Virgil Hélaine

Biocatalytic synthesis of hydroxylated natural products using aldolases and related enzymes

Synthetic building blocks bearing hydroxylated chiral centers are important targets for biocatalysis. Many C-C bond forming enzymes have recently been investigated for new applications and new strategies towards the synthesis of natural products and related oxygenated compounds. Several old catalysts have been studied to increase our functional knowledge of natural aldolase-type enzymes, and new mutated catalysts or catalytic antibodies have been tested for their synthetic utility.

A fluorogenic assay for transketolase from Saccharomyces cerevisiae

In order to generate transketolase (TK) with new or improved properties by in vitro evolution, an efficient screening system is an absolute prerequisite for identifying the evolved enzyme variants. We report here an assay allowing us to detect wild type TK activity in vitro by fluorescence. We examined the use of the fluorogenic compound 1 as donor substrate of TK, which is itself non fluorescent but releases a fluorescent product: umbelliferone.

Synthesis of 4,4‐Disubstituted L‐Glutamic Acids by Enzymatic Transamination

The syntheses of optically pure 4,4-dimethyl-L-glutamic acid as well as (2S,4R)- and (2S,4S)-4-hydroxy-4-methylglutamic acids have been achieved by transamination of the corresponding 2-oxo-4,4-dimethyl- and rac-2-oxo-4-hydroxy-4-methylglutaric acids using glutamic oxalacetic transaminase (GOT).

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.

A new access to alkyl-α-ketoglutaric acids, precursors of glutamic acid analogues by enzymatic transamination. Application to the synthesis of (2S,4R)-4-propyl-glutamic acid

Abstract 4-Alkyl-2-alkylidene-glutaric acids are easily obtained by a Claisen-Johnson rearrangement, providing a short access to 4-alkyl-α-ketoglutaric acids. These compounds are substrates of the glutamic oxalacetic transaminase (GOT), allowing the enzymatic synthesis of biologically important analogues of l -glutamate. A new analogue, (2S,4R)-4-propyl-glutamic acid, is described.

Efficient biocatalytic processes for highly valuable terminally phosphorylated C5 to C9 D-ketoses

A green enzymatic strategy for the synthesis of terminally phosphorylated C5 to C9 naturally occurring D-ketose phosphates and analogues was developed using D-fructose-6-phosphate aldolase (FSA) as a catalyst. This enzyme has stereoselectively catalysed aldol reactions between glycolaldehyde phosphate or ribose-5-phosphate as an acceptor substrate and dihydroxyacetone, hydroxyacetone or hydroxybutanone as a donor. Furthermore, D-glycero-D-altro-2-octulose 8-phosphate was obtained using a straightforward one-pot domino biocatalytic system involving FSA, ribulose-5-phosphate epimerase and ribose-5-phosphate isomerase controlling five contiguous asymmetric centres and starting from achiral material.

CHEMO-ENZYMATIC SYNTHESIS OF THE FOUR STEREOISOMERS OF 4-HYDROXY-4-METHYLGLUTAMIC ACID

Abstract A chemo-enzymatic synthesis of the four stereoisomers of 4-hydroxy-4-methylglutamic acid is described. As with other glutamate analogues these compounds can serve as probes for the investigation of glutamate receptors.

Genome Mining for Innovative Biocatalysts: New Dihydroxyacetone Aldolases for the Chemist’s Toolbox

Stereoselective carboligating enzymes were discovered by a genome mining approach to extend the biocatalysis toolbox. Seven hundred enzymes were selected by sequence comparison from diverse prokaryotic species as representatives of the aldolase (FSA) family diversity. The aldol reaction tested involved dihydroxyacetone (DHA) and glyceraldehyde‐3‐phosphate. The hexose‐6‐phosphate formation was monitored by mass spectrometry. Eighteen enzymes annotated either as transaldolases or aldolases were found to exhibit a DHA aldolase activity. Remarkably, six of them proven as aldolases, and not transaldolases, shared very limited similarities with those currently described. Multiple sequence alignment performed on all enzymes revealed a Tyr in the new DHA aldolases as found in FSAcoli instead of a Phe usually found in transaldolases. Four of these DHA aldolases were biochemically characterised in comparison with FSAcoli. In particular, an aldolase from Listeria monocytogenes exhibited interesting catalytic properties.

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.

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.