Green chemistry isolated enzymes

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

H. G. Davies, R. H. Green, D. R. Kelly and S. M. Roberts, Biotransformations in Preparative Organic Chemistry The Use of Isolated Enzymes and Whole Cell Systems, 1989... 

However from the standpoint of green chemistry, the use of isolated enzymes (or dead whole cells) is highly preferred because it avoids the generation of copious amounts of biomass. It must be emphasized that the productivity of microbial conversions is usually low, since non-natural substrates are only tolerated at concentrations as low as 0.1-0.3% [106]. The large amount of biomass present in the reaction medium causes low overall yields and makes product recovery troublesome. Therefore the E-factors for whole cell processes can be extremely high. Moreover the use of wild-type cells often causes problems because an array of enzymes is present which can interfere in the reduction of a specific ketone (giving opposite selectivities). The use of recombinant techniques, however, which only express the desired enzyme can overcome this problem [108]. 

Davies, H.G., Green, R.H., Kelly, D.R., and Roberts, S.M. (eds) Biotransformations in Preparative Organic Chemistry the Use of Isolated Enzymes and Whole Cell Systems in Synthesis. Academic Press London, 1989. 

Davies HG, Green RH, Kelly DR, Roberts SM (1989) Biotransformations in preparative orgaitic chemistry the use of isolated enzymes and whole-cell systems in synthesis. Academic, London, pp 1-23... 

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