Partial proton uptake

M thermoautotrophicum cells synthesize ATP in the presence of an artificially imposed pH gradient [18], Proton uptake is not detected following acidification. The addition of valinomycin results in the synthesis of ATP and is accompanied by the extrusion of K but not protons. ATP synthesis is unaffected by DCCD and is stimulated by uncouplers such as 2,4-dinitrophenol and m-chlorophenyl hydrazone. Membrane vesicles from M thermoautotrophicum synthesize ATP when conditions are anaerobic in response to the membrane potential since the addition of suppresses synthesis. ATP synthesis is inhibited by 100 pM CCCP and partially inhibited by DCCD (53% at 100 pM). ATP synthesis also takes place in response to a ApH produced by the oxidation of hydrogen. In this case, ATP synthesis is inhibited by lOpM DCCD and CCCP. Unlike cells, vesicles do not synthesize ATP in response to an artificially imposed ApH or in the presence of valinomycin [54]. M. thermoautotrophicum membranes have an ATPase activity that hydrolyzes ATP, GTP, and UTP at approximately the same rate. The enzyme loses activity at -90°C which is due to aggregation, and activity is restored following sonication. ATPase activity is partially inhibited by DCCD (40% at lOOpM) when membranes are incubated at pH 8 for 10 min [18]. A similar ATPase is found in a different strain of M. thermoautotrophicum [29]. The enzyme is most active at an alkaline pH and it is not significantly inhibited by ADP, 5mM NEM, or 150 pM DCCD. The absence of NEM inhibition suggests that the enzyme may not be a V-type ATPase. 

An analysis of the proton uptake upon reduction of Qb as a function of pH (19) in combination with continuum electrostatic calculations suggests, that the conformational equilibrium depends both on the redox state of Qb and on the pH of the surrounding medium (20). Qb occupies only the distal position below pH 6.5 and only the proximal position above pH 9.0 in both oxidation states. Between these pH values both positions are partially occupied. The reduced Qb has a higher occupancy in the proximal position than the oxidized Qb. 

The effect of KCl and others salts on this proton uptake in the presence of donor can be explained partially by the screening of flash-induced pK shifts of various amino acid residues. It can be qualitatively understood from the salt dependence of the Debye length, Xo, which is a measure of the screening effect in liquids (for simplicity we consider here only the case of a symmetrical z-z valent salt) ... 

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

Partially disulfonated hydroquinone-based PAES random copolymers have been synthesized and characterized for application as proton exchange membranes [128]. A copolymer with a 25% degree of disulfonafion showed the best balance between water uptake and proton conductivity. The copolymers showed substantially reduced methanol permeability compared with Nafion and a satisfactory performance of direct methanol fuel cell applications. 

The physiological effects of FC are manifold and well documented [16]. Most, if not all of them can be seen in conjunction with the marked acidification of the extracellular space and the hyperpolarization of the membrane potential observed almost immediately after addition of the toxin [16]. Drastic changes in solute transport across the plasma membrane occur (e.g. the potassium uptake by guard cells is stimulated [11]) concomitant with (as a consequence of ) the FC-induced increase in proton motive force. High-affinity FC-binding sites were characterized several years ago [for review see 2] in membranes from a number of plants, but purification of the sites proved impossible. As a consequence perhaps, interest in the binding sites faded after 1982, but has now resumed in several laboratories and this has resulted in the recent identification of the presumptive binding protein as well as its partial purification. 

---