Metabolic pathways organisation

Freeman Dyson considers that any theory on the origin of life which begins with cooperative organisation in a large population of molecules, and makes no provision for short circuits in the metabolic pathways, will be met by the criticism just described (Dyson, 1985). 

Enzymes are organised into metabolic pathways which collectively constitute metabolism. Two types of metabolism are found in cells, catabolism (breakdown pathways) and anabolism (synthetic pathways). Linking these two types of metabolic reactions are the intermediary reactions of central metabolism. Cells, which contain many complex polymers, thus have the means to generate and convert monomeric materials into the complex biological structure. The sources of these materials are the simpler components from the cell s environment, such as inorganic salts and glucose (Fig. 5.9). 

In this chapter we use the term proto-metabolism for specifying networks of reactions capable of performing chemical transformations and inducing self-organisation features in a chemical system. From this definition, there is fundamentally no difference between metabolic and proto-metabolic pathways except that enzyme catalysis makes metabolism much more efficient in achieving its function and generates multiple possibilities of feedback control almost without any limitations. 

The specific organisation is different for each individual pathway, but nevertheless there are some broad generalisations that can be made. First of all, degradation and synthesis are often physically separate. Fatty acids may be both made and used in the same tissue, e.g. liver, but the two directions of metabolism are found in... 

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