Changes in Metabolic Pathways

Metabolic shifts may occur in several fruits during ripening. For example, during ripening of bananas, there is a marked rise in aldolase and carboxylase activities and thus it appears that at this stage the Embden-Meyerhoff pathway becomes dominant and the pentose-phosphate pathway is suppressed. 

An increase in malate and pyruvate decarboxylase activities is observed in apples during the climacteric stage. The activities drop as 

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Fixing a metabolic stability problem at one position can lead to a new problem somewhere else on the molecule, a second meaning (the other being a PGx-induced changes in metabolic pathways) of the term metabolic switching. 

Allosteric enzymes show various activation and inhibition effects which are competitive in nature and related to conformational changes in the structure of the enzyme. Such allosteric enzymes are often crucial enzymes in metabolic pathways and exert control over the whole sequence of reactions. The name allostery refers to the fact that inhibition of the enzyme is by substances that are not similar in shape to the substrate. 

Signaling pathways are important for metabolic control they begin with a signaling molecule, a hormone for example, that acts through a receptor molecule with the ultimate consequence of some change in metabolism. 

The ozone treatment apparently initiated changes in the pathways of carbohydrate metabolism, with glycolysis being reduced while the activity of the pentose phosphate pathway was increased. Rat lung tissue exposed to ozone also exhibited the ozone-induced depression of GPD and enhancement of G6PD activity (, 10), The activation of the pentose phosphate pathway is a characteristic featiire of diseased plants (11, 12),... 

Three critical parameters, which together describe the responsiveness of a pathway to changes in metabolic circumstances, lie at the center of metabolic control analysis. We turn now to a qualitative description of these parameters and their meaning in the context of a living cell. In Box 15-3 we will provide a more rigorous quantitative discussion. 

Fig. 13 Relative changes in metabolic fluxes. Relative changes in the metabolic fluxes of B. megaterium WH323 carrying TFH encoding pYYBm9 after induction of recombinant protein production with either glucose (a) or pyruvate (b) as the sole carbon source are given. AcCoA acetyl-coenzyme A Activ activation F6P fructose 6-phosphate G6P glucose 6-phosphate MAL malate OAA oxaloacetate PEP phosphoenolpyruvate PPP pentose phosphate pathway PYR pyruvate TCA tricarboxylic acid. Data adapted from [88]...

Allosteric enzymes in metabolic pathways change their activity in response to molecules that either stimulate or inhibit their catalytic activity. While the end product of an eri2yme cascade is used up, the cascade continues to synthesize that product. The result is a steady-state condition in which the product is used up as it is produced and there is no significant accumulation of product. However, when the product accumulates above the steady-state level for any reason, that is, in excess of the cell s needs, the end product acts as an inhibitor of the first enzyme of the sequence. This is called allosteric inhibition, and is a type of feedback inhibition. 

Elevation of the ethanol content occurs during the maturation stage of fruit development when acidity declines, total solids increase, and the flavor compounds accumulate to characteristically ripe levels. Since ethanol is a normal product of anaerobic respiration in plants, its accumulation signals a change in the pathway of energy metabolism. This summary of research on the control of ethanol accumulation in citrus fruit describes an approach to identify the role of energy metabolism in the bioregulation of maturation. 

In considering thermodynamic parameters, e.g., heat and work, we do not need to know the exact chemical pathway taken by the reactants in conversion to products. Using thermodynamics, we can obtain information about reactions that cannot be studied directly in living systems. Thermodynamics predicts, on the basis of the known energy levels of reactants and products, whether a reaction can be expected to occur spontaneously or how much energy must be supplied to drive the reaction in one direction or another. Such information is crucial in establishing reaction routes in metabolic pathways. Thermodynamics explains how equilibrium constants are related to changes in temperature. Thermodynamics also explains the basis for enzyme catalysis. 

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