Pathways metabolic maps

FIGURE 18.2 The metabolic map as a set of dots and lines. The heavy dots and lines trace the central energy-releasing pathways known as glycolysis and the citric acid cycle. 

Pathways can be illustrated in a metabolic map as linear, branched or cyclic processes (Figure 1.3) and are often compartmentalized within particular subcellular location glycolysis in the cytosol and the Krebs tricarboxylic acid (TCA) cycle in... 

Figure 3.1. Anyone learning biochemistry is introduced to metabolic maps of the type shown here. These 2-D representations of major pathways are becoming increasingly misleading because as this chapter shows the majority of enzymatic transformations in the natural world do not stick to the simple rules implied by such a map. The great majority of chemical diversity made by organisms is made by enzymes that are much more multifunctional so they are less easily placed on a simple 2-D map.

The Absolute, Ultimate Guide combines an innovative study guide with a reliable solutions manual in one convenient volume. A poster-size Cellular Metabolic Map is packaged with the Guide, on which students can draw the reactions and pathways of metabolism in their proper compartments within the cell. 

The tricarboxylic acid cycle shown as a part of the central pathways of energy metabolism. (See Figure 8.2, p. 90 for a more detailed view of the metabolic map.)... 

Hexose monophosphate pathway shown as a component of the metabolic map (see Figure 8.2,... 

Figure 8.26 Metabolic pathways of diazinon in mammals and insects. (Redrawn from Aizawa, H., Metabolic Maps of Pesticides, Academic Press, New York, 1982.)...

Most recently, in silico methods have been apphed to the systems biology of metabolic pathways. An analysis of the human genome assigns 2709 human enzymes to 896 bioreactions 622 of the enzymes are assigned roles in 135 predicted metabolic pathways. The predicted pathways closely match the known nutritional requirements of humans. This analysis identifies probable omissions in the human genome annotation in the form of 203 pathway holes (missing enzymes within the predicted pathways). The predicted human metabolic map is described by a Pathway/Genome Database called HumanCyc, which is available at http //HumanCyc.org/. 

Kadekaro, M., Savaki, H., and Sokoloff, L., 1980, Metabolic mapping of neural pathways involved in gastrosecretory response to insulin hypoglycaemia in the rat, J. Physiol. 300 393-407. 

Phenylalanine Metabolism, As the metabolic map in Fig. 3-20 shows, phenylalanine is an essential amino acid located at the crossroads of various metabolic pathways. In the normal individual, it is used mainly for hydroxylation to tyrosine, a reaction catalyzed by phenylalanine hydroxylase. The conversion of phenylalanine to tyrosine was demonstrated by the isotopic experiments of Moss and Schoenheimer [70], who... 

Laborious studies in different laboratories elucidated the pathway used by tryptophan to yield nicotinic acid. The intermediates are outlined in the metabolic map (see Fig. 4-9). The first of these reactions is catalyzed by tryptophan pyrrolase, a heme-contain-... 

A look at the metabolic map reveals that there are two major pathways for estrogen synthesis, one involves progesterone as an intermediate, and the other bypasses the isomerization. The relative activities of the pathway (progesterone) and of the A pathway (pregnenolone) can be compared by studying the fate of labeled progesterone with that of labeled pregnenolone [71]. 

In order to do this the food materials are subjected to a variety of metabolic processes, each of which involves a well-defined sequence of reactions. Nearly every step is catalysed by a different enzyme, and results in a small but specific chemical change. This field of intermediary metabolism constitutes a major part of biochemistry. The main metabolic pathways have by now all been elucidated, and metabolic maps prepared, which show the possible origins and fates of all the major cellular constituents. 

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