Solute modulation of conformational equilibria in intrinsic membrane proteins: apparent "cooperativity" without binding.

The activity of many membrane proteins depends on a conformational transition that is often strongly influenced by small membrane-soluble solutes. This allosteric modulation may be direct, involving binding to the protein at localized sites of varying specificity, or may be indirect, resulting from altered membrane properties. In the present paper, a general expression for solute-protein titration curves is predicted, using an indirect mechanism that couples solute-induced changes in the lateral pressure profile of the bilayer to a shift in protein conformational equilibrium. When the common practice of fitting dose-response data to the Hill equation is applied to these curves, the fits are found to be reasonably good, with large Hill coefficients. Because this would commonly be interpreted as evidence of the existence of multiple sites with strong positive cooperativity, it is argued that caution must therefore be exercised in the interpretation of titration data in the absence of direct evidence of the existence of binding sites. The form of the titration curve predicted from this lateral pressure mechanism is shown to be quite general for indirect mechanisms. It is also shown that this form is the same as would be obtained from classical models of binding cooperativity, such as that of Monod, Wyman, and Changeux, in the limit of an infinite number of sites with vanishingly small site affinity.