Peroxidase-catalyzed asymmetric sulfoxidation in organic solvents versus in water

Stabilization of an enzyme cytochrome c in a metal-organic framework against denaturing organic solvents

Enzymes are promising catalysts with high selectivity and activity under mild reaction conditions. However, their practical application has largely been hindered by their high cost and poor stability. Metal-organic frameworks (MOFs) as host materials show potential in protecting proteins against denaturing conditions, but a systematic study investigating the stabilizing mechanism is still lacking. In this study, we stabilized enzyme cytochrome c (cyt c) by encapsulating it in a hierarchical mesoporous zirconium-based MOF, NU-1000 against denaturing organic solvents. Cyt c@NU-1000 showed a significantly enhanced activity compared to the native enzyme, and the composite retained this enhanced activity after treatment with five denaturing organic solvents. Moreover, the composite was recyclable without activity loss for at least three cycles. Our cyt c@NU-1000 model system demonstrates that enzyme@MOF composites prepared via post-synthetic encapsulation offer a promising route to overcome the challenges of enzyme stability and recyclability that impede the widespread adoption of biocatalysis.

Specific oxyfunctionalisations catalysed by peroxygenases: opportunities, challenges and solutions

Peroxygenases are promising oxyfunctionalisation catalysts for organic synthesis.

Biotechnological production of chiral organic sulfoxides: current state and perspectives

Chiral organic sulfoxides (COSs) are important compounds that act as chiral auxiliaries in numerous asymmetric reactions and as intermediates in chiral drug synthesis. In addition to their optical resolution, stereoselective oxidation of sulfides can be used for COS production. This reaction is facilitated by oxygenases and peroxidases from various microbial resources. To meet the current demand for esomeprazole, a proton pump inhibitor used in the treatment of gastric-acid-related disorders, and the (S)-isomer of an organic sulfoxide compound, omeprazole, a successful biotechnological production method using a Baeyer-Villiger monooxygenase (BVMO), was developed. In this review, we summarize the recent advancements in COS production using biocatalysts, including enzyme identification, protein engineering, and process development.

Enantioselective imidation of sulfides via enzyme-catalyzed intermolecular nitrogen-atom transfer

Engineering enzymes with novel reaction modes promises to expand the applications of biocatalysis in chemical synthesis and will enhance our understanding of how enzymes acquire new functions. The insertion of nitrogen-containing functional groups into unactivated C–H bonds is not catalyzed by known enzymes but was recently demonstrated using engineered variants of cytochrome P450_BM3 (CYP102A1) from Bacillus megaterium. Here, we extend this novel P450-catalyzed reaction to include intermolecular insertion of nitrogen into thioethers to form sulfimides. An examination of the reactivity of different P450_BM3 variants toward a range of substrates demonstrates that electronic properties of the substrates are important in this novel enzyme-catalyzed reaction. Moreover, amino acid substitutions have a large effect on the rate and stereoselectivity of sulfimidation, demonstrating that the protein plays a key role in determining reactivity and selectivity. These results provide a stepping stone for engineering more complex nitrogen-atom-transfer reactions in P450 enzymes and developing a more comprehensive biocatalytic repertoire.

New and classic families of secreted fungal heme peroxidases

Heme-containing peroxidases secreted by fungi are a fascinating group of biocatalysts with various ecological and biotechnological implications. For example, they are involved in the biodegradation of lignocelluloses and lignins and participate in the bioconversion of other diverse recalcitrant compounds as well as in the natural turnover of humic substances and organohalogens. The current review focuses on the most recently discovered and novel types of heme-dependent peroxidases, aromatic peroxygenases (APOs), and dye-decolorizing peroxidases (DyPs), which catalyze remarkable reactions such as peroxide-driven oxygen transfer and cleavage of anthraquinone derivatives, respectively, and represent own separate peroxidase superfamilies. Furthermore, several aspects of the "classic" fungal heme-containing peroxidases, i.e., lignin, manganese, and versatile peroxidases (LiP, MnP, and VP), phenol-oxidizing peroxidases as well as chloroperoxidase (CPO), are discussed against the background of recent scientific developments.

