Metabolic pathway/metabolite flow

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 ... 

Modulation by ligands. An important variable that regulates flow through a metabolic pathway is precursor availability (metabolite A in the case shown here). The availability of precursor A increases along with the activity of the metabolic pathways that form A (3) and it decreases with increasing activity of other pathways that also consume A (4). Transport from one cell compartment to another can also restrict the availability of A. 

The individual pathways of carbohydrate metabolism in plants overlap extensively they share pools of common intermediates, including hexose phosphates, pentose phosphates, and triose phosphates. Transporters in the membranes of chloroplasts, mitochondria, amyloplasts, and peroxisomes mediate the movement of sugar phosphates between organelles. The direction of metabolite flow through the pools changes from day to night. 

Individual enzymes in vivo have different constraints and requirements. Generally, intracellular enzymes are required to maintain a constant concentration of the various metabolites and this may be achieved by having a wide variation in the reaction flux of the material through the various metabolic pathways. The reaction rate will vary with [S] if the enzyme is working below saturation, K [S], and the rate is given by Eqn. 3. However, extracellular enzymes are often faced with dramatic changes in the concentration of their substrates and yet are required to maintain a steady flow of material for absorption and use by the cell. The reaction rate will be independent of [S] if the enzyme is working under saturation conditions,... 

For the metabolic pathway in Example 11.9, consider the steady-state flow J after changing the concentration of the first enzyme E. The rate of the first reaction is a linear function of the enzyme concentration and a nonlinear function of the concentrations of metabolites Xq, Xi, and X4. The fractional change in the steady-state rate through the pathway upon a fractional change in E is... 

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