Stoichiometric coupling

The most difficult and interesting question about H+-ATPase is how chemical reaction (ATP synthesis/ hydrolysis) is coupled with vectorial H+ conduction. The mechanism for the stoichiometric coupling between chemical reaction and vectorial transport of ions is a universal question for ion-motive ATPases. The Fo portion is a passive proton pathway but becomes a regulated pathway after the binding of Fi. Mutant analyses suggest that the y subunit has regulatory role(s) for proton conduction. 

The stoichiometric coupling of phenols to biphenols by oxidation with manganic tris(acetylacetonate) has also been reported,343 e.g.,... 

As examples of coupled counter-transport (see Figure 13.2d) and coupled cotransport (see Figure 13.2e) the transport of titanium(lV) from low acidity (pH = 1) and high acidity ([H+] = 7 M) feed solutions, respectively using the HUM system [1,2] may be presented. The di-(2-ethyUiexyl) phosphoric acid (DEHPA) carrier reacts with Ti(IV) ion to form complexes on the feed side (see Equations 13.25 and 13.26) and reversible reactions take place on the strip side (see Equations 13.27 and 13.28). Energy for the titanium uphill transport is gained from the coupled transport of protons in the direction opposite to titanium transport from the strip to the feed solutions. In the second case (high-feed acidity), Cl anion cotransported with Ti(IV) cation in the same direction. In both cases fluxes of titanium, protons, and chlorine anion are stoichiometrically coupled. 

Recent developments in catalysis at the atomic level are described. The use of transition metal halides which form intermediate metal n-complexes can promote both the catalytic and stoichiometric coupling of aryl Grignard reagents, the stereospecific coupling of 1,2 disubstituted vinyl groups and the double coupling of ethylene to dimers of butadiene. 

Sibson NR, Dhankhar A, Mason GF. Rothman DL, Behar KL, Shulman RG (1998) Stoichiometric coupling of brain glucose metabolism and glutamatergic neuronal activity, Proc Natl Acad Sci USA 95 316-321. 

Energy for the titanium uphill transport is gained from the coupled transport of protons in the opposite to titanium transport from the strip to the feed solutions. In the second case (high feed acidity) Cl anion cotransported with Ti(IV) cation in the same direction. In both cases, fluxes of titanium, protons, and chlorine anion are stoichiometrically coupled. As a rule, coupled transport used combining with the facihtated transport. 

The two enzymatic reactions are coupled by PEG-NADH and PEG-NAD+. This stoichiometric coupling does not affect enzyme kinetics but has to be considered when writing the mass balances. The discussion of this system will be continued in Sect. 7.5 where some implications of coupled enzyme systems on reactor design are described. 

In 2000, Guy reported the stoichiometric coupling of alkane thiols and arylboronic acids, which was initially thought to be mediated by Cu(ll) [71]. Liebeskind proposed that the reaction was more likely catalyzed by Cu(l), generated by oxidation of the alkane thiols into dialkyl disulfides. Based on this hypothesis, Liebeskind predicted that disulfides and disulfide equivalents should be effective reagents for thioether formation [34]. This process would constitute a modification of the Chan-Evans-Lam, which involves the coupling of arylboronic acids and amines or alcohols in the presence of tertiary amine bases, generating aryl amines and ethers, respectively. Indeed, the coupling of diphenyl disulfide with phenyl boronic acid would yield diphenyl sulfide. 

From a mathematical point of view the SBR is the most complex of the four ideal reactors because of its unsteady operation over the whole reaction period and its changing reactor volume due to the dosage. Therefore it is more convenient to use mole numbers for the description instead of concentrations. In addition it has to be observed that the actual number of moles of the fed component present in the reactor cannot be calculated directly by the stoichiometric coupling introduced but by an extended version only which accounts for conversion and time. The added reactant and the initially charged reactant will be indexed A and B, respectively, in the following text. If the conversion is known the number of moles of B present at any time can be calculated according to ... 

The stoichiometric coupling between the transport of an organic substrate (A) and a cation can be fairly easily represented by the carrier model of cotransport (Fig. 3). It is assumed that the transport is mediated by a membrane component (X) which can alternate between two states X and X", respectively. X is assumed to communicate with the -phase (e.g. extracellular phase) from which it can bind both ligands (A) and (B) to form the two binary complexes AX and BX and also a ternary complex ABX. On the other hand, the state X" communicates with the -phase (e.g. intracellular phase) from which it may bind A and/or B to form the complexes AX, BX" and ABX". The translocation of A and B between the two phases, and ", is assumed to be effected by the interconversion between the two states of each species of X, as has been described in more detail elsewhere (6,10,25,46,47). The transfer of energy between the transfers of A and B via cotransport requires that they move predominantly by ternary complex (ABX), and this implies either that the two binary complexes AX and BX are less ready to interconvert between the -state and "-state, respectively, than is the ternary complex, or that the two binary... 

The gradual transformation of the experimentally detected H -release patterns by degradative proteolysis indicates that special attention has to be paid on the functional integrity of the samples before any reliable conclusions can be drawn about the stoichiometric coupling of H -release with the Sj state transitions. 

In contrast to each of the above variations, alkyl iodides may also be utihzed directly in nickel-catalyzed conjugate additions. The mechanism of this class of reactions is not well defined however, the related stoichiometric coupling of enals (e.g., 79) with alkyl halides (e.g., 78) has been demonstrated to proceed through nickel-Jt-aUyl intermediates.l The most widely used variant employs nickel(II) chloride hexahydrate in either catal)hic or stoichiometric quantities with activated zinc as a stoichiometric reductant (Scheme g2) [144-148] pjjg organic hahde may be either sp or sp hybridized, and alkene geometry in the final product (e.g., 80) is maintained with alkenyl iodides. 

Adenine Nucleotides as Stoichiometric Coupling Agents in Metabolism and as Regulatory Modifiers ... 

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