Bioconversions

Disaccharides analogous to isomaltulose, but containing other sugars in place of ot-D-glucose, can be obtained by similar bioconversions from different disaccharides.234 Sucrose analogues (galactose-fructose, xylose-fructose, fucose-fructose) have also been prepared by isomerization, catalyzed by an engineered fructosyltransferase.235,236 

Finally, there is the important biotechnological hydrolysis of sucrose to a 1 1 mixture of glucose and fructose by the use of invertase (so-called because the dextrorotatory sucrose gives a levorotatory final mixture). Enzyme-mediated esterifications of sucrose have been discussed in the esterification section. 

The reaction of sucrose with chloroformates lead to cyclic carbonates or alkyloxycarbonyl derivatives.271 276 These reactions exhibit behavior similar to 

Other examples include, bridged interglycosidic anhydro derivatives that were identified upon treatment of sucrose with HF or by basic treatment of 

None of the precursors in the biosynthesis of quinoline alkaloids is available at a price lower than the alkaloids thus, bioconversion is not of interest, except for one step. This step would be the stereospecific conversion of quinidinone (obtained by oxidation of quinine or quinidine) to quinidine (8/ ,95) or quinine (85,97 ). This reaction offers the possibility of converting an excess of one of the naturally occurring stereoisomers to the other. It is now performed chemically as the demand for quinidine is higher than production from plant extracts. 

Feeding of tryptophan to Cinchona cell suspension cultures has been reported to result in considerable production of alkaloids 572,577,578). However, other authors reported that tryptophan inhibits the growth of the cell cultures and is converted, probably nonenzymatically, to norhar-mane and related compounds, and no increase in alkaloid production could be observed 586,587). 

Cinchona can be considered to be recalcitrant with regard to cell and tissue culture. Although it has been possible to obtain cell and tissue cultures of some Cinchona species, they often require special treatments also, growth is usually slow, and viability of cells is rapidly lost. Moreover, the cell cultures are poor producers of alkaloids. In order to arrive at 

---

Other approaches to inhibiting intramolecular cycli2ations of erythromycin have also proven successhil. Erom a series of O-alkyl derivatives of erythromycin, clarithromycin (6-0-methylerythromycin) (37) was selected for clinical development (146,147). Another approach replaced the C-8 proton of erythromycin with duorine, which was accompHshed by both chemical and bioconversion methods to yield durithromycin (38) (148). 

The enhanced activity from acylation of the 3- and 4 -hydroxyl groups of leucomycin prompted analogous studies of tylosin. Bioconversion of tylosin by S. thermotokrans yielded 3- and/or 4 -0-acyl derivatives possessing increased activity against certain resistant microorganisms and higher... 

Other macrohdes have been prepared which represent hybrids of stmctures within the 14-membered family, within the 16-membered family, or between the two families. These hybrids have been made by chemical, bioconversion, or genetic manipulations. The 9-0-[(2-methoxyethoxy)methyl]oxime of tylosin (Table 7) was synthesized, using the oxime found in roxithromycin (33) (369). 3-0-Cladinosyl derivatives of 16-membered macrohdes were synthesized in which the neutral sugar of erythromycin (Table 3) was attached to tylosin derivatives at thek 3-hydroxyl group, analogous to its position in erythromycin... 

Coffee bioconversions through enzymatic hydrolysis have been used to modify green coffee and improve the finished product (60). Similarly, enzymes have been reported which increase yield and improve flavor of instant coffee (61). Fermentation of green coffee extracts to produce diacetyl [431 -03-8] a coffee flavor compound, has also been demonstrated (62). 

Biocatalytic ledox reactions offer great synthetic utility to organic chemists. The majority of oxidase-catalyzed preparative bioconversions are still performed using a whole-ceU technique, despite the fact that the presence of more than one oxidoreductase in cells often leads to product degradation and lower selectivity. Fortunately, several efficient cofactor regeneration systems have been developed (160), making some cell-free enzymatic bioconversions economically feasible (161,162). 

Bioconversion Chemical conversion of a naturally occurring biodegradable substance using a biocatalyst. 

Fed-batch culture A cell cultivation technique in which one or more nutrients are supplied to the bioreactor in a given sequence during the growth or bioconversion process while the products remain in the vessel until the end of the run. 

Methane from renewable biological sources will never be a major energy resource, yet it can be a valuable addition to the energy supply mix. Nevertheless, whether methane comes from fossil fuel reservoirs or from bioconversions, it is certain to provide useful energy for many years to come. 

Two of the main raw materials used for bioconversion to L-phenylalanine are frous-cinnamic acid and acetamido cinnamic add (reactions 1 and 2 in Figure 8.6.)... 

Extra cost compared to direct fermentation are mainly concerned with addition of PPA (precursor addition) or ACA (bioconversion). Table 8.9 compares estimated costs for foe PPA and ACA process based on foe data considered previously (section 8.6.1 and 8.6.2). Costs are estimated for production of 100 tonnes per year. 

Hie bioconversion of a-aminonitriles, although up until now not used on an industrial scale, is of practical interest in the production of optical active a-amino adds. This, however, will only be the case if one can select a nitrilase that enantioselectively hydrolyses die aminonitrile. 

Figure A8.3 Preparation of optically active a-amino acids via bioconversion of the corresponding a-aminonitriles.

A similar approach for bioconversion, based on the date given in Table 8.8, shows that die peirentage reduction in substrate costs required is 11%. 

Bacteria, yeast and algae are produced in massive quantities of protein sources as food for animals and humans.1 SCP is considered a major source of feed for animals. The production of valuable biological products from industrial and agricultural wastes is considered through the bioconversion of solid wastes to added-value fermented product, which is easily marketable as animal feedstock. The waste streams that otherwise would cause pollution and threaten the environment can be considered raw material for CSP production using suitable strains of microorganisms. 

Several potential and mutant strains of T. viride have been identified in SCP production. Their capacity for amyloletic enzyme production was enhanced severalfold in SCP from lingnocellulosic resources. The process of bioconversion of agricultural wastes to SCP appeared to be too complex to find an economic application for agricultural waste. 

A prehminary study of the use of larch AGs in aqueous two-phase systems [394] revealed that this polysaccharide provides a low-cost alternative to fractionated dextrans for use in aqueous two-phase, two-polymer systems with polyethylene glycol (PEG). The narrow molecular-weight distribution (Mw/Mn of 1-2) and low viscosity at high concentration of AG can be exploited for reproducible separations of proteins under a variety of conditions. The AG/PEG systems were used with success for batch extractive bioconversions of cornstarch to cyclodextrin and glucose. 

This study provides a useful reference for generating chiral alcohols by selective microbial bioconversion. 

The development of bioreactor systems for the production of large-volume chemicals (see Chapter 3) could be the basis for reconsidering the production of biomass in limited quantities for fuel uses. This would require efficient microbial organisms to catalyze fermentation, digestion, and other bioconversion processes, as well as efficient separation methods to recover fuel products from process streams. 

An alternative approach to the bioconversion of sweet sorghum carbohydrates to ethanol. Biomass and Bioenergy, 8, 99-103. 

In many biological processes, e.g. the fermentation of cells and sensitive microorganisms or bioconversion with immobilised enzymes, low shear stress is of crucial importance for the optimal coimse of processes. 

---