Structural insight into activity enhancement and inhibition of H64A carbonic anhydrase II by imidazoles

Carbonic Anhydrase Activators for Neurodegeneration: An Overview

Carbonic anhydrases (CAs) are a family of ubiquitous metal enzymes catalyzing the reversible conversion of CO₂ and H₂O to HCO₃⁻ with the release of a proton. They play an important role in pH regulation and in the balance of body fluids and are involved in several functions such as homeostasis regulation and cellular respiration. For these reasons, they have been studied as targets for the development of agents for treating several pathologies. CA inhibitors have been used in therapy for a long time, especially as diuretics and for the treatment of glaucoma, and are being investigated for application in other pathologies including obesity, cancer, and epilepsy. On the contrary, CAs activators are still poorly studied. They are proposed to act as additional (other than histidine) proton shuttles in the rate-limiting step of the CA catalytic cycle, which is the generation of the active hydroxylated enzyme. Recent studies highlight the involvement of CAs activation in brain processes essential for the transmission of neuronal signals, suggesting CAs activation might represent a potential therapeutic approach for the treatment of Alzheimer’s disease and other conditions characterized by memory impairment and cognitive problems. Actually, some compounds able to activate CAs have been identified and proposed to potentially resolve problems related to neurodegeneration. This review reports on the primary literature regarding the potential of CA activators for treating neurodegeneration-related diseases.

New Histamine-Related Five-Membered N-Heterocycle Derivatives as Carbonic Anhydrase I Activators

A series of histamine (HST)-related compounds were synthesized and tested for their activating properties on five physiologically relevant human Carbonic Anhydrase (hCA) isoforms (I, II, Va, VII and XIII). The imidazole ring of HST was replaced with different 5-membered heterocycles and the length of the aliphatic chain was varied. For the most interesting compounds some modifications on the terminal amino group were also performed. The most sensitive isoform to activation was hCA I (K_A values in the low micromolar range), but surprisingly none of the new compounds displayed activity on hCA II. Some derivatives (1, 3a and 22) displayed an interesting selectivity for activating hCA I over hCA II, Va, VII and XIII.

Amine- and Amino Acid-Based Compounds as Carbonic Anhydrase Activators

After being rather neglected as a research field in the past, carbonic anhydrase activators (CAAs) were undoubtedly demonstrated to be useful in diverse pharmaceutical and industrial applications. They also improved the knowledge of the requirements to selectively interact with a CA isoform over the others and confirmed the catalytic mechanism of this class of compounds. Amino acid and amine derivatives were the most explored in in vitro, in vivo and crystallographic studies as CAAs. Most of them were able to activate human or non-human CA isoforms in the nanomolar range, being proposed as therapeutic and industrial tools. Some isoforms are better activated by amino acids than amines derivatives and the stereochemistry may exert a role. Finally, non-human CAs have been very recently tested for activation studies, paving the way to innovative industrial and environmental applications.

Post-translational modifications in tumor-associated carbonic anhydrases

Human carbonic anhydrases IX (hCA IX) and XII (hCA XII) are two proteins associated with tumor formation and development. These enzymes have been largely investigated both from a biochemical and a functional point of view. However, limited data are currently available on the characterization of their post-translational modifications (PTMs) and the functional implication of these structural changes in the tumor environment. In this review, we summarize existing literature data on PTMs of hCA IX and hCA XII, such as disulphide bond formation, phosphorylation, O-/N-linked glycosylation, acetylation and ubiquitination, highlighting, when possible, their specific role in cancer pathological processes.

Conformational Change of H64 and Substrate Transportation: Insight Into a Full Picture of Enzymatic Hydration of CO2 by Carbonic Anhydrase

