Biocatalytic Applications

Whereas SHMT in vivo has a biosynthetic function, threonine aldolase catalyzes the degradation of threonine both l- and D-spedfic ThrA enzymes are known [16,192]. Typically, ThrA enzymes show complete enantiopreference for their natural a-D- or a-t-amino configuration but, with few exceptions, have only low specificity for the relative threo/erythro-configuration (e.g. (122)/(123)) [193]. Likewise, SHMT is highly selective for the L-configuration, but has poor threo/erythro selectivity [194]. For biocatalytic applications, the knovm SHMT, d- and t-ThrA show broad substrate tolerance for various acceptor aldehydes, notably induding aromatic aldehydes [193-196] however, a,P-unsaturated aldehydes are not accepted [197]. For preparative reactions, excess of (120) must compensate for the unfavorable equilibrium constant [34] to achieve economical yields. 

The two established Hnls, those from L. usitatissimum and P. amygdalus, have found biocatalytic applications for the production of (i )-cyanohydrins. The former of these Hnls is the least widely applied, the natural substrates being acetone cyanohydrin or (i )-2-butanone cyanohydrin (Table 1) [28]. Although an improved procedure for the purification of this enzyme has been reported [27] it is still only available in limited quantities (from 100 g of seedlings approximately 350 U of enzyme are obtained). It was found that this enzyme transforms a range of aliphatic aldehyde and ketone substrates [27], the latter of which included five-membered cyclic (e.g. 2-methylcyclopentanone) and chlorinated ketone substrates. In contrast, attempts to transform substituted cyclohexanones and 3-methylcyclopentanone failed and it was even found that benzaldehyde deactivated the enzyme. 

With the continued explosion of academic and industrial activities surrounding ionic liquids for many different applications, the potential for catalytic processes using ionic liquids has been increasingly recognized 1,15-18). A number of excellent reviews have appeared 19), including the reviews related to biocatalytic applications of ionic liquids 20,21). 

Enzymes that are suited for application in biocatalysis are mostly hydrolases, bnt also oxidorednctases, lyases and, to a lesser extent, transferases are useful. Obviously, the focus of bulk enzyme producers is different from the main interests of those who want to apply enzymes in biocatalytic applications. Fortunately, a growing number of companies has become active in the field of enzyme prodnction for biotransformations and by now a large nnmber of enzymes suited for biotransformations has become commercially available (Table 5.1). 

A number of scientific consortia have taken np the formidable endeavor to seqnence the genomes of Homo sapiens and a nnmber of medically and commercially important microorganisms. Some of the seqnence projects have been completed (Table 5.9) and this provides a wealth of information which can be of nse for biocatalytic applications. 

Mention some of the advantages and disadvantages of using isolated enzymes rather than whole cells in biocatalysis. Is it always needed to purify enzymes for a biocatalytic application To what degree should one purify ... 

Biocatalytic processes increasingly penetrate the chemical industry. In a recent study, 134 industrial-scale biotransformations, on a scale of > 100 kg with whole cells or enzymes starting from a precursor other than a C-source, were analyzed. Hydrolases (44%), followed by oxido-reductases (30%), dominate industrial biocatalytic applications. Average performance data for fine chemicals (not pharmaceuticals) applications are 78% yield, a final product concentration of 108 g I.1, and a volumetric productivity of 372 g (L d) 1. 

U. T. Bornscheuer, Directed evolution of enzymes for biocatalytic applications, Biocatal. Biotransf. 2001, 19, 85-97. 

Winkler, M., Kaplan, O., Vejvoda, V. et al. 2009. Biocatalytic application of nitrilases from Fusarium solani 01 and Aspergillus niger KIO Journal of Molecular Catalysis B-Enzyme, 59 243-7. 

Lipases are of remarkable practical interest since they have been used in numerous biocatalytic applications, such as kinetic resolution of alcohols and carboxyl esters (both in water and in non-aqueous media) [1], regioselective acylations of poly-hydroxylated compounds, and the preparation of enantiopure amino acids and amides [2, 3]. Moreover, lipases are stable in organic solvents, do not require cofactors, possess broad substrate specificity, and exhibit, in general, a high enantioselectivity. All these features have contributed to make hpases the class of enzyme with the highest number of biocatalytic applications carried out in neat organic solvents. 

Veronese, F.M. Mammucari, C. Schiavon, F. Schiavon, O. Lora, S. Secundo, F. Chilin, A. Guiotto, A. Pegylated enzyme entrapped in poly(vinyl alcohol) hydrogel for biocatalytic application. II Farmaco 2001, 56, 541-547. 

Table 1. Properties of PLD from different sources for biocatalytic applications [39-51]...

Carvalho, C.M.L., Aires-Barros, M.R., and Cabral, J.M.S. (1998) Cutinase structure, function and biocatalytic applications. Electron./. Biotechnol,... 

A modular architecture is no particular hallmark of enzymes, the highest degree of modularity is typically observed in structural proteins of the extracellular matrix and in proteins involved in intracellular signal transduction. Nevertheless, there are a number of modular enzymes including some which are of interest for biocatalytic applications. 

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