Mateja Pogorevc

Selectivity-enhancement in enantioselective hydrolysis of sec-alkyl sulfates by an alkylsulfatase from Rhodococcus ruber DSM 44541

Abstract The Enantioselectivity of the biohydrolysis of sec -alkyl sulfate esters using a bacterial alkylsulfatase from Rhodococcus ruber DSM 44541 was dramatically enhanced in presence of additives (‘enhancers’) such as carbohydrates, polyethylene glycol, detergents, metal ions and through enzyme immobilization. In presence of iron, the E value for the kinetic resolution of (±)-3- and (±)-4-octyl sulfate was improved from E =3.9 to ≥200 and E =1.1 to 10, respectively.

Biocatalytic resolution of sterically hindered alcohols, carboxylic acids and esters containing fully substituted chiral centers by hydrolytic enzymes

Abstract Carboxyl esters bearing a fully substituted chiral center adjacent to the ester moiety, i.e., esters of tert -alcohols and of α,α-disubstituted carboxylates, are usually not accepted as substrates for hydrolytic enzymes such as esterases, proteases, and lipases. In order to circumvent this limitation, three strategies, which are reviewed in this paper, have been developed. (i) Several proteases and (still unspecified) microbial esterases are capable of hydrolysing esters of tert -alcohols and α,α-disubstituted carboxylic acids despite their steric bulkiness, but the number of these highly useful enzymes is rather limited. Alternatively, (ii) the use of ‘activated esters’ bearing electron-withdrawing groups enhances the electrophilic properties of the ester moiety (thus increasing the enzymatic reaction rate) may help to overcome slow reaction rates. On the other hand, (iii) spatial separation of the bulky quarternary carbon atom bearing the chiral center from the ester group to be hydrolysed by a spacer moiety led to modified (non-activated) substrates which were readily accepted.

A caveat for the use of log P values for the assessment of the biocompatibility of organic solvents

The degree of enzyme deactivation for lipases from Candida rugosa and Pseudomonas sp., hydroxynitrile lyase and mandelate racemase upon exposure to organic solvents can be correlated to their respective partition coefficients (log P values). However, three unexpected results were obtained: (1) the deactivation exerted by protic solvents, e.g., methanol, is severely underestimated; (2) little deactivation by an organic solvent cannot neccessarily be correlated to catalytic activity in this medium, and (3) in contrast to other enzymes, hydroxynitrile lyase is exceptionally stable towards deactivation by DMF.

Purification and Characterization of an Inverting Stereo- and Enantioselective sec-Alkylsulfatase from the Gram-Positive Bacterium Rhodococcus ruber DSM 44541

ABSTRACT Whole cells of Rhodococcus ruber DSM 44541 were found to hydrolyze (±)-2-octyl sulfate in a stereo- and enantiospecific fashion. When growing on a complex medium, the cells produced two sec-alkylsulfatases and (at least) one prim-alkylsulfatase in the absence of an inducer, such as a sec-alkyl sulfate or a sec-alcohol. From the crude cell-free lysate, two proteins responsible for sulfate ester hydrolysis (designated RS1 and RS2) were separated from each other based on their different hydrophobicities and were subjected to further chromatographic purification. In contrast to sulfatase RS1, enzyme RS2 proved to be reasonably stable and thus could be purified to homogeneity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single band at a molecular mass of 43 kDa. Maximal enzyme activity was observed at 30°C and at pH 7.5. Sulfatase RS2 showed a clear preference for the hydrolysis of linear secondary alkyl sulfates, such as 2-, 3-, or 4-octyl sulfate, with remarkable enantioselectivity (an enantiomeric ratio of up to 21 [23]). Enzymatic hydrolysis of (R)-2-octyl sulfate furnished (S)-2-octanol without racemization, which revealed that the enzymatic hydrolysis proceeded through inversion of the configuration at the stereogenic carbon atom. Screening of a broad palette of potential substrates showed that the enzyme exhibited limited substrate tolerance; while simple linear sec-alkyl sulfates (C7 to C10) were freely accepted, no activity was found with branched and mixed aryl-alkyl sec-sulfates. Due to the fact that prim-sulfates were not accepted, the enzyme was classified as sec-alkylsulfatase (EC 3.1.6.X).

