of CAs from Antarctic bacteria have not been identified and investigated in detail for their metabolic roles to date, this research sheds some light on these poorly understood processes.

New sulfonamides containing organometallic-acylhydrazones: synthesis, characterisation and biological evaluation as inhibitors of human carbonic anhydrases

A series of organometallic acylhydrazones was prepared, incorporating Re(CO)₃, Mn(CO)₃ and ferrocenyl moieties, which were subsequently reacted with amino-sulfonamides in order to obtain carbonic anhydrase (CA, EC 4.2.1.1) inhibitors possessing organometallic moieties in their molecules. The new derivatives were investigated as inhibitors of four human (h) CA isoforms with pharmaceutical applications, such as the cytosolic hCA I, II and VII and the mitochondrial hCA VA. An interesting inhibitory profile against these isoforms was obtained, with some of these metal complexes acting as subnanomolar or low nanomolar inhibitors. They were also thoroughly characterised from the chemical point of view, making them of interest for further developments in the field of metal complexes of sulfonamides with CA inhibitory action.

Comparison of the Sulfonamide Inhibition Profiles of the α-Carbonic Anhydrase Isoforms (SpiCA1, SpiCA2 and SpiCA3) Encoded by the Genome of the Scleractinian Coral Stylophora pistillata

The ubiquitous metalloenzymes carbonic anhydrases (CAs, EC 4.2.1.1) are responsible for the reversible hydration of CO₂ to bicarbonate (HCO₃-) and protons (H⁺). Bicarbonate may subsequently generate carbonate used in many functional activities by marine organisms. CAs play a crucial role in several physiological processes, e.g., respiration, inorganic carbon transport, intra and extra-cellular pH regulation, and bio-mineralization. Multiple transcript variants and protein isoforms exist in the organisms. Recently, 16 α-CA isoforms have been identified in the coral Stylophora pistillata. Here, we focalized the interest on three coral isoforms: SpiCA1 and SpiCA2, localized in the coral-calcifying cells; and SpiCA3, expressed in the cytoplasm of the coral cell layers. The three recombinant enzymes were heterologously expressed and investigated for their inhibition profiles with sulfonamides and sulfamates. The three coral CA isoforms differ significantly in their susceptibility to inhibition with sulfonamides. This study provides new insights into the coral physiology and the comprehension of molecular mechanisms involved in the bio-mineralization processes, since CAs interact with bicarbonate transporters, accelerating the trans-membrane bicarbonate movement and modulating the pH at both sides of the plasma membranes.

Novel 2-indolinones containing a sulfonamide moiety as selective inhibitors of candida β-carbonic anhydrase enzyme

Inhibition of the β-carbonic anhydrase (CA, EC 4.2.1.1) from pathogenic Candida glabrata (CgNce103) by 1H-indole-2,3-dione 3-[N-(4-sulfamoylphenyl)thiosemicarbazones] 4a–m was investigated. All the compounds were found to be potent inhibitors of CgNce103, with inhibition constants in the range of 6.4-63.9 nM. The 5,7-dichloro substituted derivative 4l showed the most effective inhibition (K_I of 6.4 nM) as well as the highest selectivity for inhibiting CgNce103 over the cytosolic human (h) isoforms hCA I and II. A possible binding interaction of compound 4l within the active site of CgNce103 has been proposed based on docking studies.

Activation Studies of the β-Carbonic Anhydrase from the Pathogenic Protozoan Entamoeba histolytica with Amino Acids and Amines

The β-carbonic anhydrase (CA, EC 4.2.1.1) from the pathogenic protozoan Entamoeba histolytica, EhiCA, was investigated for its activation with a panel of natural and non-natural amino acids and amines. EhiCA was potently activated by D-His, D-Phe, D-DOPA, L- and D-Trp, L- and D-Tyr, 4-amino-L-Tyr, histamine and serotonin, with K_As ranging between 1.07 and 10.1 µM. The best activator was D-Tyr (K_A of 1.07 µM). L-Phe, L-DOPA, L-adrenaline, L-Asn, L-Asp, L-Glu and L-Gln showed medium potency activation, with K_As of 16.5–25.6 µM. Some heterocyclic- alkyl amines, such as 2-pyridyl-methyl/ethyl-amine and 4-(2-aminoethyl)-morpholine, were devoid of EhiCA activating properties with K_As > 100 µM. As CA activators have poorly been investigated for their interaction with protozoan CAs, our study may be relevant for an improved understanding of the role of this enzyme in the life cycle of E. histolytica.

