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In bioorganic reactions a biocatalyst in a liquid medium is surrounded by various micro environments.
The biocatalyst may be an enzyme, or more complex systems such as microorganisms, eukaryotic cells or parts thereof.
The micro-environments consist of a thin layer of water around the biocatalyst and an interfacial region. This forms the transition between the bio-catalytic and continuous phases. Medium engineering in such cases involves modification of the micro-environment of the biocatalyst either by introducing additives or solid matrices into an existing medium or by varying the composition of the liquid medium itself.
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Presently medium engineering is a rapidly expanding area of research in which the objectives of biocatalyst engineering viz., an increased bio-catalytic reactivity, stability and hence productivity are pursued. The nature of different phases and their effects on bio-catalysis are therefore investigated intensively. In bioorganic reactions generally each biocatalyst is surrounded by a thin layer of tightly bound water. This aqueous layer is believed to be essential for bio-catalytic activity. Based on research findings following two important rules in aqueous medium engineering of bioorganic reactions have been laid down.
Rule 1: Do not distort the essential water-around the biocatalyst.
Rule 2: Try to stabilize the biocatalyst conformation by strengthening the essential water- biocatalyst interactions.
The first rule assuring the integrity of the biocatalyst provides expectation of full activity. Second rule on the other hand leads to possible way to achieve the goal in this rule. In such reactions systems immediately adjacent to the essential water layer surrounding the biocatalyst is the interfacial region. Here the associated problem remains that it is difficult to predict for any given combination of properties of microenvironment what the effect of the matrix will be on the biocatalyst activity, stability and/or productivity.
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Also, electrostatic interaction effects of organic solvents on biocatalysts, the mutual hydrophobic interactions between matrix, reactants and biocatalyst are far less specific imposing another difficulty in medium design engineering. However for various bio-catalytic systems a trend has been observed between bio-catalytic activity and polarity of the interphase/bulk phase, when the latter was expressed as the logarithm of the partition coefficient (logp). The unique feature of this trend is that it is completely system and biocatalyst independent.
For bio-catalytic systems in organic solvents forming continuous phase like enzymes entrapped in reversed micelles the medium engineering principle should be such that optimal activities were reached when the concentration of substrate in the interphase is maximal. One way of achieving this is by matching the polarity of the interphase to that of the substrate, and/or vice versa, by increasing the difference in the polarity between the continuous phase and the substrate.
In medium engineering other considerations involved are solvent toxicity, viscosity, cost, reusability, case of product isolation and bioreactor design. Additives are also important in medium engineering. These are compounds capable of stabilizing the biocatalyst against denaturation without affecting the activity to a great extent. Few examples of additives are various sugars, glycerol, ethylene glycol and polyhydric polymers. They are important to stabilize various enzymes. However, practical use of additives seems to be restricted to biocatalyst.
The findings that enzymes can function in a polar solvents like heptane dramatically expanded the range of reactions which can be approached through bio-catalytic medium engineering. The influential factors for activity and stability of enzymes in organic solvents include the ionic state of the enzyme, support characteristics and the extent of hydration of biocatalyst and solvent. In bioprocessing of oils and fats through esterification and inter esterification for industrial application medium design engineering served a potential role. Commercialization of cocoa butter is a classical example.
