The mechanisms by which variations in the lipid composition of cell membranes influence the function of membrane proteins are not yet well understood. In recent work, a nonlocal thermodynamic mechanism was suggested in which changes in lipid composition cause a redistribution of lateral pressures that in turn modulates protein conformational (or aggregation) equilibria. In the present study, results of statistical thermodynamic calculations of the equilibrium pressure profile and bilayer thickness are reported for a range of lipids and lipid mixtures. Large redistributions of lateral pressure are predicted to accompany variation in chain length, degree and position of chain unsaturation, head group repulsion, and incorporation of cholesterol and interfacially active solutes. Combinations of compositional changes are found that compensate with respect to bilayer thickness, thus eliminating effects of hydrophobic mismatch, while still effecting significant shifts of the pressure profile. It is also predicted that the effect on the pressure profile of addition of short alkanols can be reproduced with certain unnatural lipids. These results suggest possible roles of cholesterol, highly unsaturated fatty acids and small solutes in modulating membrane protein function and suggest unambiguous experimental tests of the pressure profile hypothesis. As a test of the methodology, calculated molecular areas and area elastic moduli are compared with experimental and simulation results.

Variations in the composition of cell membranes can strongly influence the function of proteins embedded therein. However, in most cases it is not known whether lipids and other membrane components act by binding directly to proteins or indirectly through changes in a structural or thermodynamic property of the fluid bilayer. In the present work, we develop a simple thermodynamic analysis based on the hypothesis that variations in membrane composition induce changes in the transverse pressure profile in lipid bilayers. If protein function involves a conformational transition accompanied by a depth-dependent change in its cross-sectional area, we predict that small changes in the lateral pressure can induce a large shift in the conformational distribution. The sensitivity of the conformational equilibrium to the lateral pressure profile arises in part from the localization of the large interfacial free energy within a domain of molecular thickness and also from the difference between the logarithmic dependence of the chemical potential of a protein conformational state on its own concentration and its linear dependence on small changes in the pressure profile.

A mechanism of general anesthesia is suggested and investigated using lattice statistical thermodynamics. Bilayer membranes are characterized by large lateral stresses that vary with depth within the membrane. Incorporation of amphiphilic and other interfacially active solutes into the bilayer is predicted to increase the lateral pressure selectively near the aqueous interfaces, compensated by decreased lateral pressure toward the center of the bilayer. General anesthesia likely involves inhibition of the opening of the ion channel in a postsynaptic ligand-gated membrane protein. If channel opening increases the cross-sectional area of the protein more near the aqueous interface than in the middle of the bilayer, then the anesthetic-induced increase in lateral pressure near the interface will shift the protein conformational equilibrium to favor the closed state, since channel opening will require greater work against this higher pressure. This hypothesis provides a truly mechanistic and thermodynamic understanding of anesthesia, not just correlations of potency with structural or thermodynamic properties. Calculations yield qualitative agreement with anesthetic potency at clinical anesthetic membrane concentrations and predict the alkanol cutoff and anomalously low potencies of strongly hydrophobic molecules with little or no attraction for the aqueous interface, such as perfluorocarbons.

No abstract

No abstract

Exceptions to the Meyer-Overton rule are commonly cited as evidence against indirect, membrane-mediated mechanisms of general anesthesia. However, another interpretation is possible within the context of an indirect mechanism in which solubilization of an anesthetic in the membrane causes a redistribution of lateral pressures in the membrane, which in turn shifts the conformational equilibrium of membrane proteins such as ligand-gated ion channels. It is suggested that compounds of different stiffness and interfacial activity have different intrinsic potencies, i.e., they cause widely different redistributions of the pressure profile (and thus different effects on protein conformational equilibria) per unit concentration of the compound in the membrane. Calculations incorporating the greater stiffness of perfluoromethylenic chains and the large interfacial attraction of hydroxyl groups predict the higher intrinsic potency of short alkanols than alkanes, the cutoffs in potency of alkanes and alkanols and the much shorter cutoffs for their perfluorinated analogues. Both effects, increased stiffness and interfacial activity, are present in unsaturated hydrocarbon solutes, and the intrinsic potencies are predicted to depend on the magnitude of both effects and on the number and locations of multiple bonds within the molecule. Most importantly, the intrinsic potencies of polymeric alkanols with regularly spaced hydroxyl groups are predicted to rise with increasing chain length, without cutoff; such molecules should serve to distinguish unambiguously between indirect mechanisms and direct binding mechanisms of anesthesia.

A mechanism of anesthesia is proposed that addresses one of the most troubling peculiarities of general anesthesia: the remarkably small variability of sensitivity within the human population and across a broad range of animal phyla. It is hypothesized that in addition to the rapid, saturable binding of a neurotransmitter to its receptor that results in activation, the neurotransmitter also acts indirectly on the receptor by diffusing into the postsynaptic membrane and changing its physical properties, causing a shift in receptor conformational equilibrium (desensitization). Unlike binding, this slower indirect mechanism is nonspecific: each neurotransmitter will, in principle, affect all receptors in the membrane. For proteins modeled as having only resting and active conformational states, time-dependent ion currents are predicted that exhibit many characteristics of desensitization for both inhibitory and excitatory channels. If receptors have been engineered to regulate the time course of ion currents by this mechanism, then (a) mutations that significantly alter receptor sensitivity to this effect would be lethal and (b) by design, excitatory receptors would be inhibited, but inhibitory receptors activated, so that their effects are not counterproductive. The wide range of exogenous molecules that affect the physical properties of membranes as do neurotransmitters, but that do not bind to receptors, would thus inhibit excitatory channels and activate inhibitory channels, i.e., they would act as anesthesics. The endogenous anesthetics would thus be the neurotransmitters, the survival advantage conferred by their proper membrane-mediated desensitization of receptors explaining the selection pressure for anesthesic sensitivity.

Some membrane peptides, such as Alamethicin, form barrel-stave aggregates with a broad probability distribution of size (number of peptides in the aggregate). This distribution has been shown to depend on the characteristics of the lipid bilayer. A mechanism for this influence is suggested, in analogy to earlier work on the effects of changes in bilayer composition on conformational equilibria in membrane proteins, that is based on coupling of shifts in the distribution of lateral pressures in the bilayer to depth-dependent changes in the lateral excluded area that accompanies the formation of an aggregate. Thermodynamic analysis is coupled with a simple geometric model of aggregates of kinked cylindrical peptides and with results of previously calculated lateral pressure distributions to predict the effects of changes in bilayer characteristics on aggregate size distributions, in qualitative agreement with experimental results.