The enzymatic hydration of CO₂ into HCO₃ ^- by carbonic anhydrase (CA) is highly efficient and environment-friendly measure for CO₂ sequestration. Here extensive MM MD and QM/MM MD simulations were used to explore the whole enzymatic process, and a full picture of the enzymatic hydration of CO₂ by CA was achieved. Prior to CO₂ hydration, the proton transfer from the water molecule (WT1) to H64 is the rate-limiting step with the free energy barrier of 10.4 kcal/mol, which leads to the ready state with the Zn-bound OH^-. The nucleophilic attack of OH^- on CO₂ produces HCO₃ ^- with the free energy barrier of 4.4 kcal/mol and the free energy release of about 8.0 kcal/mol. Q92 as the key residue manipulates both CO₂ transportation to the active site and release of HCO₃ ^-. The unprotonated H64 in CA prefers in an inward orientation, while the outward conformation is favorable energetically for its protonated counterpart. The conformational transition of H64 between inward and outward correlates with its protonation state, which is mediated by the proton transfer and the product release. The whole enzymatic cycle has the free energy span of 10.4 kcal/mol for the initial proton transfer step and the free energy change of -6.5 kcal/mol. The mechanistic details provide a comprehensive understanding of the entire reversible conversion of CO₂ into bicarbonate and roles of key residues in chemical and nonchemical steps for the enzymatic hydration of CO₂.

Human carbonic anhydrases and post-translational modifications: a hidden world possibly affecting protein properties and functions

Human carbonic anhydrases (CAs) have become a well-recognized target for the design of inhibitors and activators with biomedical applications. Accordingly, an enormous amount of literature is available on their biochemical, functional and structural aspects. Nevertheless post-translational modifications (PTMs) occurring on these enzymes and their functional implications have been poorly investigated so far. To fill this gap, in this review we have analysed all PTMs occurring on human CAs, as deriving from the search in dedicated databases, showing a widespread occurrence of modification events in this enzyme family. By combining these data with sequence alignments, inspection of 3 D structures and available literature, we have summarised the possible functional implications of these PTMs. Although in some cases a clear correlation between a specific PTM and the CA function has been highlighted, many modification events still deserve further dedicated studies.

Structural insights into the effect of active-site mutation on the catalytic mechanism of carbonic anhydrase

Enzymes are catalysts of biological processes. Significant insight into their catalytic mechanisms has been obtained by relating site-directed mutagenesis studies to kinetic activity assays. However, revealing the detailed relationship between structural modifications and functional changes remains challenging owing to the lack of information on reaction intermediates and of a systematic way of connecting them to the measured kinetic parameters. Here, a systematic approach to investigate the effect of an active-site-residue mutation on a model enzyme, human carbonic anhydrase II (CA II), is described. Firstly, structural analysis is performed on the crystallographic intermediate states of native CA II and its V143I variant. The structural comparison shows that the binding affinities and configurations of the substrate (CO2) and product (HCO3 -) are altered in the V143I variant and the water network in the water-replenishment pathway is restructured, while the proton-transfer pathway remains mostly unaffected. This structural information is then used to estimate the modifications of the reaction rate constants and the corresponding free-energy profiles of CA II catalysis. Finally, the obtained results are used to reveal the effect of the V143I mutation on the measured kinetic parameters (k cat and k cat/K m) at the atomic level. It is believed that the systematic approach outlined in this study may be used as a template to unravel the structure-function relationships of many other biologically important enzymes.

Sulfonamide-Based Azaheterocyclic Schiff Base Derivatives as Potential Carbonic Anhydrase Inhibitors: Synthesis, Cytotoxicity, and Enzyme Inhibitory Kinetics

A series of sulfonamide-bearing azaheterocyclic Schiff base derivatives 3(a-j) were synthesized as carbonic anhydrase inhibitors. The substituted benzene sulfonyl chlorides 1(a-d) were reacted with N2H4 to get aromatic sulfonyl hydrazides 2(a-d). The intermediate hydrazides 2(a-d) were treated with substituted aldehydes to afford azaheterocyclic sulfonamide Schiff bases 3(a-j). The spectral data of synthesized compounds confirmed the formation of the final products. The inhibitory effects of 3(a-j) on carbonic anhydrase activity were determined, and it was found that derivative 3c exhibited the most potent activity with IC500.84 ± 0.12 μM among all other derivatives and is also more active than standard acetazolamide (IC500.91 ± 0.12). The enzyme inhibitory kinetics results determined by Lineweaver-Burk plots revealed that compound 3c inhibits the enzyme by noncompetitive mode of inhibition with K i value 8.6 μM. The molecular docking investigations of the synthesized analogues 3(a-j) were evaluated which assured that synthesized compounds bind well inside the active binding site of the target enzyme. Cytotoxicity on human keratinocyte (HaCaT) and MCF-7 cell lines was performed, and it was found that most of the synthesized analogues were nontoxic on these cell lines and the toxic effects follow the dose-dependent manner. Based on our investigations, it was suggested that analogue 3c may serve as core structure to project carbonic anhydrase inhibitors with greater potency.

