Proton gradient generation

Under aerobic conditions, the glycolytic pathway becomes the initial phase of glucose catabolism (fig. 13.2). The other three components of respiratory metabolism are the tricarboxylic acid (TCA) cycle, which is responsible for further oxidation of pyruvate, the electron-transport chain, which is required for the reoxidation of coenzyme molecules at the expense of molecular oxygen, and the oxidative phosphorylation of ADP to ATP, which is driven by a proton gradient generated in the process of electron transport. Overall, this leads to the potential formation of approximately 30 molecules of ATP per molecule of glucose in the typical eukaryotic cell. 

When little NADP+ is available to accept electrons, an alternative electron transport pathway is used. The high-energy electron donated by photosystem I passes to ferredoxin, then the cytochrome bf complex, then plastocyanin and back to the P700 of photosystem I. The resulting proton gradient generated by the cytochrome bf complex drives ATP synthesis (cyclic photophosphorylation) but no NADPH is made and no 02 is produced. 

Thus, in conclusion, omeprazole has a highly specific action on the H+, K+-ATPase. Three factors of primary importance for this specificity are (a) the unique location of the H+, K+-ATPase in the parietal cell and the steep proton gradient generated by the H+, K+-ATPase (b) the concentration of the protonated form of omeprazole in the acidic canaliculus and (c) the conversion of omeprazole in the acidic compartments close to its target enzyme, the H+, K+-ATPase [3]. 

How does the cytochrome subunit of the reaction center regain an electron to complete the cycle The reduced quinone (QH2) is reoxidized to Q by complex III of the respiratory electron-transport chain (Section 18.3.3). The electrons from the reduced quinone are transferred through a soluble cytochrome c intermediate, called cytochrome c 2, in the periplasm to the cytochrome subunit of the reaction center. The flow of electrons is thus cyclic. The proton gradient generated in the course of this cycle drives the generation of ATP through the action of ATP synthase. 

Assume that the proton gradient generated in producing the required NADPH is sufficient to drive the synthesis of the required ATP. 

S. cerevisiae mitochondria this equilibrium appears to be shifted to supercomplex organization of Complexes III and IV (Mileykovskaya et al., 2005), which may also contain Complex II as well as two peripheral NADH dehydrogenases (Boumans et al., 1998) S. cerevisiae lack Complex I and utilize the peripheral NADH dehydrogenases. FiFq-ATP synthase (Complex V) uses the electrochemical proton gradient generated in respiration to produce ATP. 

By means of an ionophore specific for potassium (e.g. vahnomycin) or for protons (tetrachlorsalicylaniUde, TCS), Sachs et al. showed that the exchange process is electroneutral. The proton gradient generated by addition of ATP to the vesicles slowly dissipates due to the ion permeability of the vesicle. Addition of TCS leads to some increase in the rate of proton efflux, but a much faster proton release is observed upon addition of vahnomycin (Fig. 4). This means that the permeability of the vesicle for K is lower than for [66,78]. The ionophore nigericin, which exchanges H + for K, completely abolishes the proton gradient [58]. 

FIGURE 2.1.6 Schematic representation of proteopoiymersomes reconstituted with both bacteriorhodosin (BR) and F0F1-ATP synthase. Adenosine triphosphate (ATP) synthase uses an electrochemical proton gradient generated by BR to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) [100]. 

Matsushita K, Shinagawa E, Ameyama M (1982) o-Gluconate dehydrogenase from bacteria, 2-keto-D-gluconate-yielding, membrane-bound. Methods Enzymol 89 187-193 Matsushita K, Patel L, Kaback HR (1984) Cytochrome o type oxidase from Escherichia coli. Characterization of the enzyme and mechanism of electrochemical proton gradient generation. Biochemistry 23 4703-4714... 

FIGURE 10 Electron transport and ATP synthesis in chloroplasts. The jagged arrows represent light striking the two photosystems (PS I and PS II) in the thylakoid membrane. Other members of the electron transport chain shown are a quinone (Q), the cytochrome complex (heO plastocyanin (PC), and an iron-sulfur protein (FeS). The chloroplast ATP synthase is shown making ATP at the expense of the electrochemical proton gradient generated by electron transport. 

Michels PAM and Konings WN (1978) The electrochemical proton gradient generated by light in membrane vesicles and chromatophores from Rhodo-pseudomonas sphaeroides, Eur.J.Biochem. 85, 147-155. 

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