Gernot A. Strohmeier

Application of Designed Enzymes in Organic Synthesis

Application of Designed Enzymes in Organic Synthesis Gernot A. Strohmeier, Harald Pichler, Oliver May, and Mandana Gruber-Khadjawi* Austrian Centre of Industrial Biotechnology, Petersgasse 14, A-8010 Graz, Austria Institute of Molecular Biotechnology, Graz University of Technology, Petersgasse 14, A-8010 Graz, Austria DSM—Innovative Synthesis BV, Geleen, P.O. Box 18, 6160 MD Geleen, The Netherlands

Biocatalytic reduction of carboxylic acids

An increasing demand for non‐petroleum‐based products is envisaged in the near future. Carboxylic acids such as citric acid, succinic acid, fatty acids, and many others are available in abundance from renewable resources and they could serve as economic precursors for bio‐based products such as polymers, aldehyde building blocks, and alcohols. However, we are confronted with the problem that carboxylic acid reduction requires a high level of energy for activation due to the carboxylates thermodynamic stability. Catalytic processes are scarce and often their chemoselectivity is insufficient. This review points at bio‐alternatives: currently known enzyme classes and organisms that catalyze the reduction of carboxylic acids are summarized. Two totally distinct biocatalyst lines have evolved to catalyze the same reaction: aldehyde oxidoreductases from anaerobic bacteria and archea, and carboxylate reductases from aerobic sources such as bacteria, fungi, and plants. The majority of these enzymes remain to be identified and isolated from their natural background in order to evaluate their potential as industrial biocatalysts.

One-pot deracemization of sec-alcohols: enantioconvergent enzymatic hydrolysis of alkyl sulfates using stereocomplementary sulfatases.

Given the fact that the theoretically possible number of racemates is larger than that of symmetric prochiral or meso compounds, the development of deracemization methods, which yield a single stereoisomer from a racemate is an important topic. Enantioconvergent processes are based on the transformation of a pair of enantiomers through opposite stereochemical pathways affecting retention and inversion of configuration. Depending on the stereochemical course of enzymatic and chemical reactions, three types of deracemization protocols were recently classified by Feringa et al. Two chemoenzymatic methods start with a biocatalytic kinetic resolution step, which yields a heteroor homochiral 1:1 mixture of the formed product and nonconverted substrate enantiomer. The latter is subjected to a second (nonenzymatic) transformation with retention or inversion of configuration to yield a single stereoisomeric product. Although several one-pot, two-step protocols have been successfully demonstrated, they typically rely on activated species, such as sulfonates, nitrate esters, or Mitsunobu intermediates, and negatively affect the overall atom economy of the process. The most elegant method relies on one (or two) enzyme(s), which mediate the transformation of both enantiomers through stereocomplementary pathways by retention and inversion. Since the requirements of such double selectivities are very difficult to meet, successful examples are rare: This approach has been applied to the hydrolysis of epoxides using two epoxide hydrolases showing opposite enantiopreference or a single enzyme that catalyzes the enantioconvergent hydrolysis of enantiomers with opposite regioselectivity. For enzymes, the ability to act by retention or inversion is a rare feature, which has been found among epoxide hydrolases, dehalogenases, 9] and sulfatases. The latter catalyze the hydrolytic cleavage of (alkyl) sulfate esters by breakage of the S O or the C O bond leading to retention or inversion at the chiral carbon atom, and thus makes them prime candidates for enantioconvergent processes. So far, only a single inverting sec-alkylsulfatase (PISA1) was generated recombinantly and characterized biochemically, thus allowing preparative-scale applications. In combination with acid-catalyzed hydrolysis of the nonreacted substrate enantiomer under retention of configuration a chemoenzymatic two-step deracemization protocol for sec-alcohols was recently developed. However, the method suffers from serious limitations because it requires undesirably large volumes organic solvents and several molar equivalents of a strong acid (typically 2–7 equiv of p-TosOH), which pose the risk of racemization or decomposition to the functionalized substrates, especially when elevated temperatures are required for acidic hydrolysis. Moreover, it is not applicable to retaining sulfatases, because no chemical method for sulfate ester hydrolysis with inversion exists. So far, retaining-sulfatase activity was reported in whole cells of Rhodopirellula baltica DSM 10527, but the corresponding enzymes could not be identified, thus impeding the use of recombinant technology to make the enzyme available for biocatalysis. Furthermore, the retaining sulfatase of Rh. baltica would not be suitable for an enantioconvergent process with PISA1, because both proteins exhibit the same enantiopreference. During our search for a retaining secalkylsulfatase with an enantiopreference opposite to that of PISA1, we discovered that the arylsulfatase from Pseudomonas aeruginosa (PAS) exhibited activity on sec-alkylsulfates. PAS, which has been characterized on a molecular level, showed promiscuous activity on various arylic phosphates and phosphonates. On its standard model substrate (4-nitrophenyl sulfate), PAS exhibited a rate acceleration of kcat/kuncat 2.3 10, and for a less reactive substrate the highest rate enhancement (kcat/kuncat = 2 10 ) of any catalytic reaction known so far has been measured. The stereochemical [*] M. Schober, M. Toesch, Dr. M. Fuchs, Prof. K. Faber Department of Chemistry, Organic & Bioorganic Chemistry, University of Graz Heinrichstrasse 28, 8010 Graz (Austria) E-mail: kurt.faber@uni-graz.at Homepage: http://biocatalysis.uni-graz.at/