Protein engineering of toluene monooxygenases for synthesis of chiral sulfoxides

Enantiopure sulfoxides are valuable asymmetric starting materials and are important chiral auxiliaries in organic synthesis. Toluene monooxygenases (TMOs) have been shown previously to catalyze regioselective hydroxylation of substituted benzenes and phenols. Here we show that TMOs are also capable of performing enantioselective oxidation reactions of aromatic sulfides. Mutagenesis of position V106 in the alpha-hydroxylase subunit of toluene ortho-monooxygenase (TOM) of Burkholderia cepacia G4 and the analogous position I100 in toluene 4-monooxygenase (T4MO) of Pseudomonas mendocina KR1 improved both rate and enantioselectivity. Variant TomA3 V106M of TOM oxidized methyl phenyl sulfide to the corresponding sulfoxide at a rate of 3.0 nmol/min/mg protein compared with 1.6 for the wild-type enzyme, and the enantiomeric excess (pro-S) increased from 51% for the wild type to 88% for this mutant. Similarly, T4MO variant TmoA I100G increased the wild-type oxidation rate by 1.7-fold, and the enantiomeric excess rose from 86% to 98% (pro-S). Both wild-type enzymes showed lower activity with methyl para-tolyl sulfide as a substrate, but the improvement in the activity and enantioselectivity of the mutants was more dramatic. For example, T4MO variant TmoA I100G oxidized methyl para-tolyl sulfide 11 times faster than the wild type did and changed the selectivity from 41% pro-R to 77% pro-S. A correlation between regioselectivity and enantioselectivity was shown for TMOs studied in this work. Using in silico homology modeling, it is shown that residue I100 in T4MO aids in steering the substrate into the active site at the end of the long entrance channel. It is further hypothesized that the main function of V106 in TOM is the proper positioning or docking of the substrate with respect to the diiron atoms. The results from this work suggest that when the substrate is not aligned correctly in the active site, the oxidation rate is decreased and enantioselectivity is impaired, resulting in products with both chiral configurations.

Horseradish and soybean peroxidases: comparable tools for alternative niches?

Horseradish and soybean peroxidases (HRP and SBP, respectively) are useful biotechnological tools. HRP is often termed the classical plant heme peroxidase and although it has been studied for decades, our understanding has deepened since its cloning and subsequent expression, enabling numerous mutational and protein engineering studies. SBP, however, has been neglected until recently, despite offering a real alternative to HRP: SBP actually outperforms HRP in terms of stability and is now used in numerous biotechnological applications, including biosensors. Review of both is timely. This article summarizes and discusses the main insights into the structure and mechanism of HRP, with special emphasis on HRP mutagenesis, and outlines its use in a variety of applications. It also reviews the current knowledge and applications to date of SBP, particularly biosensors. The final paragraphs speculate on the future of plant heme-based peroxidases, with probable trends outlined and explored.

Co-lyophilization with d-proline Greatly Enhances Peroxidase’s Stereoselectivity in a Non-aqueous Medium

The impact of co-lyophilizing horseradish peroxidase (HRP) with numerous amino acids and other chiral excipients on the enzyme's subsequent stereoselectivity [E(S/R)] in the sulfoxidation of thioanisole in 2-propanol was systematically investigated. While many improved the stereoselectivity of (and significantly activated) HRP, the greatest effect was observed with D-proline which enhanced the E(S/R) value by over an order of magnitude from synthetically meaningless to useful.

