Buffer-facilitated proton transport. pH profile of bound enzymes (original) (raw)

In heterogeneous systems conjugate acid base pairs, besides their conventionally accepted static role as buffers, can also play a dynamic role in facilitating proton transport. When protons are generated and consumed at different locations, the conjugate base binds the proton at the "source" and the resulting acid diffuses to the "sink". 2. Both the buffering capacity of an acid base pair and its ability to facilitate proton transport are maximum when its pKa is close to the pH of the medium. The transport facilitating effect can be appreciable at much lower concentrations than those needed for buffering capacity. 3. The effect of proton transport has been theoretically investigated by using a model consisting of a H-producing enzymic surface reaction, which occur with bound enzymes both in vitro and in vivo. 4. The enhancement of proton transport is quantitatively expressed by the extent of transport facilitation, ~:. The interplay of facilitated proton diffusion and reaction kinetics is quantified by the proton modulus,/z. 5. At a given pKA the magnitude of transport facilitation increases with the acid-base concentration and the pH of the solution. The pKa value plays a dual role in determining both the concentration of available carriers and the affinity of the proton to the carrier. 6. Due to its high proton affinity OH can also facilitate proton transport already at pH 7, but in the presence of buffers its effect becomes significant only at higher pH values. 7. When the extent of transport facilitation sharply increases with the pH of the substrate solution, at relatively low buffer concentrations bound enzymes display sigmoidal pH profiles with sharp peaks. Previously reported experimental data with membrane bound enzymes can be interpreted in view of these theoretical findings. 8. It is postulated that buffers ubiquitous in biochemical and living systems can play an important role also by facilitating proton transport.