Protonophores

Carbonylcyanide-4-trilluoromethoxyphenylhydrazone is known as a protonophore or uncoupler of oxidative phosphorylation in bioelectrochemistry because it disrupts the tight coupling between electron transport and the ATP synthase. Uncouplers act by dis-... 

A relationship correlating the weak acid uncouplers activity with their A %-, pAi°, and has been presented on the basis of protonophoric theory of uncoupling activity, in which the concentration of anionic ionophore (A ) within a biomembrane is supposed to be controlled by the ionic partition of A at the biomembrane solution interface according to Eq. (28) [19]. The biomembrane solution interface could be polarized or electrogenic [37]. Experimental results on the activities of uncouplers on rat liver mitochondria [30] have been explained reasonably [19,24]. 

Antonenko, Y. N. Bulychev, A. A., Measurements of local pH changes near bilayer lipid membrane by means of a pH microelectrode and a protonophore-dependent membrane potential. Comparison of the methods, Biochim. Biophys. Acta 1070, 279-282 (1991). 

Kasianowicz, J., Benz, R. and McLaughlin, S. (1987). How do protons cross the membrane solution interface Kinetic studies on bilayer membranes exposed to the protonophore S-13 (5-chloro-3-tert-butyl-2 -chloro-4 -nitrosalicylanilide), J. Membr. Biol., 95, 73-89. 

Miyoshi, H., Nishioka, T. and Fujita, T. (1987). Quantitative relationship between protonophoric and uncoupling activities of substituted phenols, Biochim. Biophys. Acta, 891, 194-204. 

Uncoupling in hamster brown adipocytes by a natural protonophore does not appear to cause simultaneous aerobic and anaerobic processes because Nedergaard et al. (1977) reported that these cells stimulated by norepinephrine (see p. 320) had... 

B. Beauvoit, M. Rigoulet, G. Raffard, P. Canioni, and B.Guerin (1991). Differential sensitivity of the cellular compartments of Saccharomyces cerevisiae to protonophoric uncoupler under fermentative and respiratory energy supply. Biochemistry, 30, 11212-11220. 

In transportation, leaks can be found in the proton-sugar symport in bacteria where a protein mediates the transport of protons and sugar across the membrane, and adding a protonophore, a parallel pathway occurs, causing a leak in the transport. 

Each thus interacts with the other, squiggle s just p.m.f. and no more. An uncoupler simply functions by acting as protonophore. 

FIGURE 13.2. Effects of FC on respiration of rat liver MCh. Dependence of respiration rate activated by ADP (V3), calcium transport (Vca), and protonophore CCCP (Vcccp) upon concentration of FC. Substrates pymvate plus malate. 

Rees, T. A. V., Grant, C. M., Harmens, H. E., and Taylor, R. B. (1998). Measuring rates of ammoinum assimilation in marine al e Use of the protonophore carbonyl cyanide m-chlorophe-nylhydrazone to distinguish between upl e and assimilation. PhycoL 34, 264—272. 

Inhibitors have been used not only to identify pathways of N metabolism but also to distinguish between uptake and assimilation of inorganic N. For example, MSX is an irreversible inhibitor of GS and has been used to explore the role of N assimilation products (e.g., gin) and intracellular NH4 on the feedback inhibition of processes such as nitrogenase activity and NOa uptake (Arp and Zumft, 1983). In addition, the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) has been used to distinguish between uptake and assimilation of NH4 in marine algae (Rees et al., 1998). 

The above reasoning was recently confirmed by several pieces of indirect evidence. It was found that in H. halobium (i) protonophorous uncouplers decrease A/in and cause a repellent effect [45], (ii) cyanide and DCCD have no effect on the photoresponse of a mutant which possesses sensory rhodopsins but no bacteriorhodopsin [48], and (iii) in a similar mutant, A l is not involved in photosensing [49]. 

It is thus clear that the nature of the driving force which powers the flagellar motor of halobacteria, remains a problem to be solved. To discriminate between H - and Na" -motors, one should study motility in a Na -free medium. If it is the Na motor that is inherent in halobacteria, they must be motionless without added Na, and a pulse of Na must activate the motility even in the presence of a protonophore. 

Perhaps it is the Na cycle that is responsible for the effect observed by Michel and Oesterhelt[109] (see also [110]) who reported that the protonophorous uncoupler abolishes A/Ih+ at much lower concentrations than those affecting the phosphate potential. 

Heterodisulfide (CoM-S-S-HTP) reduction - coupled to primary translocation. The reaction in which ATP is synthesized during methanol reduction to CH4 could be identified with energetically competent membrane vesicles of the methanogenic strain G61. These vesicles, which are orientated more than 90% inside-out, catalyzed CH4 formation from CH3-S-C0M by reduction with H2 (Reactions 7,8) and coupled this process with the synthesis of ATP [112]. CH3-S-C0M reduction generated a ApH (inside acidic) as monitored by acridine dye quenching protonophores and ATP synthase inhibitors exerted their effects in accordance with a chemiosmotic type of ATP synthesis [113],... 

After it became clear that the reduction of CH3-S-C0M to CH4 consists of two reactions, one of these, the reduction of the heterodisulfide of CoM-S-H and H-S-HTP with H2 (Reaction 7 of Table 2), was considered to be the coupling site for ATP synthesis [14,71], Indeed, it was shown that everted vesicles of G61 also catalyzed the reduction of the heterodisulfide with H2 or with chemically reduced factor F420, F420H2, to H-S-CoM and H-S-HTP and that this reaction was coupled with the synthesis of ATP via the mechanism of electron transport phosphorylation [114-117] (Fig. 5) (i) the reduction of the heterodisulfide was associated with primary proton translocation at a ratio of up to 2H /CoM-S-S-HTP proton translocation was inhibited by protonophores rather than by DCCD (ii) reduction of the heterodisulfide was stimulated by protonophores and inhibited... 

The finding that in Methanococcus voltae protonophores did not affect ATP... 

---