Metabolic pathways pathway maps

Thousands of reactions mediated by an equal number of enzymes are occurring at any given instant within the cell. Metabolism has many branch points, cycles, and interconnections, as a glance at a metabolic pathway map reveals... 

In this chapter we will consider definitions of metabolism the biochemistry-physiology continuum. The concept of metabolic pathways and their organization and control of metabolism are likened to a road map involving flow of substrates but with mechanisms to accelerate or slow down pathways or to direct substrates through alternative routes. 

In this chapter we survey recent findings that describe the metabolic fate of the xenobiotic moieties of mercapturic acid pathway (MAP) metabolites that are excreted from the liver with the bile. We show that the products of the MAP undergo an enterohepatic circulation that is mediated by intestinal enzymes and/or intestinal microflora. 

Scheme 3 Metabolic pathways of PCB biotransformation in vertebrates. MAP, mercapturic add pathway GSH, glutathione, Cys, cysteine...

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/. 

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