Whole-cell reactions

When the whole-cell reaction was carried out in the presence of 0.1-3 M 1,3-dihydroxybenzene and 3M KHCO3, the molar conversion ratio of 1,3-dihydroxybenzene always reached 45-50% (Fig. 6). The high concentration of 1,3-dihydroxybenzene was not inhibitory on 2,6-dihydroxybenzoate decarboxylase. When 3M 1,3-dihydroxybenzene was incubated with the whole cells of Pandoraea sp. 12B-2 (43.9 mg as dry cell weight) in the presence of 3M KHCO3, 220mgmr of 2,6-dihydroxybenzoate (1.42 M) accumulated after 120 h incubation, with a conversion ratio of 48%. During the carboxylation of 1,3-dihydroxybenzene, no other product except for 2,6-dihydroxybenzoate was formed. 

Usually a large number of other reactions will occur simultaneously, some of them being beneficial for the coenzyme regeneration, whereas others lead to undesired byproducts. Also, the substrate and prodnct of the main reaction may get involved in undesirable side-reactions. Therefore, whole cell reactions may be cheaper and simpler to carry out than reactions using isolated enzymes, bnt they are less easily controlled, less reproducible and yield more waste. The most widely stndied class of reactions nsing whole cells are redactions catalyzed by baker s yeast, which is cheap and widely available (Sybesma et al 1998). 

The globotriose of Acceptor was prepared using a whole-cell reaction system as previously described (17). 

In (19-6), n has been made dimensionless. For a half-reaction, n is the number of electrons in the half-equation for the whole-cell reaction, n is the number of electrons in one of the multiplied half-equations before canceling the electrons. The Nernst equation is closely related to the laws of chemical equilibrium. Le Chatelier s principle applies to the potential of a cell in the same sense as it applies to the yield of an equilibrium process. Since Q is a fraction that has product concentrations in the numerator (top) and reactant concentrations in the denominator (bottom), an increased concentration of the product reduces the potential and an increased concentration of reactant raises the potential. 

These reactions compose a whole cell reaction given as follows ... 

Whole cell biocatalysis is a productive and practical style of conducting biocatalytic reactions. Such reactions, as implied by the term, are done with structurally intact cells. Usually, viable respiring cells are used but not exclusively. There are many reasons why a whole-cell reaction might be preferred to a... 

Wild type cells often contain several enzymes which carry out the same reaction. Unfortunately, in many cases these enzymes produce compounds with opposite stereoselectivity. Therefore, whole cells in which such enzymes are present cannot be applied for the synthesis of enantiomerically pure products. To increase the stereoselectivity of a whole cell reaction, recombinant DNA techniques need to be applied. Very common is the overexpression of the enzyme, which catalyzes the particular reaction in a suitable heterologous host such as E. coli. The simultaneous overexpression of an enzyme which catalyzes the regeneration of the consumed cofactor is highly efficient. Ideally, growing cells should provide simultaneously the enzyme for the desired reaction as well as the cofactor regenerating enzyme. Such so-called designer cells seem to be very promising for technical applications. 

Mutants of Pseudomonas putida were found to exhibit an arene dioxygenase activity, which has been exploited in whole-cell reactions for the regio- and enantioselective preparation of cw-dihydrodiols starting from benzene, substituted benzenes, and polycyclic or heteroaromatic compounds [48], The products are invaluable precursors for natural product synthesis, as exemplified in Scheme 8 [49],... 

These are the half-reactions or half-cell reactions. The total or whole-cell reaction is the sum of these, or... 

In a whole cell reaction mixture may suffice to represent the overall conversion. ... 

The conditions for enzymatic reactions are usually very mild. The solvents employed when isolated enzymes are used are typically aqueous birffer around pH of 7 or hydrophobic organic solvent such as hexane or isopropyl ether. In whole-cell reactions distilled water or tap water is normally used. 

Recently, the enzyme was expressed in baker s yeast and used for the asymmetric oxidation of alkyl cyclohexanones. The use of designer baker s yeast combined the advantages of using purified enzymes (single catalytic species, no overmetabolism) with the benefits of whole-cell reactions (experimentally simple, no cofactor regeneration necessary). The enantioselectivities observed with the recombinant enzyme were the same as those with the original, isolated enzyme. 

---