Overall Pathway

Since phytosterols contain extra carbon atoms at C-24 and often a A -bond, 

Heintz, P. Benveniste, and T. Bimpson, Biochem. Biophys. Res, Comm., 1972,46, 766. 

Efficient conversion of ergosterol into ergosta-5,7,9(ll),22-tetraen-3p-ol in Mucor rouxii has been reported.  

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The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

Figure 7.1 The overall pathway of haem biosynthesis. 5-AminolaevuIinate (ALA) is synthesized in the mitochondrion, and is transferred to the cytosol where it is converted to porphobilinogen, four molecules of which condense to form a porphyrin ring. The next three steps involve oxidation of the pyrrole ring substituents to give protoporphyrinogen fX, whose formation is accompanied by its transport back into the mitochondrion. After oxidation to protoporphyrin IX, ferrochelatase inserts Fe2+ to yield haem. A, P, M and V represent, respectively acetyl, propionyl, methyl and vinyl (—CH2=CH2) groups. From Voet and Voet, 1995. Reproduced by permission of John Wiley Sons, Inc.

A study using the gypsy moth, Lymantria dispar, illustrates the overall pathways involved in production of epoxide pheromone components (Fig. 3) [77]. This insect uses disparlure, Me2,epo7-18 H, as a pheromone component. In-... 

The overall pathways of benzene oxidation and the decompositions of possible intermediates have been well characterized via theoretical methods. Thus far, we have discussed these species mainly in the context of their oxidation mechanisms, but phenylperoxy and phenoxy radicals have also been investigated as individual experimental targets. 

In summary, although radical-anion formation in the dimerization of pyridine has been observed by ESR, the nature of the overall pathway (anionic or radical) remains to be elucidated. 

The overall pathway from tyrosine to morphine is illustrated in the following scheme. 

Outline the pathway for biosynthesis of L-leucine from glucose and NH4+ in autotrophic organisms. In addition, outline the pathways for degradation of leucine to C02, water, and NH4+ in the human body. For this overall pathway or "metabolic loop," mark the locations (one or more) at which each of the following processes occurs. 

For more detailed studies of the overall pathway, a separation of the individual enzymes was carried out. For this purpose a single general procedure, effective in measuring 3,5-epimerase activities, was developed incubation of TDP-4-keto-6-deoxyglucose-3T (35, 36) in the presence of the enzyme resulted in tritium exchange with the medium. After completion of the incubation, further tritium exchange with the medium was prevented by the addition of sodium borohydride (Figure 7). 

Three patterns for end-product inhibition. In end-product inhibition the first reaction in the pathway (a) or the first reactions after a branchpoint (b) are inhibited by the specific end products (c). Sometimes the first step in the overall pathway is inhibited by the end products from both branchpoints. 

Because of the possible alternative pathways, which probably occur simultaneously, no single set of reactions can uniquely describe the pentose phosphate pathway. One possible set of pathways is shown in figure 12.34. If the triose phosphate formed is converted to hexose phosphate, the overall pathway can be seen as regenerating five molecules of hexose phosphate for each six used initially. 

In many cases the amino acid pathway branches so that two or more amino acids are formed. Aspartate is the precursor of four other amino acids found in proteins Isoleucine, threonine, methionine, and lysine (see fig. 21.2). The first step in this overall pathway entails the conversion of aspartate to /3-aspartyl-phosphate by aspartokinase. One might imagine that all four of the amino acid end products of this pathway would act together to inhibit this enzyme. However, in E. coli a different solution has been found. In this bacterium there are three aspartokinases which appear to be parts of different multienzyme complexes leading to threonine and leucine for aspartokinase I, methionine for aspartokinase II and lysine for aspartokinase III. As might be expected threonine and isoleucine inhibit aspartokinase I,... 

Although mercapturic acids, the N-acetylcysteine conjugates of xenobiotics, have been known since the early part of the twentieth century, only since the early 1960s has the source of the cysteine moiety (glutathione) and the enzymes required for the formation of these acids been identified and characterized. The overall pathway is shown in Figure 7.19. 

The fragments are subsequently joined together. This overall pathway of assembly of new strands... 

Genes encoding enzymes involved in starch biosynthesis or other relevant pathways (e.g., synthesis of sucrose) can be used for the overexpression of enzyme activity, as described in this chapter for ADPGlc PPase. Another approach is the use of antisense (complementary) DNA or RNA to decrease gene expression, a good way to assess the role of an enzyme and whether it limits the rate of the overall pathway. 

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