Control Points in Metabolic Pathways

Enzymes that are subject to control signals generally fulfill two criteria they are present at low enzymatic activities and catalyze reactions that are not at equilibrium under cellular conditions. Both criteria arise because control enzymes are likely to be those catalyzing the slowest (rate-determining) step in a metabolic pathway. This is likely to be the case if an enzyme is present at low activity. If this is the case, the enzyme-catalyzed reaction is unlikely to be at equilibrium in vivo because there is insufficient enzyme present to allow equilibration of its reactants before they react with other compounds. 

Identification of Rate-Determining Steps in Metabolic Pathways 

Nonequilibrium reactions can be detected by determining metabolite concentrations in the tissue of interest. Conventionally, a tissue sample is rapidly frozen by compression between metal plates that have been cooled to 77 K by immersion in liquid nitrogen (freeze-clamping). This procedure rapidly halts any enzymatic processes that might alter the metabolite concentrations the concentrations can then be determined by enzymatic or chemical assays. Recently, 31P-NMR spectroscopy has shown considerable value in measuring the concentrations of such metabolites as ATP, ADP, AMP, phosphate, and phosphocreatine in living cells or tissues. 

Once the metabolite concentrations are known, the mass-action ratio (T) can be calculated. For the reaction 

A comparison of T with the equilibrium constant for the reaction will establish whether or not the reaction is near equilibrium in vivo. 

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Factors affecting the rate of synthesis include the level of induction or repression of the gene encoding the enzyme (see Topics G3 and G4 and also the rate of degradation of the mRNA produced from that gene. Many key enzymes at control points in metabolic pathways have particularly short-lived mRNAs and the rate of enzyme synthesis is thus readily controlled by factors that affect the rate of gene transcription. 

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