Enzyme approaches

Such isolated enzyme approaches for deracemization have a clear disadvantage in that they require two operational manipulations with an intermediate recovery step. A one-pot strategy is offered by employing whole-cell biotransformations with strains containing set(s) of complementary dehydrogenases operating in both biooxidative and bioreductive modes. Trace amounts of the intermediate ketone species can be isolated in several cases. In order to lead to an efficient deracemization... 

Oligosaccharide syntheses employing enzymatic reactions would in principle greatly benefit from being performed on a polymer support since the support might effectively facilitate isolation of the final product. Presumably, a water-soluble polymeric support will be preferable to any insoluble support since reaction rates could otherwise become too slow. Glycosidases as synthetic enzymes would be the best candidates to study this type of the enzymic approach to oligosaccharide synthesis. 

Perhaps the only distinct advantage of enzymic catalysts is their (occasionally) very high turnover rate in situ. Thus, the molar activity (formerly called the turnover number) of some enzymes approaches 36,000,000/min/molecule (7). This latter number pertains to carbonic anhydrase C, the enzyme that converts C02 to HC03 . However, chemists do not need enzymes to convert COz to HCO3-, as long as we are not considering in vivo reactions. Since many enzymes have molar activities as low as 1150/min/molecule, we need not consider molar activities of 100 to 500 (for nonenzymic catalysts) as a severe handicap. It is evident that enzymes and nonenzymic chiral catalysts, rather than being competitors, complement one another. 

Numerous enzyme-based biosensors have been described. Wang and Arnold reported the use of a dual-enzyme approach to induce spectral changes in NAD+ in... 

Subsequently, a combined organic-enzymic approach was proposed for the synthesis of oligosaccharides. By common organic procedures, a... 

In a biochemical pathway there will be one reaction that is non-eqnilibrinm and for which the enzyme approaches satnration with its snbstrate (pathway snbstrate). That is, the enzyme catalyses a zero order process (Fignre 3.7). 

Stafford, H.A., Flavonoid evolution an enzymic approach. Plant Physiol, 96, 680, 1991. 

The chemical-enzymic approach to the synthesis of modified polysaccharides presents a good prospect for the preparation of small quantities of these polymers, which may prove very useful for immunochemical studies. The approach is certainly not limited by the specific case of Salmonella polysaccharides 10-12, and may well be extended to other polymers. The first results from this group322 show that several analogs of O-specific polysaccharides (18) of Salmonella serogroups C2 and C3 may be prepared through this approach. 

Stafford HA. 1991. Flavonoid evolution An enzymic approach. Plant Physiol 96 680-685. 

Figure 5.9 NAD+/NADH cofactor regeneration a the coupled enzyme approach b the coupled substrate approach c, d examples [42,43] of the two approaches.

John, R. A. Enzyme-Induced Inactivation of Pyridoxal Phosphate-Dependent Enzymes Approaches to the design of specific inhibitors, in Enzyme Inhibitors as Drugs, (ed.) Sandler, M p. 73, London, Macmillan Press 1980... 

The results illustrate the ability of directed molecular evolution and site-directed mutagenesis technologies to deliver solutions about biotechnological problems of limited performance of peroxidase enzymes, approaching peroxidases as viable industrial biocatalysts. 

Interchange of H and (CH )CH results in steric hindrance to enzyme approach... 

A study of the action of beta-amylase on enzymically synthesized, branched oligosaccharides, containing 4 to 7 D-glucose residues, showed that the rate of hydrolysis is considerably lessened as the enzyme approaches the branch-points, and action stops 2 or 3 D-glucose units away from the branch-point. ... 

While the enzymic approach must be refined and rationalized, present evidence is completely in accord with the concept of linear and branched structures for the two starch components. The theory has been advanced that the individual alpha and beta Schardinger dextrins originate from specific starch fractions, but this appears to be untenable. No satisfactory mechanism has been proposed to account for the production of the several crystalline dextrins. 

Several non-natural a2,6-linked sialosides 36-40 with azide or alkyne-modified sialic acid residues were also prepared in excellent yields (86%-93%) from their C2- or C6- modified ManNAc or mannose bearing corresponding azide or alkyne functional groups 30-34 using the one-pot three-enzyme approach and GaipOMe (35) as an acceptor for Pd2,6ST (Scheme 5). 

Due to its high efficiency, the one-pot three-enzyme approach described above should also be suitable for direct transferring modified sialic acid residues to glycoconjugates containing a terminal Gal or GalNAc residue. 

Although our co-immobilized enzyme approach is able to sustain the necessary pH difference between isomerization and fermentation steps in SIF [35], the overall production rate of ethanol in SIF will still be limited by the total concentration of xylulose available to the yeast [9]. Under normal equilibrium conditions, the xylulose concentration is usually at best one fifth of the xylose concentration. Hence, other avenues of shifting the equilibrium towards higher xylulose formation will further increase the rate of ethanol production. 

---