Translocation of ions

Ernster, L. (ed.), Bioenergetics. Amsterdam Elsevier, 1984. A collection of reviews covering electron transport, the ATP-synthase, translocation of ions across the mitochondrial inner membrane, thermogenesis in brown fat, and other topics in bioenergetics. 

In this chapter, we shall discuss the distribution of ions among the extracellular phases. Extracellular accumulation is interesting and important in its own right, as it influences the value and safety of food crops, and induces or alleviates the intoxicating effect of ions whose site of action may lie in the apoplast (Horst, 1995). Extracellular accumulation of ions in the root also determines the intracellular accumulation and translocation of ions to the rest of the plant. The following diagram illustrates our conceptualization of ion interactions among phases of the rhizosphere and the plants. 

The physical concepts that are relevant to the conduction or translocation of ions, electrons, and protons in biological membranes will now be outlined. In considering protons separately from the other ions, the intention is to emphasize the unique character of these elementary particles. 

E = IR). The capacitive current is a displacement of charges on opposite sides of the membrane, and therefore, it does not involve a direct translocation of ions through the membrane. One needs to remember that the electrical current is expressed in amperes (coulombs per second). 

Schultz (2006, p. 358) noted that Structured molecular complexes of multiple protein subunits are common in biology. Ion channels are a very good example of such molecular complexes. Ion channels located in plasma membranes are known as transmembiane proteins since they permit the translocation of ions from one side of the membrane to the other. These ion channels have a multi-subunit structure that facilitates their function these snbunits include a-subunits and )-subunits. The transmembrane domains of many transmembrane proteins are known as a-helrx subunits. Mnltiple a-heUx snbunits can be organized to form a channel, or pore, through which ions can flow from one side of the membrane to the other. Other forms of snbunits, e.g., p-snbunits, do not contribute to the structure of the central pore but ate nonetheless important in providing stabilizing influences that allow the central pore to perform its function. 

Molecules added to biological systems may find their way into membranes and associate in the lipid system. Several compounds of fungal or bacterial origin (such as alamethacin, which forms micelles in aqueous systems [350]) readily incorporate into planar lipid bilayers and form channels by self-association, channels which allow the translocation of ions and other hydrophilic species through the otherwise hydrophobic membranes. Based on calculations made of the rates of association possible for channel formation by lateral diffusion of... 

From a biophysical point of view, the use of whole mitochondrion (or chloroplast) to test the "molecular" theory of Mitchell does not seem satisfying. The translocation of ions, such as protons, across a complex system, such as a cristae, is in itself ill-defined. The interactions between various fluxes (ion and water movements, electron and hole transport, etc.) are far too complex to be amenable to a simple analysis. At the present time, direct tests with the mitochondrial membranes are difficult. Experimental testing of the chemiosmotic hypothesis, using simpler model systems such as planar BLM and spherical liposomes are, therefore, in order. 

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. 

The extracellular calcium Ca -sensing receptor plays a central role in maintaining a nearly constant level of extracellular calcium by sensing small changes in Ca and directly and/or indirectly altering the translocation of calcium ions into or out of the extracellular fluid so as to normalize CaQ+. Changes in the level of expression and/or function of the CaR reset the level of CaQ+. Recently developed activators (calcimimetics)... 

Neurotransmitter transporters determine the neurotransmitter concentration in the interstitium. High-affinity transporters can efficiently remove neurotransmitter from the extracellular space because cellular uptake is typically coupled to the translocation of sodium ions. 

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... 

Mitchell s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane. The electrochemical potential difference resulting from the asymmetric dis-... 

Uptake of noradrenaline into the vesicles depends on an electrochemical gradient driven by an excess of protons inside the vesicle core. This gradient is maintained by an ATP-dependent vesicular H+-triphosphatase. Uptake of one molecule of noradrenaline into the vesicle by the transporter is balanced by the counter-transport of two H+ ions (reviewed by Schuldiner 1998). It is thought that either binding or translocation of one H+ ion increases the affinity of the transporter for noradrenaline and that binding of the second H+ actually triggers its translocation. 

The Ca transport ATPase of sarcoplasmic reticulum is an intrinsic membrane protein of 110 kDa [8-11] that controls the distribution of intracellular Ca by ATP-dependent translocation of Ca " ions from the cytoplasm into the lumen of the sarcoplasmic reticulum [12-16],... 

While many biological molecules may be targets for oxidant stress and free radicals, it is clear that the cell membrane and its associated proteins may be particularly vulnerable. The ability of the cell to control its intracellular ionic environment as well as its ability to maintain a polarized membrane potential and electrical excitability depends on the activity of ion-translocating proteins such as channels, pumps and exchangers. Either direct or indirect disturbances of the activity of these ion translocators must ultimately underlie reperfiision and oxidant stress-induced arrhythmias in the heart. A number of studies have therefore investigated the effects of free radicals and oxidant stress on cellular electrophysiology and the activity of key membrane-bound ion translocating proteins. 

Models of lipid bilayers have been employed widely to investigate diffusion properties across membranes through assisted and non-assisted mechanisms. Simple monovalent ions, e.g., Na+, K+, and Cl, have been shown to play a crucial role in intercellular communication. In order to enter the cell, the ion must preliminarily permeate the membrane that acts as an impervious wall towards the cytoplasm. Passive transport of Na+ and Cl ions across membranes has been investigated using a model lipid bilayer that undergoes severe deformations upon translocation of the ions across the aqueous interface [126]. This process is accompanied by thinning defects in the membrane and the formation of water fingers that ensure appropriate hydration of the ion as it permeates the hydrophobic environment. 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

Under basal conditions, PKC is predominantly a cytoplasmic protein. Upon activation by Ca2+ or DAG, the enzyme associates with the plasma membrane, the site of many of its known physiological substrates, including receptors and ion channels. In fact, the translocation of PKC from the cytoplasm to the membrane has long been used as an experimental measure of enzyme activation. Such translocation has often been assayed by phorbol ester binding phorbol esters are tumor-promoting agents that selectively bind to and activate PKC. The molecular basis of the translocation of PKC from the cytoplasm to the plasma membrane has been solved. Subsequent to activation, PKC binds with high affinity to a series of membrane-associated proteins, termed receptors for... 

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