Metabolic pathways, cyclic linear

We think about metabolic pathways as linear or cyclical sequences of reactions as described in Chapter 1. Individual reactions within a pathway are often dependant upon at least one other reaction. For example, we know from our studies of enzyme kinetics in Chapter 2 that the rate of an enzyme catalysed reaction is determined in part by the concentration of substrate. Remember, the substrate for one reaction is usually the product of a previous reaction, so the activity of an enzyme is affected by the activity of the preceding enzyme in the sequence. 

Some metabolic pathways are linear, some are cyclic, and some are branched, yielding multiple end products from a single precursor or converting several precursors into a single product. 

Most such metabolic pathways are linear, that is, they begin with a specific substrate and end with a specific product. However, some pathways, such as the Krebs cycle, are cyclic. Often, metabolic pathways also have branches that feed into or out of them. The specific sequences of intermediates in the pathways of cell metabolism are called intermediary metabolism. 

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 Basic principles of metabolism. The unit of metabolism is the chemical reaction, catalyzed by an enzyme (upper left). Each reaction can be studied individually in vitro to determine its mechanism and kinetics parameters. In a linear metabolic pathway (e.g., glycolysis), the reactions are coupied in series (lower left). The rate of the first reaction of the chain (A + B C) reaction is controlled by the fluxes of A and B. The flux through the whole metabolic pathway is determined by the rate of production of E. The same rules apply for a cyclic metabolic pathway such as Krebs cycle (right panel). 

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