Carbonic anhydrase modification for carbon management

Carbonic anhydrase modification (chemical and biological) is an attractive strategy for its diverse application to accelerate the absorption of CO₂ from a flue gas with improved activity and stability. This article reports various possibilities of CA modification using metal-ligand homologous chemistry, cross-linking agents, and residue- and group-specific and genetic modifications, and assesses their role in carbon management. Chemically modified carbonic anhydrase is able to improve the absorption of carbon dioxide from a gas stream into mediation compounds with enhanced sequestration and mineral formation. Genetically modified CA polypeptide can also increase carbon dioxide conversion. Chemical modification of CA can be categorized in terms of (i) residue-specific modification (involves protein-ligand interaction in terms of substitution/addition) and group-specific modifications (based on the functional groups of the target CA). For every sustainable change, there should be no/limited toxic or immunological response. In this review, several CA modification pathways and biocompatibility rules are proposed as a theoretical support for emerging research in this area.

Thermodynamic, kinetic, and structural parameterization of human carbonic anhydrase interactions toward enhanced inhibitor design

The aim of rational drug design is to develop small molecules using a quantitative approach to optimize affinity. This should enhance the development of chemical compounds that would specifically, selectively, reversibly, and with high affinity interact with a target protein. It is not yet possible to develop such compounds using computational (i.e., in silico) approach and instead the lead molecules are discovered in high-throughput screening searches of large compound libraries. The main reason why in silico methods are not capable to deliver is our poor understanding of the compound structure-thermodynamics and structure-kinetics correlations. There is a need for databases of intrinsic binding parameters (e.g., the change upon binding in standard Gibbs energy (ΔGint), enthalpy (ΔHint), entropy (ΔSint), volume (ΔVintr), heat capacity (ΔCp,int), association rate (ka,int), and dissociation rate (kd,int)) between a series of closely related proteins and a chemically diverse, but pharmacophoric group-guided library of compounds together with the co-crystal structures that could help explain the structure-energetics correlations and rationally design novel compounds. Assembly of these data will facilitate attempts to provide correlations and train data for modeling of compound binding. Here, we report large datasets of the intrinsic thermodynamic and kinetic data including over 400 primary sulfonamide compound binding to a family of 12 catalytically active human carbonic anhydrases (CA). Thermodynamic parameters have been determined by the fluorescent thermal shift assay, isothermal titration calorimetry, and by the stopped-flow assay of the inhibition of enzymatic activity. Kinetic measurements were performed using surface plasmon resonance. Intrinsic thermodynamic and kinetic parameters of binding were determined by dissecting the binding-linked protonation reactions of the protein and sulfonamide. The compound structure-thermodynamics and kinetics correlations reported here helped to discover compounds that exhibited picomolar affinities, hour-long residence times, and million-fold selectivities over non-target CA isoforms. Drug-lead compounds are suggested for anticancer target CA IX and CA XII, antiglaucoma CA IV, antiobesity CA VA and CA VB, and other isoforms. Together with 85 X-ray crystallographic structures of 60 compounds bound to six CA isoforms, the database should be of help to continue developing the principles of rational target-based drug design.

Structural Mapping of Anion Inhibitors to β-Carbonic Anhydrase psCA3 from Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative facultative anaerobe belonging to the Pseudomonadaceae family. It is a multidrug-resistant opportunistic human pathogen, a common cause of life-threatening nosocomial infections, and a key bacterial agent in cystic fibrosis and endocarditis. The bacterium exhibits intrinsic resistance to most antibacterial agents, including aminoglycosides and quinolones. Hence, the identification of new drug targets for P. aeruginosa is ongoing. PsCA3 is a β-class carbonic anhydrase (β-CA) that catalyzes the reversible hydration of carbon dioxide to bicarbonate and represents a new class of antimicrobial target. Previously, inhibitor screening studies of psCA3 have shown that a series of small anions including sulfamide (SFN), imidazole (IMD), and 4-methylimidazole (4MI), and thiocyanate (SCN) inhibit the enzyme with efficiencies in the micro- to millimolar range. Herein the X-ray crystal structures of these inhibitors in complex with psCA3 are presented and compared with human CA II. This structural survey into the binding modes of small anions forms the foundation for the development of inhibitors against β-CAs and more selective inhibitors against P. aeruginosa.