Structure-Based Mechanism of Oleate Hydratase from Elizabethkingia Meningoseptica.

Hydratases provide access to secondary and tertiary alcohols by regio‐ and/or stereospecifically adding water to carbon‐carbon double bonds. Thereby, hydroxy groups are introduced without the need for costly cofactor recycling, and that makes this approach highly interesting on an industrial scale. Here we present the first crystal structure of a recombinant oleate hydratase originating from Elizabethkingia meningoseptica in the presence of flavin adenine dinucleotide (FAD). A structure‐based mutagenesis study targeting active site residues identified E122 and Y241 as crucial for the activation of a water molecule and for protonation of the double bond, respectively. Moreover, we also observed that two‐electron reduction of FAD results in a sevenfold increase in the substrate hydration rate. We propose the first reaction mechanism for this enzyme class that explains the requirement for the flavin cofactor and the involvement of conserved amino acid residues in this regio‐ and stereoselective hydration.

Engineering Pichia pastoris for improved NADH regeneration: A novel chassis strain for whole-cell catalysis

Summary Many synthetically useful reactions are catalyzed by cofactor-dependent enzymes. As cofactors represent a major cost factor, methods for efficient cofactor regeneration are required especially for large-scale synthetic applications. In order to generate a novel and efficient host chassis for bioreductions, we engineered the methanol utilization pathway of Pichia pastoris for improved NADH regeneration. By deleting the genes coding for dihydroxyacetone synthase isoform 1 and 2 (DAS1 and DAS2), NADH regeneration via methanol oxidation (dissimilation) was increased significantly. The resulting Δdas1 Δdas2 strain performed better in butanediol dehydrogenase (BDH1) based whole-cell conversions. While the BDH1 catalyzed acetoin reduction stopped after 2 h reaching ~50% substrate conversion when performed in the wild type strain, full conversion after 6 h was obtained by employing the knock-out strain. These results suggest that the P. pastoris Δdas1 Δdas2 strain is capable of supplying the actual biocatalyst with the cofactor over a longer reaction period without the over-expression of an additional cofactor regeneration system. Thus, focusing the intrinsic carbon flux of this methylotrophic yeast on methanol oxidation to CO2 represents an efficient and easy-to-use strategy for NADH-dependent whole-cell conversions. At the same time methanol serves as co-solvent, inductor for catalyst and cofactor regeneration pathway expression and source of energy.

Discovery and structural characterisation of new fold type IV-transaminases exemplify the diversity of this enzyme fold

Transaminases are useful biocatalysts for the production of amino acids and chiral amines as intermediates for a broad range of drugs and fine chemicals. Here, we describe the discovery and characterisation of new transaminases from microorganisms which were enriched in selective media containing (R)-amines as sole nitrogen source. While most of the candidate proteins were clearly assigned to known subgroups of the fold IV family of PLP-dependent enzymes by sequence analysis and characterisation of their substrate specificity, some of them did not fit to any of these groups. The structure of one of these enzymes from Curtobacterium pusillum, which can convert d-amino acids and various (R)-amines with high enantioselectivity, was solved at a resolution of 2.4 Å. It shows significant differences especially in the active site compared to other transaminases of the fold IV family and thus indicates the existence of a new subgroup within this family. Although the discovered transaminases were not able to convert ketones in a reasonable time frame, overall, the enrichment-based approach was successful, as we identified two amine transaminases, which convert (R)-amines with high enantioselectivity, and can be used for a kinetic resolution of 1-phenylethylamine and analogues to obtain the (S)-amines with e.e.s >99%.

