Bypass reactions

My design here is to bypass reactions declared dangerous merely because they have provoked an accident, in order to provide the means of predicting the potential danger of a reaction, by virtue of global treatment of products that have structures in common. 

Figure 6.46 Pyruvate carboxylase (PC) and phosphoenotpyruvate carboxykinase (PEP CK) bypass reactions...

Answer 6. The negative AG value indicates the reaction is thermodynamically favorable (irreversible), requiring a different bypass reaction for conversion of FI, 6BP to F6P in the gluconeogenic pathway. 

We begin by considering the three bypass reactions of gluconeogenesis. (Keep in mind that bypass refers throughout to the bypass of irreversible glycolytic reactions.)... 

The overall equation for this set of bypass reactions, the sum of Equations 14-4 through 14-7, is... 

Oxaloacetate formed in the transfer of acetyl groups to the cytosol must now be returned to the mitochondria The inner mitochondrial membrane is impermeable to oxaloacetate. Hence, a series of bypass reactions are needed. Most important, these reactions generate much of the NADPH needed for fatty acid synthesis. First, oxaloacetate is reduced to malate by NADH. This reaction is catalyzed by a malate dehydrogenase in the cytosol. 

Fig. /. The four-state model used for the description of triplet energy transfer in the RC according to Angerhofer (1997). For detailed explanation, see text. The filled arrows denote the rates that have been observed and described in the literature. The broken arrows depict rates that are either unknown (from and to BS) or speculative (k3 -i for bypass reaction, and k4 -2 for tunneling). The rates defined by arrows between different molecules (P, B, and Car) are in reality second order rates, i.e. they depend on the ground state concentrations of the molecule the excited state of which they point to. In the case of low excitation densities, i.e., when double excitation of the RCs can be neglected these rates can be assumed to be first order as for example done by Frank et al. (1996b).

In the first of the bypass reactions, phosphoenolpyruvate is synthesized from the extramitochon-drial oxaloacetate by the enzyme phosphoenolpyruvate carboxykinase ... 

Again, this occurs only under conditions of high ATP and NADH concentrations, which also drive the next several reversible steps to produce fructose-1,6-bisphosphate. The formation of this compound from fructose-6-P04 through glycolysis is irreversible therefore, in the second bypass reaction, the gluconeogenesis pathway utilizes a new enzyme, fructose diphosphatase, to irre-... 

The third bypass reaction requires the hydrolysis of glucose-6-P04 to glucose, which then can leave the liver and enter the blood for distribution to other tissues. This reaction requires a third new enzyme, glucose-6-phosphatase ... 

Bypass reactions in the route from pyruvate to glucose... 

The reactions grouped in this sedion are all characterized by skeletal rearrangement. Some are generally considered to be concerted, and thus discrete electron-defident intermediates (carbonium ion, carbene, nitrene, etc.) are bypassed. Reactions in which migration is to carbon or nitrogen are the most important these are considered separately in the sections that follow. 

The cytoplasmic accumulation of acetaldehyde as a toxic by-product might be avoided by these bypass reactions in A. pasteurianus NBRC 3283. 

The synthesis of glucose from pyruvate is not simply a reversal of glycolysis despite the participation of all glycolytic intermediates (Figure 11.11). Seven reactions which are freely reversible are shared by both pathways. Three glycolytic reactions (hexokinase, 6-phosphofructokinase and pyruvate kinase) are essentially irreversible in the cell because of their standard free energies of hydrolysis. In gluconeogenesis, more favourable alternative reactions, termed the bypass reactions, are exploited. 

Lactate and alanine enter as pyruvate following the activities of lactate dehydrogenase (Figure 11.4) and alanine aminotransferase (Section 16.3). The first of the bypass reactions, the objective of which is to overcome the unfavourable energetics of a reversal of the pyruvate kinase reaction, seems a tortuous route (Figure 11.11). The reaction sequence relies on two important enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Since pyruvate carboxylase is located exclusively in the mitochondrion, pyruvate must cross the inner mitochondrial membrane (Section 12.2). Oxaloacetate produced by pyruvate carboxylase cannot traverse the inner membrane and is reduced by malate dehydrogenase into l-malate. This step is the reversal of the tricarboxylate cycle reaction (Section 12.4). Malate may, of... 

Following fructose-bisphosphate aldolase activity, the second bypass reaction circumvents the... 

Which of the following are important bypass reactions in gluconeogenesis ... 

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