The Crystal Structure of a hCA VII Variant Provides Insights into the Molecular Determinants Responsible for Its Catalytic Behavior

Although important progress has been achieved in understanding the catalytic mechanism of Carbonic Anhydrases, a detailed picture of all factors influencing the catalytic efficiency of the various human isoforms is still missing. In this paper we report a detailed structural study and theoretical pKa calculations on a hCA VII variant. The obtained data were compared with those already known for another thoroughly investigated cytosolic isoform, hCA II. Our structural studies show that in hCA VII the network of ordered water molecules, which connects the zinc bound solvent molecule to the proton shuttle His64, is altered compared to hCA II, causing a reduction of the catalytic efficiency. Theoretical calculations suggest that changes in solvent network are related to the difference in pKa of the proton shuttle in the two enzymes. The residue that plays a major role in determining the diverse pKa values of the proton shuttle is the one in position four, namely His for hCA II and Gly for hCA VII. This residue is located on the protein surface, outside of the active site cavity. These findings are in agreement with our previous studies that highlighted the importance of histidines on the protein surface of hCA II (among which His4) as crucial residues for the high catalytic efficiency of this isoform.

Carbonic anhydrase activators

Mammalian carbonic anhydrases (CAs; EC 4.2.1.1) of which 16 isoforms are known, are involved in important physiological functions. Their inhibition is exploited pharmacologically for the treatment of many diseases (glaucoma, edema, epilepsy, obesity, hypoxic tumors, neuropathic pain, etc.) but the activators were less investigated till recently. A review on the CA activation is presented, with the activation mechanism, drug design approaches of activators and comparison of the various isoforms activation profiles being discussed. Some CAs, which are abundant in the brain, were recently demonstrated to be activatable by drug-like compounds, affording the possibility to design agents that enhance cognition, with potential therapeutic applications in aging and neurodegenerative diseases as well as tissue engineering.

Crystal Structure of Carbonic Anhydrase II in Complex with an Activating Ligand: Implications in Neuronal Function

Carbonic anhydrase (CA) plays a key role in neuronal signaling, providing bicarbonate and proton ions for GABAergic and glutamatergic neuronal function. Activation of CA isoforms expressed in neurons have been shown to have implications in the prognosis of Alzheimer's disease and dementia, while inhibitors of CAs are clinically used in the treatment of epilepsy, emphasizing the importance of this family of enzymes in both disease and normal neuronal function. Previously, compounds have been reported to enhance activity of CAs in an aging rat model, but their mechanism of action was not known. We report the 1.6 Å resolution structure of an imidazole-based CA activator in complex with the ubiquitously-expressed human CA II. Based on the structure, a proposed mechanism of CA activation by the compound and its potential applications in the neurobiology of aging are discussed.

"To Be or Not to Be" Protonated: Atomic Details of Human Carbonic Anhydrase-Clinical Drug Complexes by Neutron Crystallography and Simulation

Human carbonic anhydrases (hCAs) play various roles in cells, and have been drug targets for decades. Sequence similarities of hCA isoforms necessitate designing specific inhibitors, which requires detailed structural information for hCA-inhibitor complexes. We present room temperature neutron structures of hCA II in complex with three clinical drugs that provide in-depth analysis of drug binding, including protonation states of the inhibitors, hydration water structure, and direct visualization of hydrogen-bonding networks in the enzyme's active site. All sulfonamide inhibitors studied bind to the Zn metal center in the deprotonated, anionic, form. Other chemical groups of the drugs can remain neutral or be protonated when bound to hCA II. MD simulations have shown that flexible functional groups of the inhibitors may alter their conformations at room temperature and occupy different sub-sites. This study offers insights into the design of specific drugs to target cancer-related hCA isoform IX.

