pH Effects on Membrane Transport
The majority of transport mechanisms and practically all those that have to do with the physiologically important solutes (both nonelectrolytes and ions) include a specific interaction of the transported solute with a membrane protein. One of the characteristic properties of enzymes, as well as membrane carrier proteins, is the dependence of their catalyzed reaction rate on pH which, in the vast majority of cases, is bell-shaped, there being an optimum at intermediate pH values. In a transport reaction, the pH dependence is similar but somewhat more complicated, as one must take into account the fact that the membrane protein is exposed alternately to the extracellular and to the intracellular pH. There are various indirect effects of pH on membrane transport parameters that have to do with the local interactions of membrane constituents, especially charged phospholipids. Among the more important ones one should mention the surface potential ψs.