Binding and Stabilization of a Semiquinone Radical by an Artificial Metalloenzyme Containing a Binuclear Copper (II) Cofactor.

A binuclear Cu(II) cofactor was covalently bound to a lauric acid anchor. The resulting conjugate was characterized then combined with beta-lactoglobulin (ßLG) to generate a new biohybrid following the so-called "Trojan horse" strategy. This biohybrid was examined for its effectiveness in the oxidation of a catechol derivative to the corresponding quinone. The resulting biohybrid did not exhibit the sought after catecholase activity, likely due to its ability to bind and stabilize the semiquinone radical intermediate DTB-SQ. This semi-quinone radical was stabilized only in the presence of the protein and was characterized using optical and magnetic spectroscopic techniques, demonstrating stability for over 16 hours. Molecular docking studies revealed that this stabilization could occur owing to interactions of the semi-quinone with hydrophobic amino acid residues of ßLG.

Artificial Enzymes for Diels-Alder Reactions.

The Diels-Alder (DA) reaction is a cycloaddition of a conjugated diene and an alkene (dienophile) leading to the formation of a cyclohexene derivative through a concerted mechanism. As DA reactions generally proceed with a high degree of regio- and stereoselectivity, they are widely used in synthetic organic chemistry. Considering eco-conscious public and governmental movements, efforts are now directed towards the development of synthetic processes that meet environmental concerns. Artificial enzymes, which can be developed to catalyze abiotic reactions, appear to be important synthetic tools in the synthetic biology field. This review describes the different strategies used to develop protein-based artificial enzymes for DA reactions, including for in cellulo approaches.

Antimicrobial Activity of Cationic Poly(3-hexylthiophene) Nanoparticles Coupled with Dual Fluorescent and Electrochemical Sensing: Theragnostic Prospect.

Designing therapeutic and sensor materials to diagnose and eliminate bacterial infections remains a significant challenge for active theragnostic nanoprobes. In the present work, fluorescent/electroactive poly(3-hexylthiophene) P3HT nanoparticles (NPs) stabilized with quaternary ammonium salts using cetyltrimethylammonium bromide (CTAB), (CTAB-P3HT NPs) were prepared using a simple mini-emulsion method. The morphology, spectroscopic properties and electronic properties of CTAB-P3HT NPs were characterized by DLS, zeta potential, SEM, TEM, UV-vis spectrophotometry, fluorescence spectroscopy and electrochemical impedance spectroscopy (EIS). In an aqueous solution, CTAB-P3HT NPs were revealed to be uniformly sized, highly fluorescent and present a highly positively charged NP surface with good electroactivity. Dual detection was demonstrated as the binding of the bacteria to NPs could be observed by fluorescence quenching as well as by the changes in EIS. Binding of E. coli to CTAB-P3HT NPs was demonstrated and LODs of 5 CFU/mL and 250 CFU/mL were obtained by relying on the fluorescence spectroscopy and EIS, respectively. The antimicrobial activity of CTAB-P3HT NPs on bacteria and fungi was also studied under dark and nutritive conditions. An MIC and an MBC of 2.5 µg/mL were obtained with E. coli and with S. aureus, and of 0.312 µg/mL with C. albicans. Additionally a good biocompatibility toward normal human cells (WI38) was observed, which opens the way to their possible use as a therapeutic agent.

Artificial iron hydrogenase made by covalent grafting of Knölker's complex into xylanase: Application in asymmetric hydrogenation of an aryl ketone in water.

We report a new artificial hydrogenase made by covalent anchoring of the iron Knölker's complex to a xylanase S212C variant. This artificial metalloenzyme was found to be able to catalyze efficiently the transfer hydrogenation of the benchmark substrate trifluoroacetophenone by sodium formate in water, yielding the corresponding secondary alcohol as a racemic. The reaction proceeded more than threefold faster with the XlnS212CK biohybrid than with the Knölker's complex alone. In addition, efficient conversion of trifluoroacetophenone to its corresponding alcohol was reached within 60 H with XlnS212CK, whereas a ≈2.5-fold lower conversion was observed with Knölker's complex alone as catalyst. Moreover, the data were rationalized with a computational strategy suggesting the key factors of the selectivity. These results suggested that the Knölker's complex was most likely flexible and could experience free rotational reorientation within the active-site pocket of Xln A, allowing it to access the subsite pocket populated by trifluoroacetophenone.

CuII -Containing 1-Aminocyclopropane Carboxylic Acid Oxidase Is an Efficient Stereospecific Diels-Alderase.

