Connections between Metabolic Pathways

All metabolic pathways are related, and some metabolites appear in several pathways. 

Many reactions of metabolism can take place simultaneously. 

The citric acid cycle plays a central role in metabolism, in both catabolic and anabolic pathways. The breakdown products of sugars, fatty acids, and amino acids all enter the citric acid cycle. 

In the preceding chapters, we learned about a number of individual metabolic pathways. Some metabolites, such as pyruvate, oxaloacetate, and acetyl-GoA, appear in more than one pathway. Furthermore, reactions of metabolism can take place simultaneously, and it is important to consider control mechanisms by which some reactions and pathways are turned on and off. 

A traffic officer maintains order and stops chaos in the heart of the big city. In the same way, intricate control mechanisms keep the body s biochemical pathways working smoothly together. 

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Nevertheless, there are many questions still open because of problems to detect enzyme activities corresponding to each step of the pathway. The model of biosynthesis pathway was put together by studying the metabolism of exogenously applied intermediates in cell cultures of various origins and combining these results with data of native brassinosteroid patterns. It is more or less accepted that there are three pathways in parallel, the early and the late C6 oxidation pathway, as well as the 24/ -epimers follow ing the same route. Some observations in the analysis of native brassinosteroid patterns suggest a possible connection between the pathways. It was shown that seeds of Arabidopsis contain castasterone and 24-epi-brassinolide [34]. Also members of both 24-epimers, brassinolide and 24-epi-brassinolide were detected in tomato seeds [Winter, unpublished]. 

A directed graph search is available where compounds are nodes and metabolic steps are edges. The server finds the shortest allowable path between one node and another. One can limit the search to steps that occur in a certain kingdom and can also choose to display only pathways visually connected on the metabolic pathway chart. There are thousands of connections that are not drawn on the chart but can be found by using this search. 

Connectivity has a central role in biochemistry and biology, and one imagines that the percolation model, with its focus on connectivity, should have wide application. Percolative behavior is to be expected for the coordinate functioning of systems of proteins in metabolic pathways, for functional interactions between proteins embedded in a membrane, for the interactions between domains in the folding of a polypeptide, or for the onset of function in anhydrobiotic organisms, seeds, and spores. 

Bone is a vital, dynamic connective tissue which has evolved to reflect a balance between its two major functions, provision of mechanical integrity for locomotion and protection and involvement in the metabolic pathways associated with mineral homeostatis. In addition, bone is the primary site of hemopoiesis and recent findings support its important role as a component of the immune system [1]. Bones continuously mend and rebuild themselves by opposing actions of two types of cells, the osteoblasts that form bone and the osteoclasts that resorb (destroy) bone. When the activity of the bone destroying osteoclast cell outpaces that of bone forming osteoblasts, the bottom line is bone loss and the result is osteoporosis. 

Xylitol is the probable connecting point between the D-xylose and L-arabi-nose metabolic pathways (Fig. 5). L-arabinose is the form found most abundantly in nature. Early work by Chaing and Knight showed that cell-free extracts of Penicillium chrysogenum convert L-arabinose to both L-ribose and L-xylulose through the intermediate, L-arabinitol (= L-arabitol) [80]. Only one enzyme, aldose reductase, appears to be responsible for the conversion of L-arabinose to L-arabinitol. Aldose reductase also acts on D-arabinose to produce D-arabitol. Witterveen et al. obtained a mutant of Aspergillus niger deficient in... 

The C2 pool, which is also part of glycolytic breakdown, is the starting point for lipid synthesis. In contrast, amino acids have several precursors and they are connected to a range of pools and metabolic pathways. To understand the structural and chemical similarity and possible differences between organisms, the following biochemical groups are described more in detail carbohydrates, phenylpropanes and their associated derivatives, amino acids, lipids, and the major cell wall constituents. 

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