Intermembranal water phase

Another aspect of the intermembranal water phase that can be elucidated by the time-resolved proton transfer dynamics is the intensity of the electrostatic forces. The thin layer of a high dielectric fluid sandwiched between the low dielectric lipid plates, each covered by positive and negative charges, forms a very complex medium for calculation of electrostatic interaction (24, 25). 

Bovine heart cytochrome c oxidase is in a dimer state in the asymmetric unit of the crystal as shown in Fig. 7 (see color insert) (Tsukihara et al., 1996). Thirteen different subunits were identified in each monomer in the X-ray structure of the fully oxidized enzyme at 2.8-A resolution. The top view from the intermembrane side indicates a fairly strong interaction between the two monomers. The middle portion of the side view is readily identified as the transmembrane region by the large cluster of a-helices. This part was composed mainly of 28 a-helices as had been predicted by the amino acid sequences. The Ga backbone traces show that most of the a-helices are not arranged stricdy perpendicularly to the membrane surfaces, in contrast to the prediction by the amino acid sequences. Thus, most of a-helices in the X-ray structure are longer than those predicted by the amino acid sequences. The three largest subunits, subunits I, II, and III, form a core portion and the other 10 nuclear-encoded subunits surround the core as shown in Figs. 7C and 7D. In the X-ray structure at 2.8-A resolution, 3560 of 3606 amino acid residues were identified in the asymmetric unit composed of a dimer. Only 23 of 1803 amino acid residues per monomer were not detectable in the electron density map. Most of the undetectable residues are in the N- and C-terminals, which are exposed to the bulk water phase. 

Third, dilute phases can be regarded as one-dimensional colloidal suspensions of sheets in analogy to the familiar three-dimensional suspensions of charged polystyrene spheres (e.g. polyballs). We shall see that the Poisson-Boltzmann equation in one-dimension accurately describes the intermembrane interactions for phases where the dilution is a consequence of long range electrostatic repulsion (rather than undulation forces). This happens when charged sheets are separated by water containing only the counter-ions. 

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