Optimal pH Control in Sequential Biochemical Reaction Systems via Generating a pH Gradient Across an Immobilized Enzyme Film
Multistep synthesis reactions form the basis for the manufacture of a variety of important biochemicals. However, in many cases, the optimal operating pH is much different for each individual step of these reactions. Even so, conducting all or some of these reaction steps in the same reactor often may have technical (and/or economic) benefits, as when one of the intermediate steps has a very low product to reactant ratio at equilibrium. When it is required to perform these sequential reaction steps in the same vessel, the usual method is to use a compromise pH. However, this approach is neither satisfactory nor feasible always because the rates of most enzyme-catalyzed reactions decay exponentially as one drifts away from the enzyme’s optimal pH. Thus, a technique which permits one to conduct the individual steps at their respective optimal pH and, yet, in close proximity to one another is extremely desirable. We recently proposed such a technique: a method for conducting, in a single reactor, the sequential reaction A ⇔ B → C. The first step is an enzyme-catalyzed reaction that takes place in a basic environment and the second step is either an enzyme-catalyzed reaction or a fermentation that occurs in an acidic environment. The approach suggested was to coat the pellets, within which the enzyme catalyzing the A ⇔ B step was immobilized, with a very thin (< lµm) layer of urease, before suspending the pellets in an acidic bulk solution in which the step B → C takes place. Now, adding a small concentration of urea to the bulk solution allows one to sustain the necessary pH difference between the bulk solution and the interior of the pellets, thereby enabling the two steps to take place in close proximity and also at their respective optimal pH.
KeywordsReaction Step Bulk Solution Xylose Isomerase Glucose Isomerase Jack Bean Urease
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