Novel approaches for designing drugs that interfere with pH regulation

INTRODUCTION

In all living species, pH regulation is a tightly controlled process, with a plethora of proteins involved in its regulation. These include sodium-proton exchangers, carbonic anhydrases, anion exchangers, bicarbonate transporters/cotransporters, H⁺-ATPases, and monocarboxylate transporters. All of them play crucial roles in acid-base balancing, both in eukaryotic as well as in prokaryotic organisms, making them interesting drug targets for the management of pathological events (in)directly involved in pH regulation. Areas covered: Interfering with pH regulation for the treatment of tumors and microbial infections is the main focus of this review, with particular attention paid to inhibitors targeting the above-mentioned proteins. The latest advances in each field id reviewed. Expert opinion: Interfering with the pH regulation of tumor cells is a validated approach to tackle primary tumors and metastases growth. Carbonic anhydrases are the most investigated proteins of those aforementioned, with several inhibitors in clinical development. Recent advances in the characterization of proteins involved in pH homeostasis of various pathogens evidenced their crucial role in the survival and virulence of bacterial, fungal, and protozoan microorganisms. Some encouraging results shed light on the possibility to target such proteins for obtaining new anti-infectives, overcoming the extensive drug resistance problems of clinically used drugs.

Sulfonamide Inhibition Studies of a New β-Carbonic Anhydrase from the Pathogenic Protozoan Entamoeba histolytica

A newly described β-carbonic anhydrase (CA, EC 4.2.1.1) from the pathogenic protozoan Entamoeba histolytica, EhiCA, was recently shown to possess a significant catalytic activity for the physiologic CO₂ hydration reaction (k_cat of 6.7 × 10⁵ s⁻¹ and a k_cat/K_m of 8.9 × 10⁷ M⁻¹ s⁻¹). A panel of sulfonamides and one sulfamate, some of which are clinically used drugs, were investigated for their inhibitory properties against EhiCA. The best inhibitors detected in the study were 4-hydroxymethyl/ethyl-benzenesulfonamide (K_Is of 36–89 nM), whereas some sulfanilyl-sulfonamides showed activities in the range of 285–331 nM. Acetazolamide, methazolamide, ethoxzolamide, and dichlorophenamide were less effective inhibitors (K_Is of 509–845 nM) compared to other sulfonamides investigated here. As β-CAs are not present in vertebrates, the present study may be useful for detecting lead compounds for the design of more effective inhibitors with potential to develop anti-infectives with alternative mechanisms of action.

Cloning, Characterization and Anion Inhibition Studies of a β-Carbonic Anhydrase from the Pathogenic Protozoan Entamoeba histolytica

We report the cloning and catalytic activity of a β-carbonic anhydrase (CA, EC 4.2.1.1), isolated from the pathogenic protozoan Entamoeba histolytica, EhiCA. This enzyme has a high catalytic activity for the physiologic CO₂ hydration reaction, with a k_cat of 6.7 × 10⁵ s⁻¹ and a k_cat/K_m of 8.9 × 10⁷ M⁻¹ × s⁻¹. An anion inhibition study of EhiCA with inorganic/organic anions and small molecules revealed that fluoride, chloride, cyanide, azide, pyrodiphosphate, perchlorate, tetrafluoroborate and sulfamic acid did not inhibit the enzyme activity, whereas pseudohalides (cyanate and thiocyanate), bicarbonate, nitrate, nitrite, diethyldithiocarbamate, and many complex inorganic anions showed inhibition in the millimolar range (K_Is of 0.51–8.4 mM). The best EhiCA inhibitors were fluorosulfonate, sulfamide, phenylboronic acid and phenylarsonic acid (K_Is in the range of 28–86 μM). Since β-CAs are not present in vertebrates, the present study may be useful for detecting lead compounds for the design of effective enzyme inhibitors, with potential to develop anti-infectives with alternative mechanisms of action.