Dipole-potential-mediated effects on ion pump kinetics

Abstract

The kinetics of conformational changes of P-type ATPases necessary for the occlusion or de-occlusion of transported ions are known to be sensitive to the composition of the surrounding membrane, e.g. phospholipid content, mole percentage of cholesterol, and the presence of lipid-bound anions. Research has shown that many membrane components modify the dipole potential of the lipid head-group region. Based on the observation that occlusion/de-occlusion reactions of ion pumps perturb the membrane surrounding the protein, a mechanism is suggested whereby dipole potential modifiers induce preferential stabilisation or destabilisation of occluded or non-occluded states of the protein, leading to changes in the forward and backward rate constants for the transition. The mechanism relies on the assumption that conformational changes of the protein are associated with changes in its hydrophobic thickness which requires a change in local lipid packing density to allow hydrophobic matching with the membrane. The changes in lipid packing density cause changes in local lipid dipole potential which are responsible for the dependence of conformational kinetics on dipole potential modifiers. The proposed mechanism has the potential to explain effects of lipid composition on the kinetics of any membrane protein undergoing significant changes in its membrane cross-sectional area during its activity.