Enzyme catalysis immobilized microorganisms

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

Clearly, most of the products in Table 4.1 are chiral compounds. None of the products is racemic, and only a few are achiral. The biocatalysts are (combinations) of enzymes oi cells. If the key enzyme has been indicated it may be used pure, partly purified in a cell-free extract, or in a whole cell. For each option, the biocatalyst may be used free oi immobilized. If the name of a microorganism has been indicated, usually several of its enzymes are active in the catalysis. The entries that are displayed in bold are treated in the case studies further on in this chapter, in the same order as in Table 4.1. 

Coenzymes - Many enzymes require nonprotein coenzymes for catalytic activity.8 These are cosubstrates, and must be constantly reconverted into their active form for catalysis to continue. This is not a problem for growing microorganisms since the normal metabolic processes ensure an adequate supply of coenzymes. However, with purified, or immobilized enzymes, maintaining a sufficient concentration of coenzyme can pose a major problem. Coenzymes are expensive and it is seldom economically feasible to add them in stoichiometric amounts. This is often undesirable for chemical reasons, e.g., the coenzyme may be unstable, or the eventual build-up of high concentrations of its inactive form may Induce displacement of an equilibrium reaction in the opposite direction to that desired.3 It is therefore necessary to use catalytic amounts of coenzymes and to ensure that the active forms are continuously regenerated. Some coenzymes present little or no problem in this regard since they are automatically reformed under the normal aqueous reaction conditions or in the presence of oxygen. These include biotin, pyrldoxal phosphate (PLP), thiamine pyrophosphate, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).1 ... 

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