Use of aqueous two-phase systems

Aqueous 2-phase systems (see Chapter 2) offer the opportunity to shift reaction equilibria towards product formation by ensuring that enzyme and substrate partition into one phase while the product enters, and may be removed from, the other. The theory developed (earlier) for aqueous/organic biphasic systems is equally applicable to these systems. An example of such an extractive bioconversion is a method for the conversion of starch to glucose using bacterial a-amylase and glucoamylase. Starch substrate partitions almost entirely into the lower, more hydrophilic, dextran-rich phase of a system comprising 3% polyethylene glycol (PEG 20,000) and 5% crude unfractionated dextran. The enzymes also partition largely to the bottom phase but the glucose, produced by the hydrolysis, distributes itself more evenly between the phases. A small proportion of the glucoamylase enters the upper phase and will convert any oligosaccharide entering that phase to glucose. Concentrations of up to 140 g L⁻¹ glucose may be reached in the upper phase.

Such a system offers some of the advantages of an immobilised enzyme process without some of the disadvantages. The enzymes are largely retained and are stabilised by the presence of the polymers yet catalysis is in homogeneous solution (within the phase) so no diffusion limitations to mass transfer exist. Drawbacks include the need to separate the product from the upper phase polymers and gradual loss of enzymes which enter the upper phase. Enzyme loss could be reduced, without introducing diffusion limitations, by linking them to hydrophilic polymers so as to form soluble complexes.