Quantum Chemical Studies of Proton Transport Through Biomembranes*

Document Type

Article

Journal/Book Title/Conference

Quantum Chemistry in Biomedical Sciences

Volume

367

Issue

1

Publisher

The New York Academy of Sciences

Publication Date

5-1-1981

First Page

493

Last Page

509

Abstract

Biological membranes consist basically of a lipid bilayer, the interior of which contains the lipophilic hydrocarbon portions of the molecules, while the hydrophilic polar ends lie on the inner and outer surfaces of the membrane.1,2 A variable number of protein molecules, some of which penetrate quite deeply into the lipid bilayer, are associated with the membranes. In fact, a class of "integral" proteins is known to extend completely from the interior to the exterior surface of biomembranes.

An important component in a number of diverse biological processes is the transport of protons through a cellular membrane. As an example, the direct driving force for ATP synthesis following light absorption in green plants is the pH gradient set up by the pumping of protons from the exterior to the interior surface of the cell membrane.3-8 Transmembrane proton transport is also thought to play an important role in vision,9-11 bacterial metabolism,12-18 and oxidative phosphorylation in mitochondria.19-22

We are presented with the problem of elucidating the mechanism and pathway for proton transport through the membrane. Direct passage through the hydrophobic interior seems an unlikely possibility for the proton because of its high concentration of electrical charge. Hydration of the proton by a number of water molecules would lead to some delocalization of the charge, but the resulting H+(H2O)n moiety would retain a high degree of hydrophilicity. Conduction of protons via a "pore" or "channel" through the lipid bilayer offers another possibility, but this would not be expected to provide the high degree of selectivity for protons seen in biomembranes.

A third possibility utilizes the transmembrane proteins as a pathway through the membrane. The hydrophilic regions within proteins are a less hostile domain for proton migration than the lipophilic membrane interior. More importantly, though, the large numbers of hydrogen bonds typically found in proteins suggest the possibility of a mode of proton transport similar to that known to occur in the solid and liquid phases of water.23,24 A prerequisite for this process is the existence within the protein of a hydrogen-bonded chain of the type shown below.

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