Polyamines and α-Carbonic Anhydrases

Natural products represent a straightforward source for molecular structures bearing a vast array of chemical features and potentially useful for biomedical purposes. Recent examples of this type include the discovery of the coumarins and the polyamine natural products as atypical chemotypes for the inhibition of the metalloenzymes carbonic anhydrases (CAs; EC 4.2.2.1). CA enzymes are established pharmacological targets for important pathologies, which, among others, include glaucoma, hypoxic tumors, and central nervous system (CNS)-affecting diseases. Moreover, they are expressed in many bacteria, fungi and helminths which are the etiological agents of the majority of infectious diseases. In this context, natural products represent the ideal source of new and selective druggable CA modulators for biomedical purposes. Herein we report the state of the art on polyamines of natural origin as well as of synthetic derivatives as inhibitors of human CAs.

Neutron structure of human carbonic anhydrase II in complex with methazolamide: mapping the solvent and hydrogen-bonding patterns of an effective clinical drug

Carbonic anhydrases (CAs; EC 4.2.1.1) catalyze the interconversion of CO2 and HCO3-, and their inhibitors have long been used as diuretics and as a therapeutic treatment for many disorders such as glaucoma and epilepsy. Acetazolamide (AZM) and methazolamide (MZM, a methyl derivative of AZM) are two of the classical CA inhibitory drugs that have been used clinically for decades. The jointly refined X-ray/neutron structure of MZM in complex with human CA isoform II (hCA II) has been determined to a resolution of 2.2 Å with an Rcryst of ∼16.0%. Presented in this article, along with only the second neutron structure of a clinical drug-bound hCA, is an in-depth structural comparison and analyses of differences in hydrogen-bonding network, water-molecule orientation and solvent displacement that take place upon the binding of AZM and MZM in the active site of hCA II. Even though MZM is slightly more hydrophobic and displaces more waters than AZM, the overall binding affinity (Ki) for both of the drugs against hCA II is similar (∼10 nM). The plausible reasons behind this finding have also been discussed using molecular dynamics and X-ray crystal structures of hCA II-MZM determined at cryotemperature and room temperature. This study not only allows a direct comparison of the hydrogen bonding, protonation states and solvent orientation/displacement of AZM and MZM, but also shows the significant effect that the methyl derivative has on the solvent organization in the hCA II active site.

Coupling Protein Dynamics with Proton Transport in Human Carbonic Anhydrase II

The role of protein dynamics in enzyme catalysis is one of the most highly debated topics in enzymology. The main controversy centers around what may be defined as functionally significant conformational fluctuations and how, if at all, these fluctuations couple to enzyme catalyzed events. To shed light on this debate, the conformational dynamics along the transition path surmounting the highest free energy barrier have been herein investigated for the rate limiting proton transport event in human carbonic anhydrase (HCA) II. Special attention has been placed on whether the motion of an excess proton is correlated with fluctuations in the surrounding protein and solvent matrix, which may be rare on the picosecond and subpicosecond time scales of molecular motions. It is found that several active site residues, which do not directly participate in the proton transport event, have a significant impact on the dynamics of the excess proton. These secondary participants are shown to strongly influence the active site environment, resulting in the creation of water clusters that are conducive to fast, moderately slow, or slow proton transport events. The identification and characterization of these secondary participants illuminates the role of protein dynamics in the catalytic efficiency of HCA II.

Non-Classical Inhibition of Carbonic Anhydrase

Specific isoforms from the carbonic anhydrase (CA) family of zinc metalloenzymes have been associated with a variety of diseases. Isoform-specific carbonic anhydrase inhibitors (CAIs) are therefore a major focus of attention for specific disease treatments. Classical CAIs, primarily sulfonamide-based compounds and their bioisosteres, are examined as antiglaucoma, antiepileptic, antiobesity, antineuropathic pain and anticancer compounds. However, many sulfonamide compounds inhibit all CA isoforms nonspecifically, diluting drug effectiveness and causing undesired side effects due to off-target inhibition. In addition, a small but significant percentage of the general population cannot be treated with sulfonamide-based compounds due to a sulfa allergy. Therefore, CAIs must be developed that are not only isoform specific, but also non-classical, i.e. not based on sulfonamides, sulfamates, or sulfamides. This review covers the classes of non-classical CAIs and the recent advances in the development of isoform-specific inhibitors based on phenols, polyamines, coumarins and their derivatives.

Carbonic anhydrase inhibitors: a review on the progress of patent literature (2011-2016)

INTRODUCTION

A large area of carbonic anhydrase (CA) research focuses on the inhibition of human CA IX and CA XII, as these isoforms have been designated as biomarkers and therapeutic targets for various cancer types.

