Kinetic and thermodynamic aspects of lipid translocation in biological membranes

Most lipid components of membranes of a variety of eukaryotic cells have been found to be asymmetrically distributed between the inner and the outer monolayers. In the case of red blood cells, it has been demonstrated that an ATP-dependent aminophospholipid selective transport protein may be the cause of a highly unsymmetrical distribution of phosphatidylserine (PS) and phosphatidylethanolamine (PE). The latter phenomenon of inward directed transport of PS and PE has recently been modelled on the basis of kinetic equations of a corresponding carrier mechanism taking into account competitive binding and an irreversible translocation step. The experimental fact of uneven distribution of other lipid components such as phosphatidylcholine (PC), sphingomyelin (SM) and cholesterol (Ch) may be attributed to the restrictions caused by coupling of the monolayers. The active transport of PS and PE will be accompanied by a passive redistribution of all lipid components between the monolayers. It is shown, how the latter process may be described in a thermodynamic modeling approach by making use of phenomenological linear flux-force relations (see also).

Mathematical expressions for entropic as well as mechanical forces are derived by taking into account coupling of the monolayers as a constraint. Both forces appear as functions of the lipid concentrations in the monolayers. The equilibrium state is characterized by a planar membrane consisting of symmetrically distributed ideally mixing lipids.

Making use of a general invariance principle of the flux-force relations, the dependencies of the phenomenological coefficients on the total lipid amounts are expressed. The matrix of phenomenological coefficients is shown to be decomposable into two different contributions. The first, diagonal part is linear in the total lipid concentrations with diffusion parameters as coefficients, characterizing independent (uncoupled) diffusion. The second part, which also contains nondiagonal terms, is bilinear in the total lipid concentrations, with coupling parameters as coefficients, representing interactions of the lipid fluxes.

Numerical simulation of the experimental data for steady state and transient states for the erythrocyte plasma membrane reveals that two prerequisites must be fulfilled for the choice of phenomenological parameters. First, the values of the diffusion parameters of PC, SM and Ch must be very different, with SM being the slowest diffusing species and Ch the fastest. Second, positive coupling parameters are required to yield the characteristic asymmetries for PC, SM and Ch, that is, a nearly symmetrical distribution of Ch, a pronounced preference of SM for the external monolayer, and an intermediate value for the asymmetry of PC.

To shed light into the mechanism of passive lipid redistribution, different kinetic schemes of lipid translocation are analyzed. A procedure for the derivation of phenomenological diffusion and coupling parameters in terms of kinetic constants is presented. Besides this, a proper treatment of mechanical effects in the kinetic models is demonstrated. It is shown, that an antiport mechanism of protein mediated lipid translocation cannot give rise to positive coupling coefficients. A symport mechanism may yield positive coupling parameters. However, the derived interrelation of diffusion and coupling parameters reveals that a slow diffusion of SM yields an insufficient extent of coupling. As an alternative mechanism is analyzed the redistribution of lipids through pores formed by peptides, which has been proposed recently. The main characteristic of such a transport in single-file is the effect of ordering of lipid molecules along the translocation co-ordinate. Flux equations are derived under special assumptions about the structure of the lipid-peptide pore. Linearization yields a relation between diffusion and coupling parameters that allows for slow diffusion and high positive coupling, especially for SM. Due to parameter restrictions from SM, the phenomenological analysis yields a plausible value for the pore-capacity of . In addition, a pore-independent lipid flip-flop must be taken into account to introduce different time constants of passive transmembrane movement.