Thermodynamics and kinetics of protein incorporation into membranes.

Abstract

The free energy and enthalpy of protein incorporation into membranes are calculated with special emphasis on the hitherto neglected effects of immobilization of protein and perturbation of lipid order in the membrane. The free energy change is found to be determined by the hydrophobic effect as the driving force for incorporation and the protein immobilization effect which leads to a considerable reduction of the free energy gained from the hydrophobic effect. For incorporation of a hydrophobic, bilayer-spanning a-helix, the free energy change obtained is of the order of -15 kcal/mol (1 cal = 4.184 J) in agreement with experimental results. The lipid perturbation effect yields only a small contribution to the free energy change due to an energy/entropy compensation inherent in lipid order. This effect dominates the enthalpy change, giving rise to values on the order of 100 kcal/mol with a pronounced temperature dependence around the lipid phase transition as observed experimentally. The kinetics of protein incorporation are even more strongly affected by the lipid perturbation effect, leading to an abrupt decrease of the rate of incorporation below the lipid phase transition.It is commonly accepted that the driving force for spontaneous incorporation of proteins into membranes is the hydrophobic effect. However, a number of other effects may support or counteract the hydrophobic effect. In general, the free energy of protein incorporation includes four contributions which arise from a change of (i) water structure, (ii) protein state, (iii) lipid state, and (iv) bonds between protein and water or lipid molecules.The change of water structure represents the hydrophobic effect which originates in the reduction of the mobility of water molecules surrounding the protein. The change of the protein state comprises the external translational and rotational degrees of freedom which become immobilized upon incorporation of the protein and the internal degrees of freedom whose changes involve conformational changes and changes of internal bonds. The change of the lipid state accounts for the reduction of the mobility of the lipid molecules in a fluid membrane. This parallels the reduction of the mobility of water molecules in the hydrophobic effect and may therefore be called lipophobic effect of proteins. The change of bonds at the protein surface concerns mainly polar interactions such as hydrogen bonds and electrostatic interactions between polar amino acid residues and water molecules because the interaction energy between apolar residues and water or lipid molecules is relatively small.The gain in free energy from the hydrophobic effect has been investigated thoroughly in the past (1-4). Estimates are also available for the free energy arising from the formation or breakage of hydrogen bonds and electrostatic bonds (3, 4) as well as from the change of protein conformation (5, 6). The protein immobilization effect has not been taken into account in previous work. This effect was studied in the context of enzy...

Electropermeabilization mediates a stable insertion of glycophorin A with Chinese hamster ovary cell membranes

“…Free energy of incorporation depends on many parameters : water structure, protein state, lipid state, bonds between protein and water or lipid (Jahnig, 1983). It is predicted that a membrane with higher than minimal ground state energy facilitates incorporation of proteins and promotes other processes that require a modulation of bilayer organization (fusion, leakage of polar solutes, enhanced rate of transbilayer transfer of phospholipids) (Scotto and Zakim, 1985;Jaim and Zalum, 1987).…”

“…Thermodynamics of protein insertion in lipid systems imply the occurrence of the lipid matrix perturbation. Thus, protein incorporation thermodynamics ( J i m and Zakim, 1987;Jahnig, 1983) show a favourable contribution of electrostatic forces and hydrophobic forces which occur on the membrane-spanning sequence, whereas hydration forces (Pargesian and Rau, 1984) and hydrophobic forces which occur on the protein polar segments prevent incorporation. Spontaneous insertion takes place if the first component is stronger than the second one.…”

Spontaneous insertion of polypeptide chains into membranes: a Monte Carlo model.

“…This energy is Eord = Eord sin2(6), [4] where eord is a coefficient and Ois the angle between the end-to-end vector of a polypeptide fragment and the normal to the membrane surface. eod is small, assuming values from 0.15 to 0.3 kT per residue.…”

“…Conversely, denaturation with urea or destabilization of the native structure by a point mutation accelerated the translocation process (10)(11)(12). There is a supposition that the insertion process of a membrane protein into the bilayer is spontaneous (3,5,6) and mainly driven by the hydrophobic effect (4). In particular, Engelman and Steitz (3) emphasized the role of "helical hairpin" structures in the process of protein insertion.…”

“…This process requires the insertion and transport of amphiphilic polypeptide chains across the hydrophobic lipid membrane. Despite intense investigation (1)(2)(3)(4)(5)(6)(7), the details of this process remain controversial.…”