Cations translocation

Fig. 4. E1-E2 reaction cycle of the Na,K-pump with four major occluded conformations and ping-pong sequential cation translocation. The phosphoforms can occlude Na" and dephosphoforms can occlude or Rb. Na and K without brackets are cations bound to an open form such that they can exchange with medium cations [Na ] or [K ] within brackets are occluded and prevented from exchanging with medium cations. It is proposed that release of Na cxt accompanies transition from EiP[3Na] to E2P[2Na], since the capacity for occlusion of Na in the ouabain-stabilized E2P form is lower than in the EjP form prepared by incubation with CrATP [29] or oligomycin [89].

Hundreds of mutant P-type ATPases have been generated in an effort to understand the catalytic mechanism and regulation of these enzymes. Functional analysis of these mutants has established the roles of particular domains and amino acid residues in the overall catalysis of cation translocation and allowed structure prediction (MacLennan etal., 1997) (Figs. 1 and 2). Recently the 3-D or crystal structures of Ca +, H+, and Na+/K+ P-type ATPases have been resolved, and they confirmed findings... 

Vainio H, Mela L, Chance B. 1970. Energy dependent bivalent cation translocation in rat liver mitochondria. EurJBiochem 12 387-391. 

Fig. 6. Alignment tree for selected bacterial divalent-cation translocating P-type ATPases, including the four current CadA cadmium efflux ATPases, three bacterial presumed-copper ATPases (CopA and CopB from Enterococcus and PacS from a cyanobacterium Synechococcus), along with PacL, a possibly calcium ATPase from the same Synechococcus strain. See text for explanations and literature citations...

Andersen J, Vilsen B. Structure-function relationships of cation translocation by Ca and Na,K-ATPases studied by site directed mutagenesis. FEBS Lett 1995 359 101-106. 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

Neurotransmitter transport can be electrogenic if it results in the net translocation of electrical charge (e.g. if more cations than anions are transferred into the cell interior). Moreover, some transporters may direction-ally conduct ions in a manner akin to ligand-gated ion channels this ion flux is not coupled to substrate transport and requires a separate permeation pathway associated with the transporter molecule. In the case of the monoamine transporters (DAT, NET, SERT) the sodium current triggered by amphetamine, a monoamine and psychostimulant (see Fig. 4) is considered responsible for a high internal sodium concentration... 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

It might be unexpected for cationic groups, e.g., protonated amino groups, to be important in translocation of cations across the membrane bilayer. Indeed, Grin-stein et al. [19] found that amino reagents (pyridoxal phosphate, trinitrobenzene sulfonate and diisothiocyanostilbene disulfonate) did not affect the Na /H ex-... 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

Lead may be taken up in edible plants from the soil via the root system, by direct foliar uptake and translocation within the plant, and by surface deposition of particulate matter. The amount of lead in soil that is bioavailable to a vegetable plant depends on factors such as cation exchange capacity, pH, amount of organic matter present, soil moisture content, and the type of amendments added to the soil. Back-... 

Busch AE, Karbach U, Miska D, Gorboulev V, Akhoundova A, Volk C et al. Human neurons express the polyspecific cation transporter hOCT2, which translocates monoamine neurotransmitters, amantadine, and memantine. Mol Pharmacol 1998 54(2) 342—352. 

The amino acid sequences of haptides comprise hydrophobic and cationic residues with a net charge of +4 to +5 per 19 to 21 amino acids. It was proposed that haptides could be attracted to the anionic liposomes as well as the anionic cell membrane and that the hydrophobic properties of the haptide facilitate membrane translocation (106). Haptide uptake was reported to be energy independent, occurring at 4°C. The advantage of this peptide compared to CPP such as TAT and Antp, is that, unlike the virus-derived peptides, the haptides are not recognized as foreign antigens and do not induce cell transformation (106). However, haptides have also been found to accelerate fibrin clot formation and lack cell specificity (106). 

As for the a-radical participant in this coupling reaction, the main product is surely formed as a result of radical translocation. As for the cation-radical participant, the position of the coupling is explained as follows (Begley et al. 1994) ... 

---