ATP-P, exchange

The Mg2+-activated ATPase (or ATP synthase) is made up of two parts. The Fj component is the catalytic, Mg2+-binding, extrinsic membrane protein composed of five different subunits, a, (3, y, S and e. The F0 component is an intrinsic membrane complex that contains three subunits, a, b and c, and mediates proton translocation. The F, protein is bound to the membrane through interaction with F0. The complexity of the F,F0 enzyme has presented many difficulties. Hie greatest advances have been made for the bacterial enzymes, notably for thermophiles, Escherichia coli and Rhodospirillum rubrum, where progress has been made in the purification of subunits and their reconstitution into membranes, and the identification of binding sites for Mg2+ and nucleotides on the Fi subunits.300 FiF0 preparations can be incorporated into liposomes and display H+ translocation, ATP-P, exchange and ATP synthesis.301... 

The ATP-P exchange reaction has been very useful in demonstrating the activity of the complete ATP synthase (CFy-CFi) complex, since it requires an intact system properly incorporated within a vesicular system [39]. Thus, isolated CFi or CFo-CFi, though possessing ATPase activity, do not show ATP-Pj exchange until properly incorporated into a liposomal system. 

The energy functional Etf[p] = Ttf[p] + ne />] + T[p] is known as Thomas-Fermi (TF) theory, inHiiHinv the Atp[p] exchange part (first derived by Block but commonly associated with the name of Dirac (constitutes the Thomas-Fermi-Dirac (TFD)... 

NADH-ubiquinone reductase was isolated by Hatefi et al. in 1961 (B7-29). A procedure was developed for the resolution of the mitochondrial electron transport system into four enzyme complexes. Recently, a fifth fraction, which is capable of energy conservation and ATP-P, exchange, was also isolated (SO, 31). The overall scheme for the isolation of the five component enzyme complexes of the mitochondrial electron transport-oxidative phosphorylation system is given in Fig. 1. It is seen... 

The decay of the active state of the light-triggered ATPase, in pea chloroplas membranes, is shown in Fig. 2. The process of decay is apparently biphasic -full activity is retained for several seconds, and the activity then declines precipitously. A similar biphasic decay has been reported (Bar-Zvi Shavit, 1981) for the ATP-P exchange reaction in lettuce chloroplasts. 

Actin binds ATP very tightly in the presence of a divalent metal ion that can be or The exchange-inert analogue of Mg-ATP, p,y-CrATP, can displace... 

Reactions causing PA — ATP P and other exchanges and ATPase reactions... 

How does the flow of protons drive the synthesis of ATP Isotopic-exchange experiments unexpectedly revealed that enzyme-bound ATP forms readily in the absence of a proton-motive force. When ADP and Pj were added to ATP synthase in H2 0, became incorporated into P, through the synthesis of ATP and its subsequent hydrolysis (Figure 18.27). The rate of incorporation of into P showed that about equal amounts of bound ATP and ADP are in equilibrium at the catalytic site, even in the absence of a proton gradient. However, ATP does not leave the catalytic site unless... 

In the presence of activator, pyruvate, the substrate saturation curves of the R. ruhrum ADP-Glc PPase are hyperbolic at low temperatures. Using kinetic studies its reaction mechanism was studied. The product inhibition patterns eliminated all known sequential mechanisms except the ordered BiBi or Theorell—Chance mechanisms. Small intercept effects suggested the existence of significant concentrations of central transis-tory complexes. Kinetic constants obtained in the study also favored the ordered BiBi mechanism. In addition studies using ATP-[ P]-pyrophosphate isotope exchange at equilibrium supported a sequential-ordered mechanism, which indicated that ATP is the first substrate to bind and that ADP-Glc is the last product to... 

