Proton transfer pathway, coupled

How eould this eatalytie bias be controlled One possibility is that the proton transfer pathway eould eontribute to specifieity (Peters et al., 1998). Another possibility is that differences in midpoint potential of the FeS clusters (or other redox sites) that constitute the intramolecular wire could be tuned to facilitate one of the two directions of the reaction. For example, these redox sites could best match the midpoint potentials of a particular oxidized or reduced electron carrier (Holm and Sander, 1999). Apparently, a conformational change in succinate dehydrogenase, coupled to the reduction of FAD, is responsible for its catalytic bias for fumarate reduction (Hirst et al., 1996). 

As imphed by equation (3), and by the location of the O2 reduction site in the structure, proton transfer across the cytochrome oxidase protein is required for function, which necessitates proton-conducting pathways for three specific purposes, that is, to transfer the four substrate protons from the A-side of the membrane into the site of O2 reduction, for uptake of the four pumped protons (per O2 reduced) that are translocated across the membrane coupled to the redox reaction, and for release of these protons to the opposite side of the membrane (exit pathway). Site-directed mutagenesis data indicated the presence of two proton transfer pathways from the A-side of the membrane toward the binuclear heme... 

Fig. 6. Photochemical cycles showing coupling of electron transfer to proton transfer, cytochrome oxidation and quinone exchange in (A) native reaction centers where two Cyt c are oxidized in the cycie, (B) reaction centers where uptake of the first proton is inhibited, and (C) reaction centers where uptake ofthe second proton is inhibited (shading indicates the quinone pool). Figure source (A) Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers role of aspartate-L21Z in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 734 (B) Okamura and Feher (1992) Proton transfer in reaction centers from photosynthetic bacteria. Annu Rev Biochemistry. 61 868 (C) Feher, Paddock, Rongey and Okamura (1992) Proton transfer pathways in photosynthetic reaction centers studied by site-directed mutagenesis. In A Pullman, J Jortner and B Pullman (eds) Membrane Proteins Structures, Interactions and Models, p 485. Kluwer.

Figure 10. Coupled consecutive proton transfer pathway across the membrane. The actual number of binding sites is not known. For simplicity, only five binding sites (A1 througji As) are shown. Site A3 designates the Schiff base proton binding site. It is understood that the Schiff base is neutral when deprotonated and carries a positive charge when protonated. The reverse reactions are not shown but are important for the discussion. (Reproduced with permission from reference 82. Copyright 1990.)...

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

The exchange mechanism of proton transfer in these systems is explained in terms of the Eigen model (Scheme 4) as discussed above by either hydrolysis (Mo(IV) and W(IV)) or protolysis (Tc(V) and Re(V)) pathways, coupled with direct proton transfer in the intermediate pH... 

Figure 1 shows the pulse sequence of the C HSQC experiment supplemented by a spin-lock pulse to suppress the signals from C-bound protons. The experiment is readily described in terms of Cartesian product operators [9]. For a two spin system consisting of a proton spin H coupled to a C spin C, the relevant coherence transfer pathway is... 

Very Shallow Traps. It has been proposed that the neutral Gua(Nl—H) radical, formed by proton transfer from the Gua radical by proton transfer from N1 of Gua to N3 of Cyt, is a shallow trap [143,144]. This proposal is based on projections from made on monomers in dilute aqueous solution, which predict that proton transfer is favored by 2.3 kJ/mol [22,145]. Ab initio calculations are in excellent agreement with this value [146,147]. So one expects that an energy of at least 0.025 eV is needed to activate the return of the proton to N1 Gua, reforming Gua . Once Gua is reformed, tunneling to nearby guanines is reestablished as a competitive pathway. Proton transfer therefore is a gate for hole transfer. Proton-coupled hole transfer describes the thermally driven transfer of holes from one Gua Cyt base pair to another. 

The modelling just described of the direct HC1 + CIONO2 reaction on an ice lattice to produce molecular chlorine and ionized nitric acid portrayed a relatively facile coupled proton transfer/SN2 mechanism, evidently supported in subsequent calculations.26 These results also reinforced the idea of an ionic pathway involving ionized HC1,912>21 as opposed to molecular HC1. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

The pathway for proton transfer to QB is studied in the reaction center (RC) from Rb. sphaeroides using two approaches (Adelroth et al., 2001) 1) the binding of Zn2+ or Cd2+ to the RC surface at His-H126, His-H128, and Asp-H124 and 2) the replacement of the histidines for Ala. In the double mutant RC at pH 8.5, the observed rates of proton uptake associated with both the first and the second proton-coupled electron-transfer... 

Oxidative phosphorylation occurs in the mitochondria of all animal and plant tissues, and is a coupled process between the oxidation of substrates and production of ATP. As the TCA cycle runs, hydrogen ions (or electrons) are carried by the two carrier molecules NAD or FAD to the electron transport pumps. Energy released by the electron transfer processes pumps the protons to the intermembrane region, where they accumulate in a high enough concentration to phosphorylate the ADP to ATP. The overall process is called oxidative phosphorylation. The cristae have the major coupling factors F, (a hydrophilic protein) and F0 (a hydrophobic lipoprotein complex). F, and F0 together comprise the ATPase (also called ATP synthase) complex activated by Mg2+. F0 forms a proton translocation pathway and Fj... 

IX. The E-Pathway Hypothesis of Coupled Transmembranf. Electron and Proton Transfer... 

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