New activities of a catalytic antibody with a peroxidase activity

In order to estimate the size of the cavity remaining around the heme of the 3A3-microperoxidase 8 (MP8) hemoabzyme, the formation of 3A3-MP8-Fe(II)-nitrosoalkane complexes upon oxidation of N-monosubstituted hydroxylamines was examined. This constituted a new reaction for hemoabzymes and is the first example of fully characterized Fe(II)-metabolite complexes of antibody-porphyrin. Also, via a comparison of the reactions with N-substituted hydroxylamines of various size and hydrophobicity, antibody 3A3 was confirmed to bring about a partial steric hindrance on the distal face of MP8. Subsequently, the influence of the antibody on the stereoselectivity of the S-oxidation of sulfides was examined. Our results showed that MP8 alone and the antibody-MP8 complex catalyze the oxidation of thioanisole by H(2)O(2) and tert-butyl hydroperoxide, following a peroxidase-like two-step oxygen-transfer mechanism involving a radical-cation intermediate. The best system, associating H(2)O(2) as oxidant and 3A3-MP8 as a catalyst, in the presence of 5% tert-butyl alcohol, led to the stereoselective S-oxidation of thioanisole with a 45% enantiomeric excess in favour of the R isomer. This constitutes the highest enantiomeric excess reported to date for the oxidation of sulfides catalyzed by hemoabzymes.

Asymmetric enzymatic oxidoreductions in organic solvents

It is now beyond doubt that enzymes can vigorously work even in neat organic solvents containing little or no water. Switching the enzymatic reaction medium from aqueous to nonaqueous can make previously problematic processes feasible through, for example, increased substrate solubility or diminished side reactions. Moreover, when placed in this highly unnatural milieu, enzymes exhibit new and potentially valuable properties, including greater stability, markedly altered selectivity that can be readily controlled by the solvent, and molecular memory. Consequently, novel synthetic and biotechnological opportunities ensue, as illustrated herein by those based on enzymatic oxidoreductions such as the asymmetric peroxidase-catalyzed sulfoxidation of organic sulfides.

Unexpectedly enhanced stereoselectivity of peroxidase-catalyzed sulfoxidation in branched alcohols

Lyophilized horseradish peroxidase (HRP) exhibits poor stereoselectivity in the sulfoxidation of thioanisole when the enzyme is either redissolved in water or suspended in organic solvents. However, when HRP is co-lyophilized in the presence of lyoprotectants or ligands, its stereoselectivity, although still low in most organic solvents, increases up to 4-fold if assayed in secondary or tertiary alcohols (but not in their linear isomers). A mechanistic hypothesis is presented explaining this puzzling phenomenon on the basis of a model of the active site of the enzyme-substrate complex derived from its X-ray crystal structure by means of molecular dynamics and energy minimization.

Asymmetric sulfoxidations mediated by alpha-chymotrypsin

The oxidation of aryl alkyl sulfides with H(2)O(2) in aqueous solution is a reasonably facile reaction producing racemic sulfoxides. We show that in the presence of the hydrolytic enzyme alpha-chymotrypsin such a sulfoxidation is accelerated and, more importantly, becomes stereoselective. With phenyl isobutyl sulfide as a model, the chymotrypsin-mediated, highly asymmetric oxidation is shown to occur in the hydrophobic binding pocket of the enzyme active site. The stereoselectivity of the chymotrypsin-mediated sulfoxidations is correctly explained by means of structure-based molecular modeling of the enzyme-sulfide complexes.

Peroxidase activity of in vitro-selected 2'-amino RNAs

Peroxidase activities of RNAs containing 2'-amino groups, which were selected as aptamers binding to N-methylmesoporphyrin IX, were investigated. Some clones promoted the oxidation reaction of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with hydrogen peroxide (H(2)O(2)) in the presence of iron(III)-protoporphyrin (hemin), whereas others did not. Each of them had a different substrate specificity. One of the active clones promoted the oxidation of o-dianisidine and beta-nicotinamide adenine dinucleotide reduced form (NADH) with H(2)O(2) 5 and 15 times faster than hemin only, respectively. On the other hand, one clone that was inactive on oxidation of ABTS exhibited the same level of activity on oxidation of o-dianisidine as that shown by the clone active on ABTS but no activity on NADH. By in vitro selection, we can produce various types of peroxidase-like non-natural RNAs.