It is very difficult to measure the flux of protons across the membrane either out of the mitochondria into the cytoplasm or from the cytoplasm through the ATP synthase into the mitochondria. Therefore, estimates of the stoichiometry have often been indirect. One argument is based on thermodynamics. If Ap attains values no more negative than -160 mV and Rp within mitochondria reaches at least 10 M , we must couple AGh of -15.4 kj/mol to AG of formation of ATP of +57.3 kJ/ mol. To do this four H must be translocated per ATP formed. Recent experimental measurements with chloroplast ATP synthase also favor four H+. It is often proposed that one of these protons is used to pump ADP into the mitochondria via the ATP-ADP exchange carrier (Section D). Furthermore, if Rp reaches 1(P M in the cytoplasm, it must exceed 10 M in the mitochondrial matrix. 

Fig. 5. Schematic model for the ion movements aeross gastric microsomal vesicles. The J values are the ionic fluxes with the superseiipts designating pump flux (P) or leak pathway (L). The model consists of an ATP-driven exchange pump, (K +H )-ATPase, and the passive leak pathways for the...

ATP is an absolute requirement ADP inhibits Ei has been purified fortyfold.) The role of ATP in reaction (6) was studied by P -exchange experiments. It was found that the terminal PO4 of ATP reacts with the enzyme to form a phospho-enzyme -1- ADP. The POi" of the enzyme then exchanges with the product of reaction (a) to form GSH - - P. Enz3rme-bound RNA was recently shown to participate in the GSH synthesizing system (117a). 

Hydroxy- 9-methylglutaryl CoA further yields acetyl CoA and acetoacetic acid, as was shown earlier by Coon et cU. (I48). In biotin deficiency the carboxylation reaction does not occur. It was shown by Lynen et al. that the actual carboxylation is preceded by the enzymic dehydration (rf jS-hydroxyisovaleryl CoA to /8-methylcrotonyl CoA, which is the true substrate for the entry of CO2. TTiis occurs at the expense of the hydrolysis of the terminal P04 of ATP. The unsaturated intermediate is then saturated by the addition of H2O to yield the final product. The critical step of this carboxylation is the conversion of CO2 to a reactive form. The analogy of the biochemical activation of other substances through an acyl adenylate type of compound did not fit CO2 activation. The final mechanism of the activation of CO2 was derived from the discovery that the carboxylase enzyme was a biotin-protein. This observation explains earlier work 149) which indicated that biotin is a cofactor of the fatty acid-synthesizing enzyme system. When the purified carboxylase was incubated with P and ATP an exchange reaction of phosphate occurred, which was inhibited by avidin, a protein which specifically binds biotin. This indicated that the primary reaction in CO2 fixation is the combination of ATP with the biotin-protein enzyme to yield ADP biotin-protein -f P. The active CO2 is then the product of an exchange reaction between ADP and C02 which is finally attached to the biotin-protein complex. 

A transfer of the adenylyl group i.e., a P—0 split has not been obtained so far (or maybe has not been looked for) with the adenylates in the absence of enzyme. In the presence of the activating enzymes, on the other hand, the adenylyl group is veiy readily transferred onto pyrophosphate to give ATP 116, 169, 164, 177). This reaction, however, is not specific, since tryptophan activating enzyme, for example, can synthetize ATP from all the amino acid adenylates tested 116) as well as from the adenylate of D-tiyptophan 169). The same transfer of the AMP moiety onto PP does, of course, also occur in the amino acid catalyzed ATP-PP exchange reaction. 

Phosphorus. Eighty-five percent of the phosphoms, the second most abundant element in the human body, is located in bones and teeth (24,35). Whereas there is constant exchange of calcium and phosphoms between bones and blood, there is very Httle turnover in teeth (25). The Ca P ratio in bones is constant at about 2 1. Every tissue and cell contains phosphoms, generally as a salt or ester of mono-, di-, or tribasic phosphoric acid, as phosphoHpids, or as phosphorylated sugars (24). Phosphoms is involved in a large number and wide variety of metaboHc functions. Examples are carbohydrate metaboHsm (36,37), adenosine triphosphate (ATP) from fatty acid metaboHsm (38), and oxidative phosphorylation (36,39). Common food sources rich in phosphoms are Hsted in Table 5 (see also Phosphorus compounds). 

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