Flux-generating reaction

The Flux-Generating Reaction Is the First Reaction in a Pathway That Is Saturated With Substrate... 

The Krebs cycle will only operate when the hydrogen atoms and electrons produced in the cycle enter the electron transfer chain, ultimately to react with oxygen that is, the two processes must take place simultaneously. A metabolic pathway is defined as a sequence of reactions that is initiated by a flux-generating step. In the cycle, citrate synthase catalyses the flux-generating reaction (Table 9.2) but there is no such reaction in the electron-transfer chain. Consequently, the cycle can be considered to be the first part of a longer pathway, which includes the electron transfer chain (Figure 9.3). 

Any enzymic reaction that supplies substrate to a metabolic pathway. For all subsequent steps to maintain their steady-state concentrations, flux-generating reactions must exhibit zero-order kinetics. 

It may be identified as a nonequilibrium reaction in which the of the enzyme is considerably lower than the normal substrate concentration. The first reaction in glycolysis, catalyzed by hexokinase (Figure 17-2), is such a flux-generating step because its for glucose of 0.05 mmol/L is well below the normal blood glucose concentration of 5 mmol/L. 

One of the enzymes that catalyses a non-equilibrium reaction approaches saturation with substrate, so that it is the flux-generating step, (i.e. the beginning of the pathway). 

The concentrations of the substrates in the liver of the rat indicates that there is no reaction in the cycle that approaches saturation with the pathway substrate i.e. these is to flux-generating step, so that the cycle cannot be described as a biochemical pathway, at least as defined in Chapter 3. The physiological pathway can be considered to start in either of two processes ... 

The principle underlying such a messenger system is discussed in Chapters 3 and 12.) The flux-generating step is the reaction catalysed by the phospholipase indicated by the broader arrow in the above sequence. 

The reactions of Fig. 1 are not in contention. The enzymes and intermediates of Figs. 2 and 4 are probably present in most tissues, particularly where Aid activity is greater than that of TA. No secure evidence exists, however, that the individual reactions of either Figs. 1 or 2 are so linked that they constitute a metabolic pathway, with a flux generating step, ordered reactant, and end-product stoichiometry or fixed direction of operational flux. Nor is there a basis for belief that the PPP can operate as... 

This reaction is physiologically iiTeversible and is the flux generating or first committed step of fatty acid biosynthesis. As expected it is regulated. In mammals acetyl-CoA carboxylase is a large enzyme existing as inactive protomers (560,000 MW, 4 subunits, one biotin), which can assemble into active filaments (4 -10 million MW). 

Unfortunately, the gamma fluxes generated by these reactions are very small, relative to neutron fluxes produced by reactors. 

In 2004, void formation in spherical samples was rediscovered at the nanolevel and discussed in [4-6]. HoUow nanosheUs of cobalt and iron oxides and sulfides have been obtained by means of reaction of metaUic nanopowders with oxygen or sulfur. Contrary to [2, 3], these results have been explained by the Frenkel effect - out-diffusion of metal through the formation of a spherical layer of the compound is faster than in-diffusion of oxygen or sulfur through the same phase. This inequahty of fluxes generates the inward flux of vacancies, meeting inside and forming the void in the internal part of the system. 

The form of Eqs. (6.1.27) and (6.1.29) is highly significant. In both cases, the rate of entropy density generation through irreversible processes taking place locally is specified by a sum of products of the general form 6 = > 0 wherein the represent either generalized fluxes or reaction... 

Flux generation is needed because the steady-state is characterized by a defined rate of operation of the whole pathway in the above hypothetical system this means that reactions Ex to E4 all proceed at the same rate. For this to be possible the first reaction of the sequence (Ej) must not respond to changes in its pathway substrate concentration, [A]. If it did, the rate of Ej would decline continuously as A was consumed by the pathway, so that a constant rate of Ej and hence a steady-state would be impossible. In metabolic systems, saturation of the first reaction with its pathway substrate (denoted by the symbol, / ) produces the required insensitivity to concentration of this substrate. Reactions such as E, which are saturated with their pathway substrate, can be regarded as generating the steady-state flux and have therefore been termed fluxgenerating steps — see Newsholme and Crabtree (31). 

Flux-generating steps must be nonequilibrium because of the saturation with a substrate saturation with any component of a reaction (substrate, product, cofactor) places a nonequilibrium step in the enzymic mechanism, thus making the whole reaction irreversible—see Cleland (8). 

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