Enantiocomplementary Yarrowia lipolytica Oxidoreductases: Alcohol Dehydrogenase 2 and Short Chain Dehydrogenase/Reductase.

Enzymes of the non-conventional yeast Yarrowia lipolytica seem to be tailor-made for the conversion of lipophilic substrates. Herein, we cloned and overexpressed the Zn-dependent alcohol dehydrogenase ADH2 from Yarrowia lipolytica in Escherichia coli. The purified enzyme was characterized in vitro. The substrate scope for YlADH2 mediated oxidation and reduction was investigated spectrophotometrically and the enzyme showed a broader substrate range than its homolog from Saccharomyces cerevisiae. A preference for secondary compared to primary alcohols in oxidation direction was observed for YlADH2. 2-Octanone was investigated in reduction mode in detail. Remarkably, YlADH2 displays perfect (S)-selectivity and together with a highly (R)-selective short chain dehydrogenase/ reductase from Yarrowia lipolytica it is possible to access both enantiomers of 2-octanol in >99% ee with Yarrowia lipolytica oxidoreductases.

Localization of Cholesterol within Supported Lipid Bilayers Made of a Natural Extract of Tailor-Deuterated Phosphatidylcholine

Cholesterol is an essential component of mammalian membranes and is known to induce a series of physicochemical changes in the lipid bilayer. Such changes include the formation of liquid-ordered phases with an increased thickness and a configurational order as compared to liquid-disordered phases. For saturated lipid membranes, cholesterol molecules localize close to the lipid head group-tail interface. However, the presence of polyunsaturated lipids was recently shown to promote relocation of cholesterol toward the inner interface between the two bilayer leaflets. Here, neutron reflection is used to study the location of cholesterol (both non-deuterated and per-deuterated versions are used) within supported lipid bilayers composed of a natural mixture of phosphatidylcholine (PC). The lipids were produced in a genetically modified strain of Escherichia coli and grown under specific deuterated conditions to give an overall neutron scattering length density (which depends on the level of deuteration) of the lipids matching that of D2O. The combination of solvent contrast variation method with specific deuteration shows that cholesterol is located closer to the lipid head group-tail interface in this natural PC extract rather than in the center of the core of the bilayer as seen for very thin or polyunsaturated membranes.

ADVENTURES IN MICROWAVE-ASSISTED ORGANIC SYNTHESIS: CONTRIBUTIONS FROM THE KAPPE LABORATORY 2000-2005

This review highlights work in the field of microwave-assisted organic synthesis (MAOS) originating from the Kappe research laboratories in Graz, Austria. The focus of the article is on synthetic applications in the area of heterocyclic chemistry, multicomponent reactions, transition metal- catalyzed processes, solid-phase synthesis and combinatorial chemistry.

Recombinant expression, purification and biochemical characterization of kievitone hydratase from Nectria haematococca

Kievitone hydratase catalyzes the addition of water to the double bond of the prenyl moiety of plant isoflavonoid kievitone and, thereby, forms the tertiary alcohol hydroxy-kievitone. In nature, this conversion is associated with a defense mechanism of fungal pathogens against phytoalexins generated by host plants after infection. As of today, a gene sequence coding for kievitone hydratase activity has only been identified and characterized in Fusarium solani f. sp. phaseoli. Here, we report on the identification of a putative kievitone hydratase sequence in Nectria haematococca (NhKHS), the teleomorph state of F. solani, based on in silico sequence analyses. After heterologous expression of the enzyme in the methylotrophic yeast Pichia pastoris, we have confirmed its kievitone hydration activity and have assessed its biochemical properties and substrate specificity. Purified recombinant NhKHS is obviously a homodimeric glycoprotein. Due to its good activity for the readily available chalcone derivative xanthohumol (XN), this compound was selected as a model substrate for biochemical studies. The optimal pH and temperature for hydratase activity were 6.0 and 35°C, respectively, and apparent Vmax and Km values for hydration of XN were 7.16 μmol min-1 mg-1 and 0.98 ± 0.13 mM, respectively. Due to its catalytic properties and apparent substrate promiscuity, NhKHS is a promising enzyme for the biocatalytic production of tertiary alcohols.