Biocatalysts compounds

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

After discussing the biological capability to transform steroids, we briefly examine foe biotransformation of other terpenoids to ensure that the reader develops an awareness of the potential of biotechnology to modify or produce derivatives of a wide range of natural materials that are of tremendous potential, commercial value in the food and health care sectors. We also include a brief consideration of the use of biocatalysts to transform a range of other hydrocarbon compounds. 

Despite the still growing number of available methods for the preparation of enantiopure compounds by the use ofasymmetric catalysis, kinetic resolution (KR) is still the most employed method in the industry [4], and in most cases biocatalysts (enzymes) are used. 

In the last decade, biocatalysis in nonaqueous media, using hydrolases, has been widely used for organic chemists. The possibilities that these biocatalysts offer for the preparation of different types of organic compounds, depending upon the nucleophile... 

Furthermore, the biocatalysts will be even more important with the shift of the raw materials from oil to biomass. Since biomass is a mixture of various multifunctional compounds, chemo-, regio-, and enantioselective catalysts will be... 

At present, photosynthetic organisms are not generally used as biocatalysts for bioconversion of organic compounds except for bioremediation of pollutants in the environment, although they are environment-friendly catalysts, and they may contain unusual type of enzymes to establish new reactions. Development of bioreactors specially developed for photosynthefic organism-catalyzed reaction as well as finding effective photosynthetic organisms as a biocatalyst are required in the future. 

Metabolic and enzyme engineering have received a lot of attention in academic institutions and are now being applied for the optimization of biocatalysts used in the production of a diverse range of products. Engineered microorganisms, even with non-native enzyme activities, are being used for novel products and process improvements for the production of precursors, intermediates and complete compounds, required in the pharmaceutical industry (Chartrain et ai, 2000). 

In a study looking at oxygenated metabolites of AEA and their interaction with the cannabinoid system, a series of hydroxylated alkyl chains was prepared using different lipoxygenases as biocatalysts [152-154]. Of the seven AEA derivatives prepared, only the 5R-hydroxy (208), 125 -hydroxy (209) and 155 -hydroxy (210) derivatives had any affinity for the CBi receptor. Interestingly, the IdV-hydroxy compound (211) that was inactive at the CBi receptor displayed some affinity for the CB2 receptor [152] (see Table 6.18). 

Over the years of evolution, Nature has developed enzymes which are able to catalyze a multitude of different transformations with amazing enhancements in rate [1]. Moreover, these enzyme proteins show a high specificity in most cases, allowing the enantioselective formation of chiral compounds. Therefore, it is not surprising that they have been used for decades as biocatalysts in the chemical synthesis in a flask. Besides their synthetic advantages, enzymes are also beneficial from an economical - and especially ecological - point of view, as they stand for renewable resources and biocompatible reaction conditions in most cases, which corresponds with the conception of Green Chemistry [2]. 

Chibata, L., Tosa, T. and Shibatani, T. (1992) The industrial production of optically active compounds by immobilized biocatalysts, in Chirality in Industry (eels A. X. Collins, G.N. Sheldrake and J. Crosby), John Wiley Sons, Ltd, New York, pp. 351-370. 

Biodesulfurization (BDS) is the excision (liberation or removal) of sulfur from organosul-fur compounds, including sulfur-bearing heterocycles, as a result of the selective cleavage of carbon-sulfur bonds in those compounds by the action of a biocatalyst. Biocatalysts capable of selective sulfur removal, without significant conversion of other components in the fuel are desirable. BDS can either be an oxidative or a reductive process, resulting in conversion of sulfur to sulfate in an oxidative process and conversion to hydrogen sulfide in a reductive process. However, the reductive processes have been rare and mostly remained elusive to development due to lack of reproducibility of the results. Moderate reaction conditions are employed, in both processes, such as ambient temperature (about 30°C) and pressure. 

Other Rhodococcus strains similar to those described above in terms of the desulfurization ability have also been isolated [83], The purpose of identifying such Rhodococcus strains, in several cases, appears to be the development of in-house biocatalysts for BDS application. The specificity of the desulfurizing strains of organosulfur compounds in addition to DBT has also been studied (Table 3). 

Other claimed matter DBT for enrichment, biocatalyst preparation contacting process Enzymes contacting process Pure compounds as feedstock Membrane fragments and extracts Cell-free extract (envelope and its fragments + associated enzyme) reversible emulsion microemulsion reverse micelles Cell-free enzyme preparation microemulsified process RR and derivatives and other biocatalyst concepts + any known microorganism active for C—S bond cleavage... 

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