Franck Charmantray

Interest of Acridine Derivatives in the Anticancer Chemotherapy

DNA is considered as one of the main targets for anticancer drug design. The planar structure of acridines confers to the molecules the ability to bind DNA by intercalation and therefore to interfere with metabolic processes. A large number of natural alkaloids and synthetic acridine derivatives have been tested as anticancer agents. So far, a few molecules have entered clinical trials and have been approved for chemotherapy. The mechanisms of action are not fully understood. Cytotoxicity may be related to potent enzyme inhibition. Topoisomerase and telomerase activities may be strongly affected by acridines. The affinity of acridines for DNA has also been used to design new active compounds in which a DNA modifying group is tethered to the acridine nucleus. Acridine derivatives display other pharmacological properties such as antibacterial and antimalarial activities. They are also tested for Alzheimers disease.

A pH-Based High-Throughput Assay for Transketolase: Fingerprinting of Substrate Tolerance and Quantitative Kinetics

A pH‐based high‐throughput assay method has been developed for the rapid and reliable measurement of transketolase (TK) activity. The method is based on the decarboxylation of lithium hydroxypyruvate (HPA) as a hydroxyacetyl donor with an aldehyde acceptor, using phenol red as the pH indicator. Upon release of carbon dioxide from HPA, the pH increase in the reaction mixture can be determined photometrically by the color change of the pH indicator. At low buffer concentration (2 mM triethanolamine, pH 7.5), the method is highly sensitive and allows continuous monitoring, for quantitative determination of the kinetic parameters. By using this method, the substrate specificities of the TK enzymes from Escherichia coli and Saccharomyces cerevisiae, as well as two active‐site‐modified variants of the E. coli TK (D469E, H26Y) were evaluated against a panel of substrate analogues; specific activities and kinetic constants could be rapidly determined. Substrate quality indicated by assay determination was substantiated with novel TK applications by using achiral 3‐hydroxypropanal and 4‐hydroxybutanal for preparative synthesis of chiral deoxyketose‐type products. Determination of ee for the latter could be performed by chiral GC analysis, with an unambiguous correlation of the absolute configuration from rotation data. This pH‐based assay method is broadly applicable and allows rapid, sensitive, and reliable screening of the substrate tolerance of known TK enzymes and variants obtained from directed evolution.

One-pot, two-step cascade synthesis of naturally rare L-erythro (3S,4S) ketoses by coupling a thermostable transaminase and transketolase

An efficient simultaneous cascade of two enzymatic steps catalyzed by a thermostable transaminase and transketolase was performed at elevated temperatures allowing the synthesis of naturally rare L-erythro (3S,4S) ketoses. L-ribulose, 5-deoxy-L-ribulose, D-tagatose and L-psicose, which are highly valuable chiral building blocks and display prominent biological properties, were obtained on a preparative scale with excellent stereoselectivities and good yields. A thermostable transketolase from Geobacillus stearothermophilus catalyzed at high temperatures the stereospecific synthesis of L-erythro (3S,4S)-configured ketoses from (2S)-hydroxylated aldehydes and β-hydroxypyruvate in which the latter is generated in an unprecedented manner in situ from natural L-serine and pyruvate using a novel thermostable L-α-transaminase from the thermophilic bacterium Thermosinus carboxydivorans. Overall, this cascade synthesis prevents the thermal decomposition of the labile β-hydroxypyruvate and offers an efficient and environmentally friendly procedure.

2-Aminobenzothiazole degradation by free and Ca-alginate immobilized cells of Rhodococcus rhodochrous

2-Aminobenzothiazole (ABT) degradation was investigated using free and immobilized systems during photodegradation under solar light in the presence of Fe(III)-nitrilotriacetic acid (FeNTA), biodegradation by Rhodococcus rhodochrous, and during combined conditions. Ca-alginate hydrogel was chosen as a model matrix and some complementary studies were required to characterize this new system. R. rhodochrous metabolism in this type of environment was monitored by NMR spectroscopy. Neither change in intracellular pH values nor in ATP concentrations was observed by in vivo(31)P NMR, showing that no metabolic modification occurred between free and immobilized cells. (1)H NMR demonstrated that alginate was not used as carbon source by R. rhodochrous. After establishing the pre-treatment protocol by SPE to eliminate solubilised alginate, ABT adsorption on beads and degradation were studied. The same pathways of transformation were observed in suspended and immobilized cell systems. Considering the ABT adsorption phenomenon on alginate beads (8%), the efficiency of the two systems was found to be comparable although the degradation rate was slightly lower with immobilized cells. The most important result was the finding that the positive effect of FeNTA on ABT degradation with immobilized cells was similar to that observed previously with free cells. All these results show that mechanisms observed with free cells can be extrapolated to entrapped cells, i.e. under conditions much closer to those usually encountered in the environment.

