Figure 2.3.

Thermodynamic scheme employed in the analysis of the energetics of pH-triggered membrane interactions of proteins (Posokhov et al., 2008a), such as Annexin B12 and diphtheria toxin T-domain. Environment acidification leads to the formation of a membrane-competent form (horizontal arrows), which interacts with the lipid bilayer (vertical arrows). Each of the two processes is characterized by an independent equilibrium parameter: pH-dependent fraction of the membrane-competent form (F_MC) and the free energy of membrane partitioning of this form (ΔG). Both parameters, F_MC and ΔG, can be experimentally determined via FCS methodology using the principal equations presented in the lower panel, as described in the text. Note that a transmembrane T-state and an interfacial I-state are distinguished in a separate membrane topology experiment as described in these publications (Ladokhin et al., 2002; Posokhov and Ladokhin, 2006; Posokhov et al., 2008a).