Cation transport bacterial

Zinc efflux is mediated by a zinc exporter known as ZntA (Zn + transport or tolerance), a membrane protein which was identified through studies of bacterial strains that were hypersensitive to zinc and cadmium. Sequence inspection revealed that ZntA was a member of the family of cation transport P-type ATPases, a major family of ion-translocating membrane proteins in which ATPase activity in one portion of the protein is used to phophorylate an aspartate within a highly conserved amino acid sequence, DKTG, in another portion of the protein. The cysteine rich N-terminus of these soft metal transport proteins contains several metal-binding sites. How the chemical energy released by ATP hydrolysis results in metal ion transport is not yet known, in part because there is only partial information about the structures of these proteins. The bacterial zinc exporter also pumps cadmium and lead and is therefore also involved in protection from heavy metal toxicity (see Metal Ion Toxicity). 

Early experiments showed that a transferrin-polycation complex transported bacterial DNA into cells [12]. Ions are taken up by cells as an iron-transferrin complex by receptor-mediated endocytosis. Protamine or poly-lysine ligated by disulfide bonds to transferring and mixed with a lu-ciferase-encoding plasmid may bind the DNA because of the cationic properties of the complex [12]. Subsequently, avian ery-throblasts and human K-562 cells were incubated with the transferrin-polycation peptide-DNA complex, and the complexes were recognized and transported into the cells by receptor-mediated endocytosis and taken up into endosome-Hke intracellular vesicles [12]. Treatment with chloroquine (an agent that affects the endosomal pH) enhanced the uptake considerably. In contrast to other transfection methods, the transfection of cells with transferrin-mediated endocytosis did not cause significant cell death, because of the physiologi-... 

In addition, the organic acids that are formed may be utilized in energy-yielding reactions within the plant or bacterial cells or as counterions for cation transport (35). 

These results may be viewed in the wider context of interactions between potential ligands of multifunctional xenobiotics and metal cations in aquatic environments and the subtle effects of the oxidation level of cations such as Fe. The Fe status of a bacterial culture has an important influence on synthesis of the redox systems of the cell since many of the electron transport proteins contain Fe. This is not generally evaluated systematically, although the degradation of tetrachloromethane by a strain of Pseudomonas sp. under denitrifying conditions clearly illustrated the adverse effect of Fe on the biotransformation of the substrate (Lewis and Crawford 1993 Tatara et al. 1993). This possibility should therefore be taken into account in the application of such organisms to bioremediation programs. 

In contrast to the hydrolysis and synthesis of ATP connected with proton translocation in mitochondria, chloroplasts and bacterial membranes, the energy linked movement of calcium ions gives rise to the appearance of an acid-stable phosphorylated intermediate in the membranes. A cation specific phosphorylation also occurs in the membranes of the sodium potassium transport system183. However, due to the inability to correlate phosphorylation and ion movement in the latter membranes, membrane phosphorylation has been questioned as being a step in the reaction sequence of ion translocation184,18s. Solely the sarcoplasmic calcium transport system allows to correlate directly and quantitatively ion translocation with the phosphoryl transfer reactions. 

Microorganisms also produce cyclic compounds that specifically solvate metal cations by using oxygen and nitrogen atoms to complex the ion within the ring. One of the roles of these compounds is to sequester and transport iron across the bacterial membranes. 

Amino acids involved in binding M6P are shown in O Fig. 8. The phosphate moiety is coordinated by His 105, Asn 104, Asp 103 and divalent cation. Each hydroxyl of Man is hydrogen-bonded to binding site residues, while no interactions between the hydrophobic carbohydrate backbone and aromatic side chains are observed [148]. The multiple contacts between lectin and M6P, in contrast to that observed for many lectins which mediate cell surface interactions, are reflected in pM binding constants, reminiscent of bacterial arabinose-binding protein which functions as a sugar transport protein [149]. 

Cystic fibrosis is a hereditary disorder caused by mutation in the cystic fibrosis transmembrane conductance regulator gene that encodes a cyclic adenosine monophosphate-regulated chloride channel. Defects in chloride ion transport in the airway epithelia lead to abnormal airway secretions, impaired mucociliary clearance, chronic bacterial infection, bronchiectasis, and premature death. Delivery of the cystic fibrosis transmembrane conductance regulator cDNA by adenovirus vectors or the plasmid-liposome complex resulted in transient correction of the defects in patients with cystic fibrosis. Formulations of cationic lipid-DNA complexes for aerosol delivery are being explored to improve on the gene therapy approach. 

According to Ovchinnikov and coworkers, the antibiotic activity of valinomycin is due to impairment of alkali ion transport in bacterial membranes217 The main arguments for this theory are (i) None of the synthetic non-complexing analogs has antibiotic activity (ii) enantio-valinomycin has the same antibiotic activity as valinomycin, thus excluding interaction with a stereospecific receptor (iii) valinomycin increases the cation permeability of bacterial membranes (iv) the antimicrobial action of valinomycin depends on the cation composition of the medium. 

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