Pathways parallel

Decomposition Reactions. Minute traces of acetic anhydride are formed when very dry acetic acid is distilled. Without a catalyst, equiUbrium is reached after about 7 h of boiling, but a trace of acid catalyst produces equiUbrium in 20 min. At equiUbrium, about 4.2 mmol of anhydride is present per bter of acetic acid, even at temperatures as low as 80°C (17). Thermolysis of acetic acid occurs at 442°C and 101.3 kPa (1 atm), leading by parallel pathways to methane [72-82-8] and carbon dioxide [124-38-9] and to ketene [463-51-4] and water (18). Both reactions have great industrial significance. 

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

FIGURE 18.7 Parallel pathways of catabolism and anabolism must differ in at least one metabolic step in order that they can be regulated independently. Shown here are two possible arrangements of opposing catabolic and anabolic sequences between A and P. 

An interesting case arises when different products result from parallel pathways. The hydrolysis of isopropyl bromide in alkaline solutions is instructive. The two prod-... 

The number of positive terms in the rate law is the number of independent, parallel pathways. Negative terms represent the reverse reaction. 

The order increases with increasing concentration when the reaction proceeds by parallel pathways but decreases when a series of steps occurs. 

The first situation, for parallel pathways, is illustrated by Eq. (6-9), the reaction between H2O2 and I". The limiting order with respect to [H+] increases from zero to one as [H+] increases. The second situation, for sequential reactions, is illustrated by Eq. (6-14). The order with respect to [Fe2+] falls from two to one with increasing [Fe2+], and that with respect to [Fe3 r ] from zero to negative one with increasing [Fe3+]. 

LFER. Figure 10-3a, associated with a change of RCS, features a downward bend, using the terminology from Chapter 6. On the other hand, Fig. 10-3h, with parallel pathways, features an upward bend. Discuss whether this pattern is in any sense general. 

The measured [ OH]/[ OH] branching ratio versus inverse temperature is plotted in Fig. 4. If the two species are produced by two parallel pathways, the total reaction rate is a simple sum of the two pathway-resolved rates. In this case, the data points in an Arrhenius plot should fall on a straight line with a slope proportional to the difference in activation energies for the two competing pathways. A fit to the data in Fig. 4 yields the result that the barrier to O atom abstraction is 1.0 0.4kcal mol larger than for H atom abstraction. Although... 

The reduction of Oo Is considered to proceed by two parallel pathways, namely, (A) a 2-electron process which generates peroxide and (B) a 4-electron pathway In which the presence of HO2" In the... 

The set of all intermediate steps is called the reaction pathway. A given reaction (involving the same reactants and products) may occur by a single pathway or by several parallel pathways. In the case of invertible reactions, the pathway followed in the reverse direction (e.g., the cathodic) may or may not coincide with that of the forward direction (in this example, the anodic). For instance, the relatively simple anodic oxidation of divalent manganese ions which in acidic solutions yields tetrava-lent manganese ions Mn +— Mn -l-2e , can follow these two pathways ... 

AU of the remarks above also apply to the case where a given reaction occurs along several parallel pathways. As a result of the principle of independence, the concept of a rate-determining step of the overall reaction becomes meaningless for such a reaction. 

The degree to which an electrode will influence the reaction rates is different for different electrochemical reactions, hi complex electrochemical reactions having parallel pathways, such as a reaction involving organic substances, the electrode material might selectively influence the rates of certain individual steps and thus influence the selectivity of the reaction (i.e., the overall direction of the reaction and the relative yields of primary and secondary reaction products). 

Farias MJS, Camara GA, Tanaka AA, Iwasita T. 2007. Acetaldehyde electrooxidation The influence of concentration on the yields of parallel pathways. J Electroanal Chem 600 236. 

At which stage of the reaction process does the branching into the different parallel pathways occur ... 

Camara GA, Iwasita T. 2005. Parallel pathways of ethanol oxidation The effect of ethanol concentration. J Electroanal Chem 578 315-321. 

All the salient features described here can be incorporated into a quantitative framework that takes into account stratification within the tissues and the parallel pathways [39]. It is instructive to consider a simple model based on these descriptions that embodies only transepidermal and transfollicular barrier elements. We can assume that each distinct tissue acts as a homogeneous phase, a gross distortion of reality but an assumption that nevertheless leads to a useful conceptual description. The resistance by the transepidermal route would be ... 

FIGURE 8.6 Parallel pathways to transcription and the MAP kinase family. The MAP kinases can be classified into three groups, based on the identity of the intermediate residue in their dual phosphorylation motifs (TEY, TGY, or TPY). This classification also defines three distinct signal-transduction pathways indicated as the ERK, the JNK/SAPK, and the p38/HOG pathway, each having unique protein kinases acting upstream. 

Since this is an equilibrium reaction and the concentrations of all substances involved are changing during the potential scan, it is difficult to predict what percentage of reacting methanol follows this path. Nevertheless the situation presents some advantages at least. We can use the reaction product of Eq. 2.11 to demonstrate the existence of a parallel pathway involving formic acid. 

In Fig. 2.13 the mass intensities for carbon dioxide (m/e = 44) and methyl formate (m/e = 60) during a potential scan are given. While the signal for C02 follows the current pattern in the whole potential range that for HCOOCH3 does not. This indicates the existence of parallel pathways. Methyl formate was also detected as an electrolysis product in long duration experiments [66],... 

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

It is also agreed that further reaction goes through two parallel pathways,... 

Brown, H.S., Ito, K., Galetin, A. and Houston, J.B. (2005) Prediction of in vivo drug—drug interactions from in vitro data impact of incorporating parallel pathways of drug elimination and inhibitor absorption rate constant British Journal of Clinical Pharmacology, 60 (5), 508-518. 

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