Reactions biocatalytic

Various peroxidases and monooxygenases have been used as biocatalysts for the oxidation of sulfides to sulfoxides [74, 75]. Haloperoxidases have been studied in 

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In this chapter, we try to summarize the work so far reported in this field. We first give a short introduction into the different forms of biocatalytic reactions, highlighting some special properties of biocatalysts. 

Thanks to their special properties and potential advantages, ionic liquids may be interesting solvents for biocatalytic reactions to solve some of the problems discussed above. After initial trials more than 15 years ago, in which ethylammonium nitrate was used in salt/water mixtures [29], results from the use of ionic liquids as pure solvent, as co-solvent, or for biphasic systems have recently been reported. The reaction systems are summarized in Tables 8.3-1 and 8.3-2, below. Table 8.3-1 compiles all biocatalytic systems except lipases, which are shown separately in 8.3-2. Some of the entries are discussed in more detail below. 

Toxin (Enzyme Inhibition) Biosensors Enzyme affectors (inhibitors and activators) that influence the rate of biocatalytic reactions can also be measured. Sensing probes for organophosphate and carbamate pesticides, for the respiratory... 

The enantioselectivity of biocatalytic reactions is normally expressed as the enantiomeric ratio or the E value [la], a biochemical constant intrinsic to each enzyme that, contrary to enantiomeric excess, is independent of the extent of conversion. In an enzymatic resolution of a racemic substrate, the E value can be considered equal to the ratio of the rates of reaction for the two enantiomers, when the conversion is close to zero. More precisely, the value is defined as the ratio between the specificity constants (k st/Ku) for tho two enantiomers and can be obtained by determination of the k<-at and Km of a given enzyme for the two individual enantiomers. 

Preparation of optically active P-aminoesters, P-aminonitriles, and P-aminocarbox-amides are of special relevance for the synthesis of enantiomerically pure P-aminoacids compounds of special relevance in several areas of medicinal chemistry. The resolution of P-aminoesters can be carried out by acylation of the amino groups or by other biocatalytic reactions of the ester groups, such as hydrolysis, transesterification, or aminolysis. The resolution of ethyl ( )-3-aminobutyrate... 

Comparison of whole cell biocatalytic reaction kinetics for recombinant Escherichia coli with periplasmic-secreting or cytoplasmic-expressing organophosphorus hydrolase... 

In the present work, for detail kinetic studies, we compared biocatalytic reaction kinetics for four types of whole cell biocatalyst systems whole cells with periplasmic-secreting OPH under trc or T7 promoters and whole cells with cytoplasmic-expressing OPH imder trc or T7 promoters. 

Fig. 2. Whole cell biocatalytic reactions for four types of recombinant whole cell systems. Bioconversion reactions were performed in resting cell condition. All data were based on unit cell concentration (1 mg-dry cell weight ml ). Each value and error bar represents the mean of two independent experiments and its standard deviation.

Fig. 3. Lineweaver-Burk plot analyses for four types of recombinant whole cell biocatalytic reactions.

Since the beginning of the 20th century, organic solvents have been used in enzymatic reaction media [30]. Biocatalytic reactions in water-organic biphasic media were first carried out by Cremonesi et al. [31] and by Buckland et al. [32] less than 30 years ago. Their work aimed at the conversion of high concentrations of poorly water soluble components, particularly steroids. Later, biphasic systems were used for enzyme-catalyzed synthesis reactions that were unfavored in water, changing the reaction equilibrium towards the higher yield of the product, such as esters or peptides. 

The majority of biocatalytic reactions are thermodynamically controlled. Product yield is thus dependent on the equilibrium position of a reaction. Optimization of the product yield requires knowledge of the equilibrium position in different organic solvents. Several works described and compared models for the prediction of the equilibrium position in two-phase media [6, 28, 29, 33]. 

Many interesting biocatalytic reactions involve organic components that are poorly water-soluble. When using organic-aqueous biphasic bioreactor, availability of poorly water-soluble reactants to cells and enzymes is improved, and product extraction can be coupled to the bioreaction. Many applications in two-phase media can use the existing standard-type bioreactors, such as stirred-tank, fluidized-bed, and column reactors with minor adjustments. 

Hill, E.M., Broering, J.M., Hallett, J.P., Bommarius, A.S., Liotta, C.L. and Eckert, C.A. (2007) Coupling chiral homogeneous biocatalytic reactions with benign heterogeneous separation. Green Chemistry, 9 (8), 888—893. 

W09617940 [45] desulfurization of fossil fuel with flavoprotein. biodesulfurization of a fossil fuel by adding an amount of a flavoprotein to the biocatalytic reaction mixture. Incubation and separation complete the process scheme. 

A major aspect to be overcome in the integration of biocatalysis and chemocatalysis through cascade conversions is the lack of compatibility of the various procedures, both mutually for the many chemocatalytic reactions and between the chemocatalytic and biocatalytic conversions. This is in contrast to biocatalytic reactions, which are, by far, more mutually compatible and can be much more easily combined in a multi-step cascade, as will be shown below. 

Integration of biocatalysts into industrial processes and biocatalytic reaction engineering... 

Straightforward. We have therefore employed XAD-4 to combine biocatalytic synthesis with simultaneous product extraction. The system (Figure 15.8) comprises a continuously stirred tank reactor, a starting material feed pump, a product recovery loop with a (semi-) fluidized bed of XAD-4, and a pump to circulate the entire reaction mixture through the loop." ° Preliminary studies indicated that XAD-4 had no detrimental effects on E. coli JMlOl (pHBP461), hence, separation of biomass and reaction liquid prior to catechol extraction was not required. The biocatalytic reaction was carried out at very low concentrations of the toxic substrate and product. This was achieved by feeding the substrate at a rate lower than the potential bioconversion rate in the reactor. 

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