Biocatalytic conversion

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. 

Biocatalytic conversion of lignocellulose into bioethanol, which requires upgrading of existing processes of fermenting sugars by using enzymatic-enhanced pretreatment of (hemi)cellulose. New, improved biocatalysts are needed for this route. 

Here, we describe, as a representative example, an efficient chemical synthesis of 4-fluorophenylpyruvic acid (Procedure 1, Section 10.5.1) followed by its biocatalytic conversion to L-4-fluorophenylalanine catalysed by the N145V mutant of PheDH (Procedure 2, Section 10.5.2). 

Figure 5.14 Chemical and biocatalytic conversion of penicillin G to 6-aminopenicillanic acid, the core structure of various antibiotics.

Any or all of these conditions may limit productivity, and whichever of the previously listed sequences occur in a particular process, the biocatalytic conversion needs to be selected and designed to cope with these issues. Such issues may be solved by process techniques (using purification or a change of medium/catalyst between the reactions) or potentially by alteration of the properties of the biocatalyst via selection or directed evolution. A further approach is to combine operations in a single-pot operation using process techniques, biocatalyst modification, and a degree of compromise. 

Many chemical and biocatalytic conversions involve reactions with an unfavorable equilibrium such as condensations.8 In both cases the Law of Mass Action will apply such that the removal of one species from the reaction mixture will shift the equilibrium position. This is particularly useful for biocatalytic conversions where the alternative approach of using a reactant in excess may have deleterious effects on the biocatalyst. 

Scheme 4.102 Indirect chemical regeneration of NADH using FAD/FADH2 as the mediator and subsequent biocatalytic conversion [425]. Reprinted with permission from [425]. Copyright 2005 American Chemical Society.

Most biocatalytic conversions are performed with the enzyme immobilized in the microreactor. Miyazaki et al. [426] developed a simple noncovalent immobilization method for His-tagged enzymes on a microchannel surface. These enzymes contain a polyhistidine-tag motif that consists of at least six histidine residues, often located at the N- or C-terminus. The H is-tag has a strong affinity for nickel and can be reversibly immobilized by a nickel-nitrilotriacetic acid (Ni-NTA) complex (Scheme 4.103), a strategy commonly used in affinity chromatography. 

Several synthetic examples that successfully exploit TK for biocatalytic conversions have been reported. For example, the total synthesis of the beetle pheromone (+)-exo-brevicomin utilizes TK from Baker s yeast as the sole chiral reagent. Starting with racemic 2-hydroxybutanol, the ability of TK to effect kinetic resolution of substrates was exploited (Scheme 5.56). The smooth reaction of 2-hydroxybutanol with HPA was catalyzed by TK at pH 7.5 to yield the enantioenriched ketone in 90% yield. This intermediate was chemically converted into (+)-exo-brevicomin.101... 

Another interesting commercial nonstereoselective application represents the biocatalytic conversion of methyl groups into carboxylic acids. This new technology, also developed at Lonza, is already successfully applied on an industrial scale in the production of 5-methylpyrazine-2-carboxylic acid, a versatile building block for pharmaceuticals [85]. 

Yang, F. and Russell, A. J., Two-step biocatalytic conversion of an ester to an aldehyde in reverse micelles, Biotechnol. Bioeng., 43, 232-241, 1994. 

The biotransformation has the advantages that no recovering of unreacted nitrile is necessary since the conversion is 100% and no copper catalyst removal is needed. This is also the first case of a biocatalytic conversion of a bulk fiber monomer. 

Below are examples, taken primarily from the ACS Symposium on Biocatalysis in Polymer Science at the National Meeting in San Francisco (September 2006), that highlight new biocatalytic methodologies. Indeed, throughout this book, many newly developed methodologies are reported that enable enzymes to do new or improved biocatalytic conversions. 

Biocatalytic Conversion of Renewable Feedstocks to Industrial Chemicals... 

Advantages of Genencor s biocatalytic conversion of biomass to value-added chemicals include, a) commercial viability, simplicity and economic feasibility b) prevention of product inhibition of cellulosic enzymes by concurrent conversion to bioproducts c) feasibility of quantitative conversion, d) elimination of byproducts e) higher productivity and yield on carbon J) production capacity enhancement. This biocatal) ic conversion concept is novel because a multienzyme process for converting renewable biomass to value-added commercial ingredients has not yet been commercially demonstrated. 

In-vitro Biocatalytic Conversion of Lignocellulosic Feed-stock... 

Figure 2 In vitro Biocatalytic Conversion of Cellulose (Avicel, Lattice 20) with Cellulose (Spezyme) to Glucose...

Figure 5 In vitro biocatalytic conversion of acid-pretreated corn-stover to gluconic acid with Spezyme, OxyGO and Fermcolase.

Figure 6 Biocatalytic conversion and concomitant fermentation of cellulose to 1, 3-Propanediol...

Economic Perspectives of Biocatalytic Conversion of Aromatics to Optical Pure Synthons for the Pharmaceutical Industry... 

Antibody catalysts, role in environmentally benign synthesis of chemicals, 125-126 Aqueous-supercritical carbon dioxide medium, phase-transfer catalytic oxidation, 144-145 Arene cw-dihydrodiols, biocatalytic conversion of aromatics to optically pure synthons for pharmaceutical industry, 180-195... 

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