Ion translocation

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. 

The Na/K ATPase has been extensively purified and characterized, and consists of a catalytic a subunit of around 95 kDa and a glycoprotein 0 subunit of approximately 45 kDa (Skou, 1992). The functional transporter exists as a dimer with each monomer consisting of an a and /3 subunit. Hiatt aal. (1984) have su ested that the non-catalytic jS subunit may be involved in the cottect insertion of the a subunit into the lipid bilayer and, therefore, it is conceivable that a modification of the 0 subunit structure may be reflected by changes in the catalytic activity of the a subunit. Therefore, in studies involving the manipulation of tissue glutathione levels, alterations of intracellular redox state may have an effect on substrate binding at an extracellular site on this ion-translocating protein. 

Figure 4.14 Diagrammatic representation of (a) oxy-radical>mediated S-thioiation and (b) thiol/disulphide-initiated S-thiolation of protein suiphydryl groups. Under both circumstances mixed disuiphides are formed between glutathione and protein thiois iocated on the ion-translocator protein resulting in an alteration of protein structure and function. Both of these mechanisms are completely reversible by the addition of a suitabie reducing agent, such as reduced glutathione, returning the protein to its native form.

In environments lacking a suitable external electron acceptor - such as dioxygen, sulfate, or ferric iron - respiration is not possible. Here, many organic compounds may be metabolized by fermenting microorganisms. Microbes of this class may create ATP by a direct coupling mechanism, using a process known as substrate level phosphorylation, SLP with an ion translocation mechanism like that employed by respirers, as already described or by a combination of SLP and ion translocation.1... 

Buckel, W. (2001). Sodium ion-translocating decarboxylases, Biochim. Biophys. 

In a series of papers, we have proposed the torsional mechanism of energy transduction and ATP synthesis, the only unified and detailed molecular mechanism of ATP synthesis to date [16-20,56] which addresses the issues of ion translocation in Fq [16, 20, 56], ionmotive torque generation in Fq [16, 20, 56], torque transmission from Fq to Fj [17,18], energy storage in the enzyme [17], conformational changes in Fj [18], and the catalytic cycle of ATP synthesis [18, 19]. We have also studied the thermodynamic and kinetic aspects of ATP synthesis [19,20,41,42,56]. A kinetic scheme has been developed and mathematically analyzed to obtain a kinetic model relating the rate of ATP synthesis to pHjn and pH m in the Fq portion and the adenine nucleotide concentrations in the Fj portion of ATP synthase. Analysis of these kinetic models reveals a wealth of mechanistic details such as the absence of cooperativity in the Fj portion of ATP synthase, order of substrate binding and product release events, and kinetic inequivalence of ApH and Aip. 

In glycerol monooleate/decane bilayers we find the steady-state conductance at zero current to be proportional to the first power of the ion concentration and to the second power of the ionophore concentration, as illustrated in Fig. 1. (The current-voltage characteristic is hyperbolic for all ionic species indicating that this molecule is in the equilibrium domain for the interfacial reactions, with the rate-limiting step being the ion translocation across the membrane interior.) The conductance selectivity sequence is seen to be Na>K>Rb>Cs, Li. 

Molecular Motions Driven by Transition Metal Redox Couples Ion Translocation and Assembling-Disassembling of Dinuclear Double-Strand Helicates... 

Figure 2.11 Thermodynamic cycle that connects equilibrium (2.1) (the Ni"-to-Ni " redox change and chloride ion translocation) and equilibrium (2.2) (simple Nin-to-Ninl oxidation) through chloride complexation equilibria involving Cu11 and Ni111, in the conjugate system...

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. 

A unique feature of the F/V/A-ATPases is that they are rotary molecular motor enzymes. This has been shown by experiment for members of the F-and V-ATPase subfamilies and is generally assumed to be true for the closely related A-ATPases as well. The two enzymatic processes, ATP synthesis/hydrolysis and ion translocation, are coupled via a rotational motion of a central domain of the complex (the rotor) relative to a static domain (the stator). The A-, F-, and V-ATPases represent the smallest rotary motors found in the living cell so far. Most of what we know about the structure and mechanism of these microscopic energy converters comes from studies conducted with the F-ATPase. In the following review, current structural knowledge for all three members of the family of F-, V-,... 

Structural Characterization of Conformational Changes Associated with Ion Translocation 41 Change in Exposure to Modifying Agents 41... 

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