Proton pump model

III.E. Some Other Photosynthetic Proton-Pump Models... 

Lanyi, J.K. Bacteriorhodopsin as a model for proton pumps. Nature 375 461-464, 1995. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

Proton pump inhibitors are extremely safe. Diarrhea, headache, and abdominal pain are reported in 1-5% of patients, although the frequency of these events is only slightly increased compared with placebo. Proton pump inhibitors do not have teratogenicity in animal models however, safety during pregnancy has not been established. 

Photoprotonic signals can be generated by light induced proton release, this might happen for the quaternary N-R+ derivatives of 117, for instance [8.233]. The photoproduction of proton gradients across membranes could serve as a light-powered proton pump for inducing vectorial processes such as the transport of protons [8.233, 8.234] or H+-ATPase model reactions [8.274]. 

Schematic diagram of one model of the physiologic control of hydrogen ion secretion by the gastric parietal cell. ECL cell, enterochromaffin-like cell G(CCK-B), gastrin-cholecystokinin-B receptor H, histamine H2, histamine H2 receptor Mi, M3, muscarinic receptors ST2, somatostatin2 receptor ATPase, K /H ATPase proton pump. Some investigators place histamine receptors—and possibly cholinoceptors—on nearby tissue cells rather than on the parietal cell itself. (Modified and redrawn from Sachs G, Prinz C Gastric enterochromaffin-like cells and the regulation of acid secretion. News Physiol Sci 1996 11 57, and other sources.)...

The irreversible inactivation of endogenous enzymes caused by drugs, e.g., the antiplatelet effect of aspirin after oral administration [442], the 5o-reductase inhibition by a new nonsteroidal inhibitor [443], and the H+, K+-ATPase inactivation by proton pump inhibitors [444], is modeled with turnover models. The simplest model [442] includes terms for the production rate kt and loss rate ka of the response E, coupled with a function g (c) representing the change of plasma or effect-compartment drug concentration ... 

Complex IV catalyzes electron transfer from cytochrome c to O2 this process appears to be coupled to proton translocation, with an H+/e value of 2. Two models have been developed to account for these values (Fig. 14-7). Current understanding is that complex IV is capable of acting as a proton pump. 

In addition to the mechanism of dioxygen reduction, an understanding of how Cco pumps protons is also desirable. Models have been proposed that allow for linkage of the proton pumping to the dioxygen reduction reaction (50). One attractive model involves the CuA site and is shown in Figure 6 (10). In this mechanism, the CuA center is ligated to two histidines and two thiolates and receives the initial electron from... 

Figure 6. Model for redox-linked proton pumping in cytochrome oxidase involving the CuA site. (Reproduced from reference 10. Copyright 1990 American Chemical Society.)...

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

Figure 6.6-17 Model of proton pump mechanism of bacteriorhodopsin.

The percolation model suggests that it may not be necessary to have a rigid geometry and definite pathway for conduction, as implied by the proton-wire model of membrane transport (Nagle and Mille, 1981). For proton pumps the fluctuating random percolation networks would serve for diffusion of the ion across the water-poor protein surface, to where the active site would apply a vectorial kick. In this view the special nonrandom structure of the active site would be limited in size to a dimension commensurate with that found for active sites of proteins such as enzymes. Control is possible conduction could be switched on or off by the addition or subtraction of a few elements, shifting the fractional occupancy up or down across the percolation threshold. Statistical assemblies of conducting elements need only partially fill a surface or volume to obtain conduction. For a surface the percolation threshold is at half-saturation of the sites. For a three-dimensional pore only one-sixth of the sites need be filled. 

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