In the context of developing ecofriendly chemistry, artificial enzymes are now considered as promising tools for synthesis. They are prepared in particular with the aim to catalyze reactions that are rarely, if ever, catalyzed by natural enzymes. We discovered that 1-aminocyclopropane carboxylic acid oxidase reconstituted with CuII served as an efficient artificial Diels-Alderase. The kinetic parameters of the catalysis of the cycloaddition of cyclopentadiene and 2-azachalcone were determined (KM =230 µm, kapp =3 h-1 ), which gave access to reaction conditions that provided quantitative yield and >99 % ee of the (1S,2R,3R,4R) product isomer. This unprecedented performance was rationalized by molecular modeling as only one docking pose of 2-azachalcone was possible in the active site of the enzyme and this was the one that leads to the (1S,2R,3R,4R) product isomer.

Receptor-Based Artificial Metalloenzymes on Living Human Cells.

Artificial metalloenzymes are known to be promising tools for biocatalysis, but their recent compartmentalization has led to compatibly with cell components thus shedding light on possible therapeutic applications. We prepared and characterized artificial metalloenzymes based on the A2A adenosine receptor embedded in the cytoplasmic membranes of living human cells. The wild type receptor was chemically engineered into metalloenzymes by its association with strong antagonists that were covalently bound to copper(II) catalysts. The resulting cells enantioselectively catalyzed the abiotic Diels-Alder cycloaddition reaction of cyclopentadiene and azachalcone. The prospects of this strategy lie in the organ-confined in vivo preparation of receptor-based artificial metalloenzymes for the catalysis of reactions exogenous to the human metabolism. These could be used for the targeted synthesis of either drugs or deficient metabolites and for the activation of prodrugs, leading to therapeutic tools with unforeseen applications.

αRep A3: A Versatile Artificial Scaffold for Metalloenzyme Design.

αRep refers to a new family of artificial proteins based on a thermostable α-helical repeated motif. One of its members, αRep A3, forms a stable homo-dimer with a wide cleft that is able to accommodate metal complexes and thus appears to be suitable for generating new artificial biocatalysts. Based on the crystal structure of αRep A3, two positions (F119 and Y26) were chosen, and independently changed into cysteine residues. A phenanthroline ligand was covalently attached to the unique cysteine residue of each protein variant, and the corresponding biohybrids were purified and characterized. Once mutated and coupled to phenanthroline, the protein remained folded and dimeric. Copper(II) was specifically bound by the two biohybrids with two different binding modes. Furthermore, the holo-biohybrid A3F119NPH was found to be capable of enantioselectively catalyzing Diels-Alder (D-A) cycloadditions with up to 62 % ee. This study validates the choice of the αRep A3 dimer as a protein scaffold and provides a promising new route for the design and production of new enantioselective biohybrids based on entirely artificial proteins obtained from a highly diverse library.

Artificial Metalloenzymes with the Neocarzinostatin Scaffold: Toward a Biocatalyst for the Diels-Alder Reaction.

A copper(II) cofactor coupled to a testosterone anchor, copper(II)-(5-(Piperazin-1-yl)-1,10-phenanthroline)testosterone-17-hemisuccinamide (10) was synthesized and associated with a neocarzinostatin variant, NCS-3.24 (KD =3 µm), thus generating a new artificial metalloenzyme by following a "Trojan horse" strategy. Interestingly, the artificial enzyme was able to efficiently catalyze the Diels-Alder cyclization reaction of cyclopentadiene (1) with 2-azachalcone (2). In comparison with what was observed with cofactor 10 alone, the artificial enzymes favored formation of the exo products (endo/exo ratios of 84:16 and 62:38, respectively, after 12 h). Molecular modeling studies assigned the synergy between the copper complex and the testosterone (KD =13 µm) moieties in the binding of 10 to good van der Waals complementarity. Moreover, by pushing the modeling exercise to its limits, we hypothesize on the molecular grounds that are responsible for the observed selectivity.

Neocarzinostatin-based hybrid biocatalysts with a RNase like activity.

A new zinc(II)-cofactor coupled to a testosterone anchor, zinc(II)-N,N-bis(2-pyridylmethyl)-1,3-diamino-propa-2-ol-N'(17'-succinimidyltestosterone) (Zn-Testo-BisPyPol) 1-Zn has been synthesized and fully characterized. It has been further associated with a neocarzinostatin variant, NCS-3.24, to generate a new artificial metalloenzyme following the so-called 'Trojan horse' strategy. This new 1-Zn-NCS-3.24 biocatalyst showed an interesting catalytic activity as it was found able to catalyze the hydrolysis of the RNA model HPNP with a good catalytic efficiency (kcat/KM=13.6M(-1)s(-1) at pH 7) that places it among the best artificial catalysts for this reaction. Molecular modeling studies showed that a synergy between the binding of the steroid moiety and that of the BisPyPol into the protein binding site can explain the experimental results, indicating a better affinity of 1-Zn for the NCS-3.24 variant than testosterone and testosterone-hemisuccinate themselves. They also show that the artificial cofactor entirely fills the cavity, the testosterone part of 1-Zn being bound to one the two subdomains of the protein providing with good complementarities whereas its metal ion remains widely exposed to the solvent which made it a valuable tool for the catalysis of hydrolysis reactions, such as that of HPNP. Some possible improvements in the 'Trojan horse' strategy for obtaining better catalysts of selective reactions will be further studied.

Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.

Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion.

An artificial metalloprotein with metal-adaptive coordination sites and Ni-dependent quercetinase activity.

Engineering non-native metal active sites into proteins using canonical amino acids offers many advantages but is hampered by significant challenges. The TIM barrel protein, imidazole glycerol phosphate synthase from the hyperthermophilic organism Thermotoga maritima (tHisF), is well-suited for the construction of artificial metalloenzymes by this approach. To this end, we have generated a tHisF variant (tHisFEHH) with a Glu/His/His motif for metal ion coordination. Crystal structures of ZnII:tHisFEHH and NiII:tHisFEHH reveal that both metal ions bind to the engineered histidines. However, the two metals bind at distinct sites with different geometries, demonstrating the adaptability of tHisF. Only ZnII additionally ligates the Glu residue and adopts a tetrahedral geometry. The pseudo-octahedral NiII site comprises the two His and a native Ser residue. NiII:tHisFEHH catalyzes the oxidative cleavage of the flavanols quercetin and myricetin, providing an unprecedented example of an artificial metalloprotein with quercetinase activity.

Synthesis and characterization of a binuclear iron(III) complex bridged by 1-aminocyclopropane-1-carboxylic acid. Ethylene production in the presence of hydrogen peroxide.

A mu-oxo-diiron(III) complex bridged by two molecules of 1-aminocyclopropane-1-carboxylic acid (ACCH) was prepared with the ligand 1,4,7-triazacyclononane (TACN): [(TACN)Fe(2)(mu-O)(mu-ACCH)(2)](ClO(4))(4) x 2 H(2)O (1). This complex was characterized, and its crystal structure was solved. The bridging amino acid moieties were found in their zwitterionic forms (noted as ACCH). Reactivity assays were performed in the presence of hydrogen peroxide, and 1 turned out to be the first example of a well-characterized iron-ACCH complex able to produce ethylene from the bound ACCH moiety. The reaction requires the presence of a few equivalents of base, probably involved in the deprotonation of the amine groups of the ACCH bridges.

Binding of 2-hydroxypyridine-N-oxide on dicopper(II) centers: insights into tyrosinase inhibition mechanism by transition-state analogs.

2-Hydroxypyridine-N-oxide (HOPNO) is described as a new and efficient transition-state analog (TS-analog) inhibitor for the mushroom tyrosinase with an IC(50) = 1.16 microM and a K(I) = 1.8 microM. Using the binuclear copper(II) complex [Cu(2)(BPMP)(mu-OH)](ClO(4))(2) (2) known as a functional model for the tyrosinase catecholase activity, we isolated and fully characterized a 1:1 (2)/OPNO adduct in which the HOPNO is deprotonated and chelates only one Cu-atom of the binuclear site in a bidentate mode. On the basis of these results, a structural model for the tyrosinase inhibition by HOPNO is proposed.

Crystal structure of phosphomannose isomerase from Candida albicans complexed with 5-phospho-d-arabinonhydrazide.

Type I phosphomannose isomerases (PMIs) are zinc-dependent monofunctional metalloenzymes catalysing the reversible isomerization of d-mannose 6-phosphate to d-fructose 6-phosphate. 5-Phospho-d-arabinonhydrazide (5PAHz), designed as an analogue of the enediolate high-energy intermediate, strongly inhibits PMI from Candida albicans (CaPMI). In this study, we report the 3D crystal structure of CaPMI complexed with 5PAHz at 1.85 Å resolution. The high-resolution structure suggests that Glu294 is the catalytic base that transfers a proton between the C1 and C2 carbon atoms of the substrate. Bidentate coordination of the inhibitor explains the stereochemistry of the isomerase activity, as well as the absence of both anomerase and C2-epimerase activities for Type I PMIs. A detailed mechanism of the reversible isomerization is proposed.

ACC-Oxidase like activity of a copper (II)-ACC complex in the presence of hydrogen peroxide. Detection of a reaction intermediate at low temperature.

A Cu(II)-ACC complex [(Bpy)Cu(ACC)(H2O)]ClO4 (1) was prepared and its treatment with hydrogen peroxide gave rise to ethylene production in an ACC-Oxidase like activity. A brown species that could be a key intermediate in the reaction was detected at low temperature.

