Control of Acid-Base Conditions in Low-Water Media

Abstract

The catalytic activity of an enzyme is profoundly affected by its ionization state, whether it is dissolved in aqueous solution or suspended in low-water organic media. In aqueous solution, counterions can freely move around in a solution. Because they are not closely associated with opposite charges, their identity does not effect the protonation state of the enzyme; thus, pH alone governs the protonation state. When a biocatalyst is suspended in a low-water organic solvent, the situation is more complex. In this case, counterions are in closer contact with the opposite charges on the enzyme because of the lower dielectric constant of the medium. Thus, protonation of ionizable groups on the enzyme will be controlled by the type and availability of these ions as well as hydrogen ions. Changes in ionization state of the protein can therefore be described by two equilibria that can, in theory, be controlled independently (,): 1. Exchange of hydrogen ions and cations with acidic groups of the protein. For example, carboxyl groups require simultaneous exchange of H+with a cation such as Open image in new window Such equilibria can be characterized by the ratio of thermodynamic activities, aH+/aNa+(this may also be represented as pH-pNa). 2. Transfer of both a hydrogen ion and an anion onto basic groups of the protein. For example, amino groups bind or release H+and an anion such as Cl- together. Open image in new window Such equilibria can be characterized by the product of thermodynamic activities, H+aCl- (or pH+pCl).