Biocatalytic strategies for the asymmetric synthesis of alpha-hydroxy ketones

The development of efficient syntheses for enantiomerically enriched alpha-hydroxy ketones is an important research focus in the pharmaceutical industry. For example, alpha-hydroxy ketones are found in antidepressants, in selective inhibitors of amyloid-beta protein production (used in the treatment of Alzheimer's), in farnesyl transferase inhibitors (Kurasoin A and B), and in antitumor antibiotics (Olivomycin A and Chromomycin A3). Moreover, alpha-hydroxy ketones are of particular value as fine chemicals because of their utility as building blocks for the production of larger molecules. They can also be used in preparing many other important structures, such as amino alcohols, diols, and so forth. Several purely chemical synthetic approaches have been proposed to afford these compounds, together with some organocatalytic strategies (thiazolium-based carboligations, proline alpha-hydroxylations, and so forth). However, many of these chemical approaches are not straightforward, lack selectivity, or are economically unattractive because of the large number of chemical steps required (usually combined with low enantioselectivities). In this Account, we describe three different biocatalytic approaches that have been developed to efficiently produce alpha-hydroxy ketones: (i) The use of thiamine diphosphate-dependent lyases (ThDP-lyases) to catalyze the umpolung carboligation of aldehydes. Enantiopure alpha-hydroxy ketones are formed from inexpensive aldehydes with this method. Some lyases with a broad substrate spectrum have been successfully characterized. Furthermore, the use of biphasic media with recombinant whole cells overexpressing lyases leads to productivities of approximately 80-100 g/L with high enantiomeric excesses (up to >99%). (ii) The use of hydrolases to produce alpha-hydroxy ketones by means of (in situ) dynamic kinetic resolutions (DKRs). Lipases are able to successfully resolve racemates, and many outstanding examples have been reported. However, this approach leads to a maximum theoretical yield of 50%. As a means of overcoming this problem, these traditional lipase-catalyzed kinetic resolutions are combined with racemization of remnant substrate, which can be done in situ or in separate compartments. Examples showing high conversions (>90%) and enantiomeric excesses (>99%) are described. (iii) Whole-cell redox processes, catalyzed by several microorganisms, either by means of free enzymes (applying a cofactor regeneration system) or by whole cells. Through the use of redox machineries, different strategies can lead to high yields and enantiomeric excesses. Some enantiopure alpha-hydroxy ketones can be formed by reductions of diketones and by selective oxidations of vicinal diols. Likewise, some redox processes involving sugar chemistry (involving alpha-hydroxy ketones) have been developed on the industrial scale. Finally, the redox whole-cell concept allows racemizations (and deracemizations) as well. These three strategies provide a useful and environmentally friendly synthetic toolbox. Likewise, the field represents an illustrative example of how biocatalysis can assist practical synthetic processes, and how problems derived from the integration of natural tools in synthetic pathways can be efficiently tackled to afford high yields and enantioselectivities

Immobilization of different protein fractions from Rhizomucor miehei lipase crude extract - Enzymatic resolution of (R,S)-2-Tetralol

The hydrolytic enzymes contained in a crude extract from Rhizomucor miehei (RML) were immobilized onto different supports. The catalytic behavior of the different enzyme derivatives in the resolution of esters of racemic 2-tetralol and structurally related secondary alcohols was investigated. We observed that, when the immobilization occurs by adsorption on highly hydrophobic solid surfaces, such as octyl-agarose or octadecyl-Sepabeads, only the lipase fraction (36 kDa) was immobilized and the resulting catalysts showed good lipasic activity and high enantioselectivity. By contrast, when immobilization was performed by ionic or covalent attachment, all proteins contained in the crude extract were immobilized and both activity and enantioselectivity were found to be much lower. The different enantioselectivity seems to be related to conformational changes of the lipase fraction (36 kDa) in the different immobilization approaches. (R)-2-Tetralol was obtained with high enantiomeric excess (89% at 50% of conversion, E = 51) by hydrolysis of the corresponding butyric acid ester using RML on octyl-agarose

Structural bases for understanding the stereoselectivity in ketone reductions with ADH from Thermus thermophilus: A quantitative model

Automated structural analysis of alcohol dehydrogenase from Thermus thermophilus (ADHTt), a new carbonyl reductase recently described belonging to the SDR superfamily, allows to identify the aminoacidic residues responsible for the reductive catalytic activity, namely Ser-135, Tyr-148 and Lys-152. A series of acetophenone like compounds reduced with such enzyme was docked showing a distinct preference for binding to the active center. Favorable docking conformations calculated with two different protocols fall into two low-energy ensembles. These conformational ensembles are distinguished by the relative position of a given structure, presenting either the si- or re-face of the ketone to the nicotinamide mononucleotide (NMN) moiety reductant. For the ketones presented here, there is a correspondence between the major enantiomer obtained from the experimental data and the conformer found to have the most stable interaction energy with the receptor site in all cases. Furthermore, based on these two energy data sets we were able to build a reliable quantitative model (R2 = 0.98; crossvalidation q2 = 0.78) to predict the percentage of conversion from docking energy and the nature of the substrate with the following equation: conversion (%) = 30.80 Ed 1272.84 + 224.34, where Ed is docking energy and is the Mulliken charge of the adjacent group of the ketone. The receptor site modeling, docking simulations, and enzyme\u2013substrate geometry optimizations lead to a model for understanding the enantioselectivity of this NADH dependent carbonyl reductase

Endothelial cell signalling induced by trans-sialidase from Trypanosoma cruzi.

International audienceThe protozoan responsible for Chagas' disease, Trypanosoma cruzi, expresses on its surface an unusual trans-sialidase enzyme thought to play an important role in host-parasite interactions. Trans-sialidase is the product of a multigene family encoding both active and inactive proteins. We have demonstrated that despite lacking enzymatic activity due to a single mutation, Tyr342-His, inactive trans-sialidase displays sialic acid binding activity, with identical specificity to that of its active analogue. In this work we demonstrate that binding of a recombinant inactive trans-sialidase to molecules containing alpha2,3-linked sialic acid on endothelial cell surface triggers NF-kappaB activation, expression of adhesion molecules and upregulation of parasite entry into host cells. Furthermore, inactive recombinant trans-sialidase blocks endothelial cell apoptosis induced by growth factor deprivation. These results suggest that inactive members of the trans-sialidase family play a role in endothelial cell responses to T. cruzi infection