Biocatalytic systems

A variety of monooxygenases (see above) can perform epoxidations. Some biocatalytic methods come into sight, which will become attractive for industrial use. In these cases chiral epoxides are the targeted products, and therefore this subject will be dealt with in Section 4.6. 

---

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. 

In the biocatalytic system, a second organic phase consisting of bis(2-ethyUiexyl)phthalate and containing the substrate is added at a phase ratio of 1 1. This procedure enables in situ product extraction and protects the microbial cells from toxic effects of the substrate and... 

II. ORGANIC SOLVENTS IN THE BIOCATALYTIC SYSTEMS POSSIBLE CONFIGURATIONS... 

Considering only the aqueous phase of the biocatalytic system, the equilibrium constant for the reaction is given as a function of thermodynamic activities of the components shown ... 

Clearly, quantum mechanics can be applied to biocatalytic systems in a variety of ways and scales. We hope that the methods presented in this article can further expand the scope of applications to biomolecular systems. 

Due to the water requirement of biocatalytic systems, BDS is typically carried out as a two-phase aqueous-oil process. However, increased sulfur removal rates could be accomplished by using an aqueous-alkane solvent catalytic system [46,203,220,255], The BDS catalytic activity depends on both, the biocatalysts and the nature of the feedstock. It can vary from low activity for crude oil to as high as 60% removal for light gas-oil type feedstocks [27,203,256], or 70% for middle distillates, 90% for diesel, 70% for hydrotreated diesel, and 90% for cracked feedstocks [203,256], The viscosity of the crude oil poses mixing issues in the two-phase oil-water systems however, such issues are minimal for distillate feedstocks, such as diesel or gasoline [257]. 

Improving functionality may involve a complex biocatalytic system including more than one biocatalyst, as it is the case in BDM reactions. Additionally to the BDS for instance, another biocatalyst active for BDM [395,406], which consists of a heme oxygenase or a Cytochrome reductase could be used to widen up the functionality. 

The patents, however, protected the microorganisms (biocatalysts/biocatalytic systems) [86,87] as well as process to use the microorganism [87], So far, there are no records of any other international protection. The patent reports new cultures of Rhodococcus strains, and a method to improve biocatalyst stability, which allows recycling. 

The first two patents [150,151] protect the biocatalytic system and its use in the form of whole cells or cell-free extracts or enzymes purified from the organisms. The other claimed organisms in the patent include the thermophilic culture Aneurinibacillus sp. IGTN4T (ATCC N° PTA-4581), P. stutzeri, Yokenella sp. and P. nitroreducens. The following text gives the details of the strain PTA-806, since the others have not been reported in much detail. 

Y.L. Khmelnitsky, A.V. Levashov, N.L. Klyachko, and K. Martinek, Engineering biocatalytic systems in organic media with low water content. Enzyme Microb. Technol. 10, 710-724 (1988). 

Chemistries such as gasification, carboxylation, carbonylation, partial oxidation, and salt splitting may see much greater emphasis in manufacturing. These chemistries will need concurrent development of more selective catalytic and biocatalytic systems and promoters, as well as processes requiring much less ex-... 

Microbial biocatalytic systems, 76 398 Microbial biocontrol agents, 73 347-348 foliar application of, 73 349 phytotoxin production and, 73 351 problems associated with, 73 348-349 shelf life and storage of, 73 350 Microbial biomass, 26 471 474 substrates for, 26 473-474 Microbial catalysts, rapid screening of, 76 405... 

A critical consideration in the development of biocatalytic systems is the form in which the enzyme or enzyme system is going to be used. There are two general approaches. One is to use isolated enzymes. If these are inexpensive, they can be used as disposable biocatalysts, as is the case for glucose isomerase, ° which is the key biocatalyst in the production of high-fructose corn syrups from starch, or the lipases and proteases that are present in detergents. Alternatively, if enzymes are expensive to produce, they can be immobilized and used repeatedly by recovering the enzyme particles after each use. 

Presently, specific immobilization of various enzymes is studied under the aspect of the orientation and the local surface environments. The deeper understanding of biocatalytic systems together with suitable surface coating techniques may lead to biologically inspired and more complex catalytic systems grafted on solid supports. 

Bioprocesses incorporating more than one redox enzyme in an oxidative reaction system might involve, in the simplest case, two oxidizing enzymes coupled so that they act sequentially to effect two oxidation steps. A key issue in the development of such oxidative biocatalytic systems would be the determination of the values, for each enzyme involved, of the redox potentials. These can be determined by potentiometric titration using redox mediators (such as NADH) and techniques such as cyclic voltammetry or electrophoresis [44]. Knowledge of the redox potentials would facilitate the design and engineering of a process in which the two... 

An additional condition may be imposed, even when a cofactor-independent enzyme is used, if a mediator molecule is involved in the electron transfer process, as is often the case with oxidases. Laccases, for example, may employ small-molecule diffusible mediator compounds in their redox cycle to shuttle electrons between the redox center of the enzyme and the substrate or electrode (Scheme 3.1) [1, 2]. Similarly, certain dehydrogenases utiHze pyrroloquinoline quinone. In biocatalytic systems, mediators based on metal complexes are often used. 

Cofactor Recycle in Multi-Step Oxidizing Biocatalytic Systems... 

Biocatalytic Systems Involving Coupled Oxidizing Enzymes... 

Diphasic Biocatalytic System for the Synthesis of Conducting Polymers... 

Scheme 6 Biphasic biocatalytic system in conducting polymer synthesis. (Reprinted with permission from Marcilla et al. [48]. 2009, WUey)...

In all the solvent-containing biocatalytic systems, the nature of the solvent influences the reaction to a large extent. The solvent can affect the activity and the stability of the enzyme and the maximal yield in the reaction. 

Starting from the findings of the racemic cross-benzoin condensation [66], and assuming that aldehydes not accepted as donor substrates might still be suitable acceptor substrates, and vice versa, a mixed enzyme-substrate screening was performed in order to identify a biocatalytic system for the asymmetric cross-carboligation of aromatic aldehydes. For this purpose the reactions of 2-chloro-(40a), 2-methoxy- (40b) and 2-methylbenzaldehyde (40c), respectively, were studied with different enzymes in combination with benzaldehyde (Scheme 2.2.7.23) [67]. The three ortho-substituted benzaldehyde derivatives 40a-40c were... 

Increasing consumers demand for organic , bio , healthy , natural High chemo-, regio- and stereoselectivities of biocatalytic systems... 

---