2-Amino[1,2,4]triazolo[1,5-c]quinazolines and Derived Novel Heterocycles: Syntheses and Structure-Activity Relationships of Potent Adenosine Receptor Antagonists.

2-Amino[1,2,4]triazolo[1,5-c]quinazolines were identified as potent adenosine receptor (AR) antagonists. Synthetic strategies were devised to gain access to a broad range of derivatives including novel polyheterocyclic compounds. Potent and selective A3 AR antagonists were discovered, including 3,5-diphenyl[1,2,4]triazolo[4,3-c]quinazoline (17, Ki human A3 AR 1.16 nm) and 5'-phenyl-1,2-dihydro-3'H-spiro[indole-3,2'-[1,2,4]triazolo[1,5-c]quinazolin]-2-one (20, Ki human A3 AR 6.94 nm). In addition, multitarget antagonists were obtained, such as the dual A1 /A3 antagonist 2,5-diphenyl[1,2,4]triazolo[1,5-c]quinazoline (13 b, Ki human A1 AR 51.6 nm, human A3 AR 11.1 nm), and the balanced pan-AR antagonists 5-(2-thienyl)[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 c, Ki human A1 AR 131 nm, A2A AR 32.7 nm, A2B AR 150 nm, A3 AR 47.5 nm) and 9-bromo-5-phenyl[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 q, Ki human A1 AR 67.7 nm, A2A AR 13.6 nm, A2B AR 75.0 nm, A3 AR 703 nm). In many cases, significantly different affinities for human and rat receptors were observed, which emphasizes the need for caution in extrapolating conclusions between different species.

A unique 1-amino-1-cyclopropane carboxylate cupric-cryptate hosting sodium.

The first cluster containing acc was prepared via supramolecular self-assembly. This Cu(II) cluster traps Na(+), as shown in the solid state by the crystal structure and in solution by ESI-MS. Further characterisations revealed a ferromagnetic intracluster exchange and an irreversible reduction with a rapid intracluster electron transfer.

Neocarzinostatin-based hybrid biocatalysts for oxidation reactions.

An anionic iron(III)-porphyrin-testosterone conjugate 1-Fe has been synthesized and fully characterized. It has been further associated with a neocarzinostatin variant, NCS-3.24, to generate a new artificial metalloenzyme following the so-called 'Trojan Horse' strategy. This new 1-Fe-NCS-3.24 biocatalyst showed an interesting catalytic activity as it was found able to catalyze the chemoselective and slightly enantioselective (ee = 13%) sulfoxidation of thioanisole by H2O2. Molecular modelling studies show that a synergy between the binding of the steroid moiety and that of the porphyrin macrocycle into the protein binding site can explain the experimental results, indicating a better affinity of 1-Fe for the NCS-3.24 variant than testosterone and testosterone-hemisuccinate themselves. They also show that the Fe-porphyrin complex is sandwiched between the two subdomains of the protein providing with good complementarities. However, the artificial cofactor entirely fills the cavity and its metal ion remains widely exposed to the solvent which explains the moderate enantioselectivity observed. Some possible improvements in the "Trojan Horse" strategy for obtaining better catalysts of selective oxidations are presented.

Tunable single-site ruthenium catalysts for efficient water oxidation.

The catalytic water oxidation activity of mononuclear ruthenium complexes comprising a pyridine-functionalized abnormal triazolylidene ligand can be adjusted by modification of the triazolylidene substituents, which is readily achieved through click-type cycloaddition chemistry, affording some of the most active ruthenium catalysts known thus far for water oxidation (TONs > 400, TOFs close to 7000 h(-1)).

Probing the peptidylglycine alpha-hydroxylating monooxygenase active site with novel 4-phenyl-3-butenoic acid based inhibitors.

Specific inhibition of the copper-containing peptidylglycine alpha-hydroxylating monooxygenase (PHM), which catalyzes the post-translational modification of peptides involved in carcinogenesis and tumor progression, constitutes a new approach for combating cancer. We carried out a structure-activity study of new compounds derived from a well-known PHM substrate analogue, the olefinic compound 4-phenyl-3-butenoic acid (PBA). We designed, synthesized, and tested various PBA derivatives both in vitro and in silico. We show that it is possible to increase PBA affinity for PHM by appropriate functionalization of its aromatic nucleus. Compound 2 d, for example, bears a meta-benzyloxy substituent, and exhibits better inhibition features (K(i)=3.9 microM, k(inact)/K(i)=427 M(-1) s(-1)) than the parent PBA (K(i)=19 microM, k(inact)/K(i)=82 M(-1) s(-1)). Docking calculations also suggest two different binding modes for PBA derivatives; these results will aid in the development of further PHM inhibitors with improved features.