Medium-biocatalyst interface

In the case of immobilized enzymes, partition at the biocatalyst-medium interface can be the consequence of the different physicochemical properties of the bulk medium and the enzyme environment within the support. Mostly relevant is partition due to electric charges. This situation occurs when the enzyme support bears some charge in that case, partition of reactive species occurs as long as they are charged molecules at the conditions of reaction and it always occur with respect to protons, so that it has an effect on the pH dependence on the immobilized enzyme (Goldstein 1976). 

Biocatalysis localization in the biphasic medium depends on physicochemical properties of the reactants. When all the chemical species involved in the reaction are hydro-phobic, catalysis occurs at the liquid-liquid interface. However, when the substrate is hydrophobic (initially dissolved in the apolar phase) and the product is hydrophilic (remains in the aqueous phase), the reaction occurs in the aqueous phase [25]. The majority of biphasic systems use sparingly water-soluble substrates and yield hydrophobic products therefore, the aqueous phase serves as a biocatalyst container [34,35] [Fig. 2(a)]. Nevertheless, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38] (Fig. 2(b), (c)). 

Nevertheless, development of processes for commercial purposes is still limited, particularly with interfacial effects the loss of activity of the biocatalyst, the slow coalescence, the biocatalyst aggregation, and accumulation of medium components at the interface. 

Partition and mass transfer limitation make the substrate (and product) concentration in contact with the enzyme different from that in the bulk reaction medium producing the corresponding profiles, as shown in Eig. 4.5. Partition produces a discontinuity of the profiles at the medium-biocatalyst interface while mass transfer limitations produce profiles in the immediate vicinity of that surface and on the inside of the biocatalyst support (if allowed to host the enzyme). 

Boundary condition I) assumes that EDR is negligible with respect to IDR, as may frequently occur for enzymes immobilized inside solid supports. If not, it is wrong since at the surface of the biocatalyst particle s = ss so and boundary condition I) should be replaced by an equation of continuity at the medium-particle interface. This situation will be analyzed afterwards. 

SO that in the second batch productivity was severely reduced and conversion yield was significantly lower even at prolonged operation. SDS-PAGE electrophoresis revealed that most of the desorbed protein had a molecular weight of 31,000 Da, which corresponds to that of Alcaligenes faecalis lipase. Most of the activity lost from batch to batch corresponded to protein desorption from the matrix, enzyme inactivation being quite low. Therefore, it made very little sense to use QLG instead of QL, but the free enzyme was not recoverable from the reaction medium and cost estimates indicated that the enzyme should be used at least five times to make the process economically attractive. Therefore, the next goal was to construct an immobilized lipase biocatalyst from soluble QL. The hydrophobic nature of the active site and the requirement of a hydrophobic interface for lipase action made reasonable to use hydrophobic supports however to test the validity of this hypothesis several immobilization systems were tested. The results obtained are summarized in... 

Another important factor in macro-heterogeneous biphasic systems with the substrates/products in one phase and enzyme in another is partitioning, which produces discontinuity of the profiles at the medium-biocatalyst interface (Fig. 6.70)... 

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