[33] Barrier models for the description of proton transport across membranes

This chapter describes the barrier models used for the description of proton transport across membranes. The intrinsic proton permeability of lipid bilayers is extremely low, and it is therefore generally assumed that proton transport across biomembranes involves specialized structures representing low-energy pathways through the apolar core of the membrane. Proton moving through a transmembrane channel is thought to interact sequentially with a series of binding sites, such as carboxyl residues. It is feasible that in part of the transport pathways the proton migrates in the form of the hydronium ion (H3O+). A general description of proton permeation is given on the basis of potential profile on the proton along the transport pathway, consisting of a series of wells separated by energy barriers. Recent X-ray diffraction studies and spectroscopic experiments have shown that proteins may exist in a large number of conformational states and may rapidly move from one state to the other at physiological temperatures. A minimum model of a proton pump driven by adenosine triphosphate (ATP) hydrolysis is also discussed in the chapter.