Metabolite flow enzymic mechanisms

Metabolite flow along a metabolic pathway is mainly determined by the activities of the enzymes involved (see p. 88). To regulate the pathway, it is suf cient to change the activity of the enzyme that catalyzes the slowest step in the reaction chain. Most metabolic pathways have key enzymes of this type on which the regulatory mechanisms operate. The activity of key enzymes is regulated at three independent levels ... 

Enzymic Mechanisms Underlying Control of Metabolite Flow... 

Just as physiological patterns of control over metabolite flow can be recognized, so can physiological patterns of control over the amount of enzyme. Table III lists a convenient classification. Clearly, the amount of enzyme at any time is a summation of the amount formed and the amount destroyed. Two specific patterns controlling formation might be recognized repression by endproducts and induction by substrate or a precursor. It should be stressed that the terms "repression" and "induction" are operational and should carry no connotations regarding the mechanisms. 

In the preceding discussion of enzyme activity, little was said about the overall regulation and coordination of these activities within a smoothly functioning cell. Clearly, the flow of metabolites through anabolic and catabolic pathways must be consistent with the requirements of the cell (and the organism) and effectively respond to both short- and long-term perturbations in the intracellular and extracellular environment. There are many mechanisms that affect enzyme activity, and the more important ones are summarized below. 

The enzyme DAHP synthase regulates the carbon flow in the shikimate pathway. Different biotic and abiotic stresses, including mechanical wounding and fungal elicitation, induce the accumulation of DAHP synthase mRNA, and, therefore, of phenolic metabolites. 

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