Proton channels, oxidase activation

At least three types of proton channel systems are recognized in animal cells. These include the Na+/H+ exchanger, the H+-ATPase, and the HCOj/Cl- exchanger. It is clear that a major part of proton release by some cells in response to transplasma membrane electron transport is by activation of the Na+/H+ exchanger. This is clear from the characteristics of the proton movement elicited and the magnitude of H+ release in relation to electron flow when electron transport is activated. Activation of electron transport can be elicited by addition of di-ferric transferrin to activate the transmembrane NADH oxidase activity or by electron flow to external ferricyanide from internal NADH. Addition of di-ferric transferrin to certain cells, especially pineal cells, elicits a remarkable proton release and internal alkaliniza-tion. The stoichiometry of H+ release to iron reduced is more than 100 to 1 (Sun et... 

FIGURE 14. Structure of E coli amine oxidase active site in the equilibrium turnover complex. Interactions and labelling as in Figure 12 except that the reduced form of the cofactor is labelled TPR and carries a nitrogen at the 5-position. The aldehyde product, labelled Prod, is retained in the channel to the enzyme surface. Water W4 is positioned for nucleophihc attack on C5 of the cofactor and W2 is retained as a link in a proton shuttle from 04 to dioxygen. 

Figure 1. The channels which can be available for proton release in different cells. These may be activated by ligands attached to receptors or signals generated by the electron transport system. The electron transport across the membrane can also be accompanied by proton movement, depending on the orientation of electron transport, but th is movement would be I imited because of the slow rate of electron transport compared to the rapid rate which can be elicited through channels. Any possible relation of oxidase control to the H+-ATPase or the H -K+-ATPase has not been tested by inhibitors such as bafilomycin or omeprazole, respectively (Swallow et al., 1990).

The plasma membranes of all cells contain a unique ligand-activated NADH oxidase, the components of which are largely undefined (Brightman et al., 1992). This oxidase has been shown to be associated with proton movement in all types of cells (plant and animal), so it is a basis either for proton transfer itself or for activation of proton transfer channels of other types with independent energization. 

Other oxidases also derive function from bidirectional PCET pathways at the enzyme active site. The recent crystal structures of PSII [206, 207] support suggestions that as the oxygen evolving complex (OEC) steps through its various S-states [208, 209], substrate derived protons are shuttled to the lumen via a proton exit channel, the headwater of which appears to be the Dgi residue hydrogen-bonded to Mn-bound water [210]. The protons are liberated with the proton-coupled oxidation of the Mn-OH2 site. As shown by the structure reproduced in Fig. 17.23, Dgj is diametrically opposite to Y, which has long been known [148, 151, 152] to be the electron relay between the PS II reaction center and OEC. Notwithstanding,... 

Heller J., Heller A. Loss of activity or gain in stability of oxidases upon their immobilization in hydrated sihca Significance of the electrostatic interactions of surface arginine residues at the entrances of the reaction channels. J Am Chem. Soc. 1998 120(19) 4586-4590 Honma L., Takeda Y., Bae J.M. Protonic conducting properties of sol-gel derived organic/inorganic nanocomposite membranes doped with acidic functional molecules. Solid State Ionics 1999 120(1-1) 255-264... 

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