Constricting zone

Figure 8-20 MolScript ribbon drawings of the OmpF porin of E. coli. (A) View of the 340-residue monomer. (B) View of the trimer looking down the threefold axis. From Wa-tanabe et al3is From atomic coordinates of Cowan et al.3i9 (C) Molecular model of the constriction zone of the PhoE porin. Locations of key residues are shown, with positions of homologous residues in OmpF given in parentheses. Extracellular loops have been omitted. Constructed from coordinates of Cowan et al.349 by Samartzidou and Del-cour.342 Courtesy of Anne Delcour.

A in diameter. The only aqueous links between the inner vestibule and the cell interior are the narrow (<8-9 A wide) "windows" between the subunits lying directly under the membrane surface. The gate of the channel, made by the pore-lining segments, M2, is near the middle of the membrane (upper arrow), and the constriction zone (the narrowest part of the open channel) is at the cytoplasmic membrane surface (lower arrow). 

From Figure 6.3, we know that a spherical electrode alone has a larger sensitivity in the constricted zone of the electrode for purely geometrical reasons. Using a surface disk M electrode, tissue near the electrode edge also influences the result to a larger extent than tissue elsewhere (Section 6.2.2). 

One of the first to introduce the method was Thomasset (1965), using a two-electrode method and 1 kHz signal frequency. With just two electrodes, it is important to use large-area band electrodes to reduce the contribution from the current constrictional zones near the electrodes. With a tetrapolar electrode system, it is easier to select the preferred... 

Without the protective action of the skin, the internal body resistance may be divided into a constrictional zone resistance with increased current density near an electrode, and segmental resistances of each body segment with rather uniform current density. With small area electrode contact, the constrictional zone resistance will dominate (see Section 6.2.2). The segmental resistance may be estimated from the equation Rsr = L/aA. With constant a, the segmental resistance depends on the ratio L/A, and accordingly varies according to body or limb size. 

Figure 3 The OmpF porin channel (left) emhedded in an explicit POPC membrane, and (right) the corresponding top view of the OmpF. The L3 loop in the constriction zone of the three monomers is represented by the large shaded cylinder. The structure has been obtained from the protein database (lompf.pdb) and the plot has been rendered with...

Fig. 1. Detail of reaction zone of the metal-atom reactor. Suitable reactor dimensions are 15-18 cm diameter, 5 mm wall thickness and 36-46 cm depth. The water-cooled electrodes are 7.5 cm apart. The central substrate inlet tube, a 6 mm od Pyrex slightly constricted at the end, extends 5 cm below the liquid nitrogen level. A 14 mm od Pyrex tube which serves as a substrate deflector is positioned 5 cm below the inlet nozzle and is suspended horizontally between the electrodes. A built-in Pyrex syphon tube extends to the bottom of the reactor for the removal of air sensitive products under an inert atmosphere.

The lining of the vestibule, which contains a zone of short stiff hairs (vibrissae), acts as a baffle system for filtering out large (>10 pm) airborne particles. The stratified squamous epithelium of the vestibule is resistant to dehydration and inhaled irritants and poorly permeable. Between the vestibule and atrium, there is a constriction known as the internal... 

The porin monomers associate to form trimeric channels as is shown in Fig. 8-20B. They all have a central water-filled, elliptical channel that is constricted in the center to an "eye" -0.8 x 1.1 nm in size. In this restriction zone the channel is lined with polar residues that provide the substrate discrimination and gating. For example, in OmpF and FhoE there are many positively and negatively charged side chains that form the edge of the eye (Fig. 8-20C). The electrostatic potential difference across the outer membrane is small, but apparently determines whether the porins are in an open or a closed state. The voltage difference has opposite effects on OmpF and FhoE, apparently as a result of the differing distribution of charged... 

---