Electrochemical detection of transketolase activity using a tyrosinase biosensor

This paper proposes a new concept of transketolase (TK) activity profiling. A tyrosinase (PPO) biosensor, based on the immobilization of this enzyme in a Mg(2)Al-Cl layered double hydroxide, was developed for the amperometric detection of N-acetyl-l-tyrosine ethyl ester monohydrate (N-Ac-Tyr-OEt) at -0.2V. This compound was released during an enzymatic reaction catalyzed by TK with N-acetyl-O-(2R, 3S, 5-trihydroxy-4-oxopentyl)-l-tyrosine ethyl ester used as donor substrate. This tyrosinase biosensor was optimized for the detection of TK activity, including PPO optimum substrate concentration, electrolyte nature, pH, and influence of bovine serum albumin (BSA). It was found that N-Ac-Tyr-OEt release is dependent on TK concentration (U/mL) in the electrolyte medium. These results demonstrate the sensitivity and specificity of the tyrosinase biosensor designed for in vitro detection of TK activity, which is known to be involved in several diseases.

Chiral Polyol Synthesis Catalyzed by a Thermostable Transketolase Immobilized on Layered Double Hydroxides in Ionic liquids

In this work we set out to study the activity of a thermostable Transketolase (TK) from Geobacillus stearothermophilus (TKgst) in an ionic liquid as cosolvent, which has never been investigated before with this enzyme. 1‐Butyl‐3‐methylimidazolium chloride ([BMIm][Cl]) in the range 30–50 % in water maintained the total activity of TKgst and increased the reaction rate in the presence of pentoses as acceptor substrates, particularly d‐ribose. To improve the synthetic process, TKgst was immobilized on an inorganic support, layered double hydroxides (LDHs), with excellent immobilization yield and catalytic activity using a simple, eco‐compatible, efficient coprecipitation procedure. The biohybrid MgAl@TKgst was tested in 30 % [BMIm][Cl] for the synthesis of a rare, very costly commercially available sugar, d‐sedoheptulose, which was obtained in one step from d‐ribose with an isolated yield of 82 %. This biohybrid was reusable over four cycles with no loss of enzymatic activity. The particular activity of free and immobilized TKgst in [BMIm][Cl] holds promise to extend the applications of TKgst in other ionic liquids and unusual media in biocatalysis.

Synthesis of specially designed probes to broaden transketolase scope

Efficient, biocompatible, stereospecific strategies were developed to prepare eight probes to assay transketolase (TK) variants with new substrate specificities. The structure of these probes combines a sugar moiety (D‐threo or L‐erythro ketose, or D‐threo aldose) with the side chain of an amino acid (Ala, Leu, Val, Met, Thr) for in vivo detection of new TK activities using amino acid auxotrophs. To obtain D‐threo ketose probes, biocatalysts, such as transketolase and fructose‐6‐phosphate aldolase Ala129Ser, were used whereas L‐erythro ketoses and D‐threo aldose probes were synthesized by the way of organocatalysis or Sharpless dihydroxylation as sustainable alternative key steps to biocatalysis.

Engineering a thermostable transketolase for arylated substrates

Aromatic components are difficult substrates for enzymes catalyzing stereoselective carboligation reactions. We have engineered transketolase from Geobacillus stearothermophilus by directed evolution to utilize arylalkanals and benzaldehyde as the electrophilic substrate in highly stereoselective C–C bond forming conversions. Enzyme variants were discovered with rate accelerations up to 28-fold that convert 2-phenylethanal, 3-phenylpropanal, phenyloxyethanal, benzyloxyethanal, and (N-Cbz)-3-aminopropanal with formation of the corresponding aryl-substituted 1,3-dihydroxyketones in good yields (60–72%) and virtually complete (3S)-stereoselectivity (>99% ee). Novel double-site variants were also found for the conversion of benzaldehyde.

An efficient amperometric transketolase assay: Towards inhibitor screening

This paper describes an innovative amperometric biosensor for the in vitro determination of activity of transketolase from Escherichia coli (TKec) using commercially available TK substrates, namely d-fructose-6-phosphate a physiological donor and glycolaldehyde the best non-phosphorylated acceptor. A galactose oxidase (GAOx) biosensor, based on the immobilization of this enzyme within laponite clay, allows amperometric detection of L-erythrulose released upon TK-catalyzed reaction. A calibration curve has been established from 0.01 to 0.1 U ml(-1) TKec concentration in solution. These data are comparable to that obtained by a fluorometric method. In order to ensure a higher sensitivity and re-usability of the system, an original bienzymatic sensing system was further developed based on apoenzyme TKec and GAOx separately immobilized on the electrode surface. The inner sensing layer contains GAOx@laponite and the outer layer TKec@layered double hydroxide biohybrid. The biosensor response was validated by the determination of KD(app) for thiamine diphosphate, the TK cofactor and the inhibition action of two commercially available products, pyrophosphate, a TK cofactor analog and d-arabinose-5-phosphate, a substrate analog.

Fluorogenic substrates for the screening assay of transketolase through beta-elimination of umbelliferone—Development, scope and limitations

5-O-Coumarinyl-d-xylulose was studied as a fluorogenic substrate for the stereospecific assay of transketolase enzyme. Enzymatic C2-C3 cleavage released an alpha-hydroxyl, beta-coumarinyl substituted aldehyde. Although the subsequent beta-elimination step was rate limiting under chemical or enzymatic catalysis, we detected a TK activity as low as 0.7mIU. To improve the fluorescence signal release, kinetic and product distribution analyses of this reaction were performed by LC/UV/MS coupling.