ATP-driven reactions

The simplest ATP-driven reaction, ATP-driven proton uptake, has already been discussed. After activation the membrane-bound ATP synthase pumps protons into the inner thylakoid space, coupled to ATP hydrolysis. Both 4pH and Aip are produced with magnitudes similar to those produced by light-driven proton transport [51.77]. 

under similar experimental conditions, has also been shown to drive reverse electron flow, leading to the oxidation of an external electron donor, such as DTT or hydroquinone, and the reduction of Q. The reaction seems to involve. 

ATP-induced luminescence was also demonstrated to occur in intact chloroplasts [80]. 

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Activated acetate. The formation of acetyl CoA from acetate is an ATP-driven reaction ... 

Bacteria and plants can synthesize acetyl CoA from acetate and CoA by an ATP-driven reaction that is catalyzed by... 

This reaction can also be driven by light [9,10,88] or ATP [9,10,89]. The rates of the ATP and PPj-driven NAD reduction are about 20-30 and 6-12% of the light-driven reduction, respectively. The addition of both PPj and ATP causes a synergistic stimulation. Oligomycin inhibits the ATP-driven reaction but stimulates the PPj- as well as the light-driven reactions with 60-70 and 20-80%, respectively. The PPj-driven reaction is inhibited by uncouplers but not by electron-transport inhibitors. 

The biosynthesis is accomplished by several ATP-driven reactions. Ring formation (cyclization) is achieved by incorporating nucleophilic amino groups and electrophilic carbonyl groups at the appropriate positions. 

Amino acids must be "activated" by ATP-driven reaction to be incorporated into proteins (Figure 5.19)... 

However, it must be admitted that, of all the synthetic reactions in which ATP is involved, the observation of a phosphorylated substrate is relatively rare. In the majority of the ATP-driven reactions attempts to demonstrate the intermediate formation of a phosphorylated substrate molecule have been unsuccessful. Outstanding examples of this are (1) synthesis of glutamine from glutamic acid, ATP, and ammonia and (2) protein synthesis. 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

FIGURE 11-40 Reversibility of F-type ATPases. An ATP-driven proton transporter also can catalyze ATP synthesis (red arrows) as protons flow down their electrochemical gradient. This is the central reaction in the processes of oxidative phosphorylation and photophosphorylation, both described in detail in Chapter 19. 

In the dark, the production of ATP and NADPH by photophosphorylation, and the incorporation of C02 into triose phosphate (by the so-called dark reactions), cease. The dark reactions of photosynthesis were so named to distinguish them from the primary light-driven reactions of electron transfer to NADP+ and synthesis of ATP, described in Chapter 19. They do not, in... 

Show how this reaction can be incorporated into an ATP-driven cyclic pathway for generating NADPH from NADH. 

V). The centers resemble PSII of chloroplasts and have a high midpoint electrode potential E° of 0.46 V. The initial electron acceptor is the Mg2+-free bacteriopheophytin (see Fig. 23-20) whose midpoint potential is -0.7 V. Electrons flow from reduced bacteriopheophytin to menaquinone or ubiquinone or both via a cytochrome bct complex, similar to that of mitochondria, then back to the reaction center P870. This is primarily a cyclic process coupled to ATP synthesis. Needed reducing equivalents can be formed by ATP-driven reverse electron transport involving electrons removed from succinate. Similarly, the purple sulfur bacteria can use electrons from H2S. 

Three modifications of the conventional oxidative citric acid cycle are needed, which substitute irreversible enzyme steps. Succinate dehydrogenase is replaced by fumarate reductase, 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarate oxidoreductase (2-oxoglutarate synthase), and citrate synthase by ATP-citrate lyase [3, 16] it should be noted that the carboxylases of the cycle catalyze the reductive carboxylation reactions. There are variants of the ATP-driven cleavage of citrate as well as of isocitrate formation [7]. The reductive citric acid... 

Figure 16.32. Substrate Cycle. This ATP-driven cycle operates at two different rates. A small change in the rates of the two opposing reactions results in a large change in the net flux of product B.

The flux-force relations for (enzyme-catalyzed) chemical reactions were derived above. We consider now the case of a chemical reaction that is coupled to the vectorial movement of a solute across a membrane. An example is the ATP-driven pump present in the mitochondrial inner membrane. It catalyzes the hydrolysis of ATP to ADP and phosphate (P ), with concomitant translocation of a number of H ions from the mitochondrial matrix to the external medium. The reaction can be written down as ... 

Two major ATP synthesizing reactions in living organisms are oxidative phosphorylation and photophosphorylation. Both reactions take place in H -ATPase (FqF,), which is driven by an electrochemical potential difference of protons across the biomembrane, as predicted by Mitchell [1]. In Racket s laboratory, ATPases related to oxidative phosphorylation were prepared, but their relationship to Mitchell s chemiosmotic hypothesis [1] was not described [2], Later, an insoluble ATPase (H -ATPase) was shown to translocate protons across the membrane when it was reconstituted into liposomes [3], H -ATPase was shown to be composed of a catalytic moiety called F, (coupling factor 1) [4], and a membrane moiety called Fq [5], which confers inhibitor sensitivity to F,. F was shown to be a proton channel, which translocates down an electrochemical potential gradient across the membrane when Fg is reconstituted into liposomes (Fig. 5.1) [6]. Thus, -ATPase was called FqFj or ATP synthetase. 

The energy transfer through conformational change in F Fj may be compared with that of flux-driven flagellar motor rotation [211] and ATP-driven bending of myosin head [212]. Both the latter reactions are loosely coupled, since their torque... 

The A/Xh+ can be generated by light-induced or dark electron transport, as well as by hydrolysis of ATP, GTP or PPj [7,8], The rates of the ATP- and PP -driven reactions are about 70 and 56% of the light-driven reaction, respectively. The addition of both ATP and PPj stimulates the rate more than either alone, and gives a rate almost as high as light. The PP-driven reaction is inhibited by uncouplers of phosphorylation but not by electron-transport inhibitors or oligomycin. The stoicheiometries of ATP and PP hydrolyzed to NADP reduced are 1 and 10, respectively. 

The energy expenditure of the mitochondrial ATP-driven transhydrogenase reaction has been estimated to one ATP per NADPH formed [29,30,46,53,60-62]. Since the equilibrium constant of the nonenergy-linked transhydrogenase reaction is 0.79 [2] and that of ATP hydrolysis is about 10 M [63], the equilibrium constant of the overall reaction is also of the order of 10 M. In spite of the very unfavourable equilibrium a reversibility of the energy-Unked transhydrogenase reaction has been demonstrated using ATP production [64] or uptake of lipophilic anions [65-67] as assay. 

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