AREAS COVERED

Recently, the majority of CA inhibitor (CAI) patents cover compound design, synthesis, and delivery methods for the treatment of glaucoma and cancer. The analysis of included patents highlights the need for isoform specific inhibitors. This review covers the patents of medically relevant carbonic anhydrase inhibitors between 2011-2016.

EXPERT OPINION

The improvement of structure-based drug design methods and access to the crystal structures of human CA isoforms have improved inhibitor development. This progress can be observed in relation to the selective inhibition of CA IX for cancer treatments, with one inhibitor in clinical trials. However, the design of nonclassical CAIs is essential to further improve isoform specificity and prevent sulfur allergies.

Carbon Dioxide "Trapped" in a β-Carbonic Anhydrase

Carbonic anhydrases (CAs) are enzymes that catalyze the hydration/dehydration of CO2/HCO3(-) with rates approaching diffusion-controlled limits (kcat/KM ∼ 10(8) M(-1) s(-1)). This family of enzymes has evolved disparate protein folds that all perform the same reaction at near catalytic perfection. Presented here is a structural study of a β-CA (psCA3) expressed in Pseudomonas aeruginosa, in complex with CO2, using pressurized cryo-cooled crystallography. The structure has been refined to 1.6 Å resolution with R(cryst) and R(free) values of 17.3 and 19.9%, respectively, and is compared with the α-CA, human CA isoform II (hCA II), the only other CA to have CO2 captured in its active site. Despite the lack of structural similarity between psCA3 and hCA II, the CO2 binding orientation relative to the zinc-bound solvent is identical. In addition, a second CO2 binding site was located at the dimer interface of psCA3. Interestingly, all β-CAs function as dimers or higher-order oligomeric states, and the CO2 bound at the interface may contribute to the allosteric nature of this family of enzymes or may be a convenient alternative binding site as this pocket has been previously shown to be a promiscuous site for a variety of ligands, including bicarbonate, sulfate, and phosphate ions.

Crystal structure of the most catalytically effective carbonic anhydrase enzyme known, SazCA from the thermophilic bacterium Sulfurihydrogenibium azorense

Two thermostable α-carbonic anhydrases (α-CAs) isolated from thermophilic Sulfurihydrogenibium spp., namely SspCA (from S. yellowstonensis) and SazCA (from S. azorense), were shown in a previous work to possess interesting complementary properties. SspCA was shown to have an exceptional thermal stability, whereas SazCA demonstrated to be the most active α-CA known to date for the CO2 hydration reaction. Here we report the crystallographic structure of SazCA and the identification of the structural features responsible for its high catalytic activity, by comparing it with SspCA structure. These data are of relevance for the design of engineered proteins showing higher stability and catalytic activity than other α-CAs known to date.

Out of the active site binding pocket for carbonic anhydrase inhibitors

2-Benzylsulfinylbenzoic acid binds to human carbonic anhydrase II in a mode completely different from any other class of carbonic anhydrase inhibitors investigated so far.

NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy. The possible hydrogen-bond clusters formed and the mechanism of the tautomerism are discussed. Information about the imidazole hydrogen-bond geometries is obtained by establishing a correlation between published (1) H and (15) N chemical shifts of the imidazole rings of histidines in proteins. This correlation is useful to distinguish histidines embedded in the interior of proteins and those at the surface, embedded in water. Moreover, evidence is obtained that the hydrogen-bond geometries of His64 in the active site of HCA II and of 4-MI in H64A HCA II are similar. Finally, the degeneracy of the rapid tautomerism of the neutral imidazole ring His64 reported by Shimahara et al. (J. Biol. Chem.- 2007, 282, 9646) can be explained with a wet, polar, nonaqueous active-site conformation in the inward conformation, similar to the properties of 4-MI in the Freon solution. The biological implications for the enzyme mechanism are discussed.

Ethylene bis-imidazoles are highly potent and selective activators for isozymes VA and VII of carbonic anhydrase, with a potential nootropic effect

A series of ethylene bis-imidazoles was synthesized via a novel microwave-mediated synthesis. Biological testing on eight isozymes of carbonic anhydrase (CA) present in the human brain revealed compounds with nanomolar potency against CA VA and CA VII, also displaying excellent selectivity against other CA isozymes present in this organ.