Porins selective

Benz, R., Schmid, A., van der Ley, P. and Tommassen, J. (1989). Molecular basis of porin selectivity membrane experiments with OmpC-PhoE and OmpF-PhoE hybrid proteins of Escherichia coli K-12, Biochim. Biophys. Acta, 981, 8-14. 

Each porin molecule has three channels Ion channels combine ion selectivity with high levels of ion conductance The K+ channel is a tetrameric molecule with one ion pore in the interface between the four subunits... 

In Gram-negative bacteria, diffusion of (3-lactam antibiotics into the periplasm (where the activity of PBPs takes place) occurs via the channels that porins create in the outer membrane. The number and properties of the porin molecules are such that diffusion is relatively rapid in E. coli but much slower in Enterobacter and Pseudomonas. Mutants can be selected after the permeability of porin channels or their number has been decreased. A slow diffusion into... 

Typically, functional porins are homotrimers, which assemble from monomers and then integrate into the outer membrane. The general porins, water-filled diffusion pores, allow the passage of hydrophilic molecules up to a size of approximately 600 Daltons. They do not show particular substrate specificity, but display some selectivity for either anions or cations, and some discrimination with respect to the size of the solutes. The first published crystal structure of a bacterial porin was that of R. capsulatus [48]. Together with the atomic structures of two proteins from E. coli, the phosphate limitation-induced anion-selective PhoE porin and the osmotically regulated cation-selective OmpF porin, a common scheme was found [49]. Each monomer consists of 16 (3-strands spanning the outer membrane and forming a barrel-like structure. 

Figure 4 (Plate 5). Atomic structure of the sucrose specific selective porin ScrY isolated from the outer membrane. Longer loops are directed to the outside, shorter turns are facing the periplasm. Monomer and assembled homotrimer in side view (left and middle) top view of assembled trimer (right). (Reproduced by permission of W. Welte and A. Brosig)...

Channel proteins have a polar pore through which ions and other hydrophilic compounds can pass. For example, there are channels that allow selected ions to pass (ion channels see p. 222) and porins that allow molecules below a specific size to pass in a more or less nonspecific fashion (see p. 212). 

Fig. 5.3. Structure of the OmpF porin of E. coli. The porin is a bacterial membrane protein with P-sheet structures as transmembrane elements. The structure of a monomer of the OmpF porin is shown. In total, 16 P-bands are configured in the form of a cylinder and form the waUs of a pore through which selective passage of ions takes place. LI—L8 are long loops, Tl,2,3 and T7,8 are short bends (T turn) that fink the P-sheets. According to Cowan et al. (1992), with per-...

Solutes enter mitochondria through pores in thousands of molecules of the voltage-gated anion-selective channel VDAC, also known as mitochondrial porin.1516 288 289 In the absence of a membrane potential these pores allow free diffusion to molecules up to 1.2 kDa in mass and may selectively permit passage of anions of 3- to 5-kDa mass. However, a membrane potential greater than 20 mV causes the pores to close. NADH also decreases permeability. 

While gramicidin and other channel formers can show high transport rates, they do not show the high selectivity that characterizes natural channels. There is much interest at present in a class of proteins called porins, which form natural pores in the outer membranes of Gram-negative bacteria. Several different porin proteins have been isolated from Escherichia coli. These form water-filled channels of various sizes in membranes. Thus the proteins OmpC and OmpF seem to be cation-specific channels while other proteins give larger diameter channels that seem to be specific for anions.34,35... 

R. E. Hancock, C. Egli, R. Benz and R. J. Siehnel (1992). Overexpression in Escherichia coli and functional analysis of a novel PPi-selective porin, oprO, from Pseudomonas aeruginosa. J. 

Channels are of course also formed by all porins. A general porin contains 16 /1-strands and has a shear number of 20 and a nearly circular cross section (Table II). Three parallel barrels associate to form trimers. The type of residues outlining the channel determines the specificity of a porin which, however, is usually not very strict. The two 18-stranded porins are very specific. Their channel cross-sections are actually smaller than those of the general porins in agreement with their higher selectivity. The 22-stranded barrels of the iron transporter proteins have circular cross sections and would form a very wide channel if they were not filled with the globular N-terminal 150-residue domain. 

All general porins contain pores with sizes allowing the permeation of molecules up to molecular masses of about 600 Da (Nikaido, 1994). The pores come with various selectivities. The porin from Rhodobacter capsulatus, for instance, contains a rather nonpolar binding site near the external end of the pore eyelet, indicating that it may pick up molecules... 

The diffusion of small solutes through porins is passive. The diameters of the pore eyelets range from 10 A for the general porins to 6 A for the highly selective porins. Larger pores are usually decorated with... 

Saxena, K., Drosou, V., Maier, E., Benz, R., and Ludwig, B. (1999). Ion selectivity reversal and induction of voltage-gating by site-directed mutations in the Paracoccus denitrifi-cam porin. Biochemistry 38, 2206-2212. 

Ulmke, D., Kreth, J., Lengeler, J. W., Welte, W., and Schmid, T. (1999). Site-directed mutagenesis of loop L3 of sucrose porin ScrYleads to changes in substrate selectivity. J. Bacteriol. 181, 1920-1923. 

Zeth, K., Diederichs, K., Welte, W., and Engelhardt, H. (2000). Crystal structure of Omp32, the anion-selective porin from Comamonas acidovorans, in complex with a periplasmic peptide at 2.1 A resolution. Structures, 981-992. 

Are the proteins identified in the previous section selectively preserved in DOM In general, porins are expected to be resistant to bacterial degradation (Powell et al. (2005) and reference therein), and modification of these proteins, by saccharide moieties for example, may further retard their bacterial degradation. For example, in a laboratory experiment Keil and Kirchman (1993) noted longer turnover times for glycosylated ribulose 1,5-bisphosphate carboxylase (RuBPCase) when compared with unmodified RuBPCase. In accordance with this report Suzuki et al. (1997) suggested that the glycosylation of porins could help to explain their enhanced preservation/accumulation in seawater. Subsequently Yamada and Tanoue (2003)... 

The distinction between facilitated diffusion through channels and carrier-mediated transport is somewhat artificial/ but may be justified on the basis of specificity. For example/ 3-lactams in general can pass through nonselective bacterial outer membrane porin (e.g./ OmpF) channels via passive diffusion/ whereas imipenem (and related zwitterionic carbapenems) can also utilize OprD channels/ which preferentially recognize basic amino acids and dipeptides. The identification of mutants that selectively confer imipenem resistance suggests that more intimate protein-drug associations are involved in carrier-mediated transport than in facilitated diffusion/ which may be limited only by pore diameter. 

The 48 kDa protein was purified for Al-terminal sequencing and shown to have significant homology with porin-P from the gram negative bacterium Pseudomonas aeruginosa. Tanoue s data are the most direct evidence to date that resistant biopolymers selectively survive degradation and accumulate as oceanic DOM. 

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