Comparison of cyclohexanone monooxygenase as an isolated enzyme and whole cell biocatalyst for the enantioselective oxidation of 1,3-dithiane

Neutrophil Immunomodulatory Activity of Natural Organosulfur Compounds

Organosulfur compounds are bioactive components of garlic essential oil (EO), mustard oil, Ferula EOs, asafoetida, and other plant and food extracts. Traditionally, garlic (Allium sativum) is used to boost the immune system; however, the mechanisms involved in the putative immunomodulatory effects of garlic are unknown. We investigated the effects of garlic EO and 22 organosulfur compounds on human neutrophil responses. Garlic EO, allyl propyl disulfide, dipropyl disulfide, diallyl disulfide, and allyl isothiocyanate (AITC) directly activated Ca²⁺ flux in neutrophils, with the most potent being AITC. Although 1,3-dithiane did not activate neutrophil Ca²⁺ flux, this minor constituent of garlic EO stimulated neutrophil reactive oxygen species (ROS) production. In contrast, a close analog (1,4-dithiane) was unable to activate neutrophil ROS production. Although 1,3-dithiane-1-oxide also stimulated neutrophil ROS production, only traces of this oxidation product were generated after a 5 h treatment of HL60 cells with 1,3-dithiane. Evaluation of several phosphatidylinositol-3 kinase (PI3K) inhibitors with different subtype specificities (A-66, TGX 221, AS605240, and PI 3065) showed that the PI3K p110δ inhibitor PI 3065 was the most potent inhibitor of 1,3-dithiane-induced neutrophil ROS production. Furthermore, 1,3-dithiane enhanced the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), glycogen synthase kinase 3 α/β (GSK-3α/β), and cAMP response element binding (CREB) protein in differentiated neutrophil-like HL60 cells. Density functional theory (DFT) calculations confirmed the reactivity of 1,3-dithiane vs. 1,4-dithiane, based on the frontier molecular orbital analysis. Our results demonstrate that certain organosulfur compounds can activate neutrophil functional activity and may serve as biological response modifiers by augmenting phagocyte functions.

Intensification of β-poly(L: -malic acid) production by Aureobasidium pullulans ipe-1 in the late exponential growth phase

β-Poly(malic acid) (PMLA) has attracted industrial interest because this polyester can be used as a prodrug or for drug delivery systems. In PMLA production by Aureobasidium pullulans ipe-1, it was found that PLMA production was associated with cell growth in the early exponential growth phase and dissociated from cell growth in the late exponential growth phase. To enhance PMLA production in the late phase, different fermentation modes and strategies for controlling culture redox potential (CRP) were studied. The results showed that high concentrations of produced PMLA (above 40 g/l) not only inhibited PMLA production, but also was detrimental to cell growth. Moreover, when CRP increased from 57 to 100 mV in the late exponential growth phase, the lack of reducing power in the broth also decreased PMLA productivity. PMLA productivity could be enhanced by repeated-batch culture to maintain cell growth in the exponential growth phase, or by cell-recycle culture with membrane to remove the produced PMLA, or by maintaining CRP below 70 mV no matter which kind of fermentation mode was adopted. Repeated-batch culture afforded a high PMLA concentration (up to 63.2 g/l) with a productivity of 1.15 g l(-1) h(-1). Cell-recycle culture also confirmed that PMLA production by the strain ipe-1 was associated with cell growth.

Recent developments in the application of Baeyer-Villiger monooxygenases as biocatalysts

Baeyer-Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer-Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin-containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio- and enantioselective Baeyer-Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO-mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.

