Proton-conducting channels

F0 appears to contain the proton-conducting channel of the ATP-synthase. If F0 alone is incorporated into lipo-somes, it makes the membranes leaky to protons. The multiple copies of one of the small subunits of F0 may assemble to form a tubular channel across the membrane (see fig. 14.23). 

Linear arrays of protonatable or hydrogen bonded sites may allow the directed long range transfer of protons, thus functioning as proton-conducting channel, i.e., as proton wire. Relevant systems would be linear polyamines or polyphenolic condensed aromatic units [8.218], self-assembled hydrogen bonded heterocyclic ribbons such as 116 (see Section 9.4.4) or polyelectrolyte membranes [8.219] in which collective proton motion may take place and lead to proton conductivity. 

There are three main reasons to suggest a specific function of subunit III in proton translocation. First, Casey et al. [171] showed that modification of this subunit with dicyclohexylcarbodiimide (DCCD) blocks proton translocation, but has little effect on electron transfer. Similar results have been obtained with the reconstituted oxidase from the thermophilic bacterium PS3 [164]. Prochaska et al. [160] showed that DCCD binds mainly to Glu-90 of the bovine subunit III, which is predicted to lie within the membrane domain and hence to be a site analogous to the DCCD binding site in the membranous fj, sector of the ATP-synthase (Fig. 3.8 see also Ref. 85). Since the latter is a part of a proton-conducting channel in ATP synthase, subunit III was thought to have the same function. However, there is one essential difference between the two phenomena. Modification of the membranous glutamic residue in by DCCD leads also to inhibition of ATP hydrolysis in the complex, as expected for two linked reactions. In contrast, DCCD has little or no effect on electron transfer in cytochrome oxidase under conditions where H translocation is abolished. Hence, DCCD cannot simply be judged to block a proton channel in the oxidase. More appropriately, it decouples proton translocation from electron transfer. 

There is a strong resemblance between the mechanism of ion motion next to the protein and the proton-collecting antenna reported for bacteriorhodopsin [78, 79] or cytochrome c oxidase [2]. These domains consist of a cluster of carboxylates that function as proton binding sites. The protonation on any carboxylate of the cluster leads to rapid proton exchange reactions that finally deliver the proton to the immediate vicinity of the proton-conducting channel of the protein. 

Checovee, S., et al., Mechanism of proton entry into the cytoplasmic section of the proton-conducting channel of bacteriorhodopsin, Biochemistry, 1997, 36, 13919-13928. 

V- and F-class ATPases, which transport protons exclusively, are large, multlsubunlt complexes with a proton-conducting channel in the transmembrane domain and ATP-blndlng sites in the cytosolic domain. 

In the electron transport chain, CcO receives electrons from cytochrome c, a water-soluble heme protein, on the cytoplasmic side of the membrane, and transfers them through a series of electron transfer steps to the active site, which contains a heme iron and a copper, where the electrons are used to reduce the molecular oxygen. The protons needed for this reaction are taken from the mitochondrion matrix side throngh two proton-conducting channels. In addition to these chemical protons, four more protons, per every oxygen molecule reduced, are translocated across the membrane. The overall enzymatic reaction of CcO is... 

Which ionizable amino acids are participating in the translocation of protons (water chains are carrying protons between such sites) is one of the key questions. Several such side chains have been identified in experimental studies, in particular in the K- and D- proton conducting channels, which are the entry points of the protons. These are the sites roughly below the level of heme a/heme a3 (see Figure 4.2). The identity of such sites above the line heme a/heme a3 is mainly unknown at present. 

Pumping mechanism, cytochrome c oxidase basic physical principle, 79—80 Coulomb machine gun, 80 le/lH-t pumping ratio, 78 membrane potential, 83—84 pK quantum mechanics/molecular mechanics calculations, 84 proton collecting antenna and proton conducting channels, 83 proton loading site (PLS), 80, 82—83 pump element, heme a, 78 water chains, 82... 

L. Wu, Z. Zhang, J. Ran, D. Zhou, C. Li, T. Xu, Advances in proton-exchange membranes for fuel cells an overview on proton conductive channels (PCCs), Phys. Chem. Chem. Phys. 15 (14) (2013) 4870-4887. 

The MEA is composed of three main parts, e.g., polymer electrolyte membrane (PEM), gas diffusion medium, and catalyst layer (CL). The membrane, with hydrophilic proton-conducting channels embedded in a hydrophobic structural matrix, plays a key role in the operation of PEFCs. The PEMs for PEFCs commonly use perfluorosulfonic acid (PFSA) electrolytes such as Nation , with the chemical structure shown in Fig. 2, because of its high proton conductivity as well as chemical and thermal stability [1]. The gas diffusion medium (GDM), including both the microporous layer (MPL) and the gas diffusion layer (GDL), which typically is based on carbon fibers, is also an important component. The GDM is designed with three distinct... 

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