Enantioselective stereoinversion of sec-alkyl sulfates by an alkylsulfatase from Rhodococcus ruber DSM 44541

Enantioselective biohydrolysis of sec-alkyl sulfate esters using a bacterial alkylsulfatase from Rhodococcus ruber DSM 44541 proceeded in a stereoselective fashion though inversion of configuration. Thus, from racemic substrates, the corresponding (R)-enantiomers were hydrolyzed selectively to furnish the corresponding sec-alcohol and non-reacted sulfate ester, both of (S)-configuration, which represents a homochiral product mixture. The enantioselectivities were found to depend on the substrate structure and were optimal for sec-sulfate esters in the ω-1 position (up to E=21). Since the enzyme was inactive on prim-sulfate esters, it can be classified as a sec-alkylsulfatase [EC 3.1.6.X].

Novel Carboxyl Esterase Preparations for the Resolution of Linalyl Acetate

Summary. Biocatalytic resolution of the tertiary terpene alcohol (±)-linalool was accomplished via hydrolysis of its corresponding acetate ester using two highly enantiospecific enzymes (E > 100). The latter were identified in a crude cell-free extract of Rhodococcus ruber DSM 43338 and could be separated by (partial) protein purification. Since they showed opposite enantiopreference, they were termed (R)- and (S)-linalyl acetate hydrolase (LAH). The activity and selectivity of the enzyme preparations was markedly dependent on the fermentation conditions.

Synthesis of 2,4-diketoacids and their aqueous solution structures

The synthesis of several 2,4-diketo carboxylic acids by standard methods was undertaken to study the substrate specificity of the carbon-carbon bond hydrolases. It was shown by 1H- and 13C-NMR experiments that compounds with 4-alkyl, 4-alkenyl and 4-alicyclic substituents exist in three main forms: 2,4-diketo, 2-enol-4-keto and 2-hydrate-4-keto. The equilibrium ratios of these aqueous solution structures were similar, but were markedly affected by the pH values (1.5–10.5). At pH 7.5 the ratio of these structures was approximately 4:5:1, but at low pH values the 2-hydrate predominated (≈50%) and at high pH values the 2-enolate carboxylate was dominant (≈80%) while the 2-hydrate was not detected. 4-Aryl substituents gave one pH-independent isomer formulated with C-2, C-3 and C-4 electrons delocalized in conjugation with the arene at C-4. This interpretation of a very rapid equilibrium between 2- and 4-enolate isomers to give a π-delocalized six-membered ring in conjugation with arene substituents is supported by the effect of divalent metal ions on the structural forms. Rate and equilibrium constants for several of these solution interconversions are influenced by pH. Mg2+ coordinates to the 2-enolate dianion of alkyl analogues, whereas Cu2+ forms a six-membered π-delocalized ring with the 2- and 4-oxo atoms in conjugation with the arenes. Exchange of 2H from 2H2O-enriched solvent occurs with the protons at C-3. The dimers of the 4-alkyl analogues inaqua were characterized as a product of self-aldol condensations. These data have facilitated enzyme mechanism studies of C–C bond hydrolysases (β-ketolases).

Preparation of an epoxide-hydrolyzing biocatalyst: Rhodococcus ruber DSM 44540—an activity-growth study

The constitutive epoxide hydrolase activity of Rhodococcus ruber DSM 44540 strongly depends on the status of the cells and appears to be regulated by a catabolic switch: activity peaked when glucose was exhausted and peptone/yeast extract consumption started. The activity-maximum for the kinetic resolution of a 2,2- and the enantioconvergent asymmetric biohydrolysis of a 2,3-disubstituted oxirane coincided. In order to obtain a maximum yield, cells should be harvested after ca. 17 h.