Porins cell membrane

Fig. 9.1 Schematic representation of possible mechanisms of resistance in Gram-negative and Gram-positive bacteria. 1, antibiotic-inactivating enzymes 2, antibiotic efflux proteins 3, alteration or duplication of intracellular targets 4, alteration of the cell membrane reducing antibiotic uptake 5, alterations in porins or lipopolysaccharide reducing antibiotic uptake or binding.

Some of the previous sections have described the acquisition of low-level resistance to various antibiotics by alterations in the cell membrane causing decreased uptake of the drugs. These have normally have characterized as changes in components such as porins which result in a decrease of penetration by antibiotics. 

The mechanism of toxicity for aminoglycosides has not been fully explained and is therefore unclear. It is known that the drug attaches to a bacterial cell wall and is drawn into the cell via channels made up of a protein, porin. Once inside the cell, the aminoglycoside attaches to the 30S bacterial ribosomes. Ribosomes are the intracellular structures responsible for manufacturing proteins. This attachment either inhibits protein biosynthesis or causes the cell to produce abnormal, ineffective proteins. The bacterial cell cannot survive with this impediment. This explanation, however, does not account for the potent bactericidal properties of these agents, since other antibiotics that inhibit the synthesis of proteins (such as tetracycline) are not bactericidal. Recent experimental studies show that the initial site of action is the outer bacterial membrane. The cationic antibiotic molecules create fissures in the outer cell membrane, resulting in leakage of intracellular contents and enhanced antibiotic uptake. This rapid action at the outer membrane probably accounts for most of the bactericidal activity. 

Many small molecules can penetrate the outer cell membrane by diffusion through channels created by outer-membrane proteins called porins. 

Figure 4 Two examples of membrane proteins, (a) Bacteriorhodopsin is mainly an a-protein containing seven helices. It is a membrane protein serving as an ion pump and is found in bacteria that can survive in high salt concentrations, (b) Porin is a P-barrel. Porins work as channels in cell membranes, which let small metabolites such as ions and amino acids in and out of the cell. Figure drawn using MOLSCRIPT. ...

Generally, integral membrane proteins are vectorial molecules with distinct cytoplasmic and extracellular parts. They insert into cell membranes with a specific orientation. A reconstitution of aquaporin 0 with preferred orientation has been achieved by using vesicles consisting of an asjunmetric PEO-PDMS-PMOX bilayer (94). The amino terminus of the porin orients to the (ytoplasmatic side... 

Hollow structures can also be prepared by the self-assembly of stave or rod-like subunits into barrel or bundle-shaped frameworks. This is one of the most common strategies in nature for channel formation, where the rod-like molecules of the barrel-stave type are /S-sheets or a-helices of amphipathic character. The central cavity has hydrophilic properties, while the lipophilic area is oriented outward, in contact with the membrane of the cell. A natural example of this type of protein is a-hemolysin, a bacterium toxin formed by seven identical subunits that self-assemble when in contact with the cell membrane. This assembly gives rise to a mushroom-shaped structure, whose trunk is formed by a -barrel that is inserted into the cell membrane. The resulting channel has a diameter of 13 A at its narrowest point and can transport ions and other molecules. Other natural examples based on this model —but using an a-helix instead of f-sheets—include cholera toxin, potassium channels, porins, aquaporins, and the most powerful toxin known to date, botulinum neurotoxin (BoNT, known as Botox), broadly studied by Mental s group. ... 

Model HA301), a function generator (Hokuto Denko, Model HB104) and an X-Y recorder (Riken Denshi, F35). The measurement cell was of all-glass construction, approximately 10 ml in volume, incorporating a conventional three-electrode system. An anion-selective polymer membrane electrode was also coated with a PhoE porin-lecithin membrane. Potentiometric measurements were made with an electrometer (Hokuto Denko, Model HE-IOIA) in conjunction with a recorder (Riken Denshi, Model SP-J3C). 

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

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