Overcoming the thermodynamic limitation in asymmetric hydrogen transfer reactions catalyzed by whole cells

Whole lyophilized cells of an Escherichia coli overexpressing the alcohol dehydrogenase (ADH-'A') from Rhodococcus ruber DSM 44541 were used for the asymmetric reduction of ketones to secondary alcohols. The recycling of the required nicotinamide cofactor (NADH) was achieved in a coupled-substrate process. In the course of the reaction the ketone is reduced to the alcohol and the hydrogen donor 2-propanol is oxidized to acetone by one enzyme. This leads to a thermodynamic equilibrium between all four components determining the maximum achievable conversion. To overcome this limitation an in situ product removal technique (ISPR) for the application with whole cells was developed. In this method the most volatile compound is separated from the reaction vessel by an air flow resulting in a shift of the equilibrium towards the desired secondary alcohol. The so-called stripping process represents a simple and efficient method to overcome the thermodynamic limitation in biocatalytic reactions. Employing this method, the conversion of selected biotransformations was increased up to completeness.

Microbial transformations 59: first kilogram scale asymmetric microbial Baeyer-Villiger oxidation with optimized productivity using a resin-based in situ SFPR strategy

This study is demonstrating the scale up of asymmetric microbial Baeyer-Villiger oxidation of racemic bicyclo[3.2.0]hept-2-en-6-one (1) to the kilogram scale using a 50 L bioreactor. The process has been optimized with respect to bottlenecks identified in downscaled experiments. A high productivity was obtained combining a resin-based in situ substrate feeding and product removal methodology (in situ SFPR), a glycerol feed control, and an improved oxygenation device (using a sintered-metal sparger). As expected both regioisomeric lactones [(-)-(1S,5R)-2 and (-)-(1R,5S)-3] were obtained in nearly enantiopure form (ee > 98%) and good yield. This represents the first example of such an asymmetric Baeyer-Villiger biooxidation reaction ever operated at that scale. This novel resin-based in situ SFPR technology therefore clearly opens the way to further (industrial) upscaling of this highly valuable (asymmetric) reaction.

Non-enzymatic regeneration of nicotinamide and flavin cofactors for monooxygenase catalysis

Biocatalytic oxygenation chemistry is a rapidly evolving field in which monooxygenases are the tools of choice. Monooxygenases catalyze many industrially important synthetic transformations; however, their use in preparative applications is hampered by their intrinsic requirement for reducing equivalents. As a result, non-enzymatic strategies--where the reducing equivalents are introduced directly into the catalytic cycle--are being developed to supersede the well-established enzymatic NAD(P)H regeneration systems currently in use. In this review we summarize and evaluate recent achievements in this area.

On oxygen limitation in a whole cell biocatalytic Baeyer–Villiger oxidation process

In this article, a recombinant cyclohexanone monooxygenase (CHMO), overexpressed in Escherichia coli has been used to study the oxidation of bicyclo[3.2.0]hept-2-en-6-one to its two corresponding lactones at very high enantiomeric excess. The reaction is a useful model for the study of biocatalytic oxidations to create optically pure molecules. The major limitations to a highly productive biocatalytic oxidation in this case are oxygen supply, product inhibition, and biocatalyst stability. In this article, we investigate the effects of whole cell biocatalyst concentration on the rate of reaction at a range of scales from shake flasks to 75 L bioreactors. At low cell concentrations (<2 g(dcw)/L) the maximum specific rate (0.65 g/g(dcw).h) is observed. However, at higher cell concentrations (> 2 g(dcw)/L), the reaction becomes oxygen limited and both the specific rate and absolute rate decrease with further increases in cell concentration. The role of oxygen limitation in reducing the rate of reaction with scale was investigated by increasing the maximum oxygen transfer rate in the reactor at a high cell concentration and observing the increase in product formation rate. We propose a qualitative model demonstrating the relationship between oxygen limitation, biocatalyst concentration, and the rate of reaction. This conceptual model will be a useful guide in the industrial scale-up of whole cell mediated Baeyer-Villiger biocatalysis.

[Cp*Rh(bpy)(H2O)]2+ as a coenzyme substitute in enzymatic oxidations catalyzed by Baeyer–Villiger monooxygenases

[Cp*Rh(bpy)(H2O)]2+ was applied as a flavin regenerating reagent in BVMO catalyzed oxidations of organic sulfides to chiral sulfoxides.