Hofmeister effect on enzymatic catalysis and colloidal structures

An attempt is made to decouple the effects that occur when different electrolytes are added to proteins, especially enzymes, in buffer solutions. In particular, direct molecular ‘chemical’ interactions between ions and specific sites on the enzyme are distinguished from ‘physical’ interactions due to electrostatic and dispersion forces. The latter are often related to a Hofmeister series. It is shown that bulk effects, usually on the pH via a shift of the buffer equilibria, must be separated from direct enzyme–ion interactions. ABTS oxidation with horseradish peroxidase is taken as an example to show how this separation can be done. The different concentration ranges in which these phenomena occur are also discussed. Finally, the influence of ions on a particular enzyme, the NADH oxidase, is compared to their influence on mixed catanionic micelles. The striking similarities can be associated with the structure-making and -breaking behaviour of the ions and a balanced effect for ‘neutral’ Hofmeister ions.