Outer membrane proteins Gram-negative bacteria

Figure 2.7. The outer membrane of Gram-negative bacteria. Abbreviations LB, LamB protein LP, lipoprotein O, OmpA protein MP, membrane protein. The peptidoglycan backbone consists of alternating residues of A-acetylglucosamine and /V-acetylmuramic acid, which are cross-linked via short peptides.

When deficient in iron, bacteria and fungi produce and excrete to the extracellular medium low molecular weight, specific iron-carrier molecules, called siderophores. These siderophores bind ferric ions, to form soluble complexes. The complexed ferric ions are transported into the cell through high-affinity and energy-dependent receptor proteins located on the outer membrane. In Gram-negative bacteria, such as E. coli, the most studied system, siderophore-iron complexes are transported initially to the periplasm. 

The insertion of (i-barrel precursors is one of the two translocation processes, besides the sorting of inner membrane and IMS proteins, that are clearly derived from a eubacterial translocation system. (1-Barrel proteins are exclusively found in the outer membranes of Gram-negative bacteria and of endosymbiotic organelles such as mitochondria and plastids (Wim-... 

The lipid membranes are a few nanometers thick. They contain proteins whose role is to actively transport particular target chemicals across these nonpolar barriers. The outer membranes of gram negative bacteria also have protein channels called porins that allow passage of small polar and charged substrates. 

Porins. The outer membranes of gram-negative bacteria contain several 34- to 38-kDa proteins known... 

Transport and Cell Penetration. One of the causes of bacterial resistance to the cephalosporins is poor transport of the antibiotic through tlie outer membrane of gram-negative bacteria. This lipid-bilayer membrane carries receptor proteins for the recognition and transport of essential nutrients, but provides an effective barrier to large molecules. In the case of the cephalosporins there can be a considerable difference between the concentration required to inhibit intact cells and the concentrations required to saturate the target enzymes in broken cell piepaiations. 

Some membranes contain relatively large pores which allow for the free passage of molecules with molecular weights up to about 600. For example the outer membranes of gram negative bacteria contain pores with diameters of about 10 A which are formed from proteins called porins. 

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... 

The outer membrane of Gram-negative bacteria is spanned by porins, trimeric proteins that form inlet channels between the outer membrane and the periplasmic space. The geometry of the porins defines the substrate uptake limit, which is on the order of 600 daltons (Weiss et al., 1991), approximately equivalent to a trisaccharide. Substrates larger than this limit (with few exceptions) must be hydrolyzed outside the outer membrane prior to uptake. Extracellular hydrolysis is carried out by means of extracellular en-... 

Fluoroquinolones must penetrate bacteria to reach their target, DNA gyrase. The second mechanism of fluoroquinolone resistance is decreased cell wall permeability. The fluoroquinolones diffuse through porin channels in the outer membrane of Gram-negative bacteria. Mutation results in a decrease in porin channel proteins, resulting in decreased uptake of the fluoroquinolones into bacterial cells. Alterations in a wide range of outer membrane proteins in Pseudomonas spp. result in resistance. From these mutations, the increase in MIC of the fluoroquinolones is relatively low (2-to 32-fold). Flowever, there is cross-resistance with unrelated antibiotics, most frequently cefoxitin, chloramphenicol, trimethoprim and tetracycline. 

The porins are a class of transmembrane proteins whose structure differs radically from that of other integral proteins. Several types of porin are found in the outer membrane of gram-negative bacteria such as E. coli and in the outer membranes of mitochondria and chloroplasts. The outer membrane protects an intestinal bacterium from harmful agents (e.g., antibiotics, bile salts, and proteases) but permits the uptake and disposal of small hydrophilic molecules including nutrients and waste products. The porins in the outer membrane of an E. coli cell provide channels for the passage of disaccharides and other small molecules as well as phosphate. 

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