Fundamentals of Biochemical Engineering, Part 1 - Bailey J., Ollis D. 1989

Stoichiometry and Energetics of Metabolic Transformations
Organization and Regulation of Metabolism
Enzyme-Level Regulation of Metabolism

Allosteric Enzymes regulate the distribution of metabolic fluxes across various pathways. The regulatory signals governing The activity of these enzymes are provided by the concentrations of ATP, NADH, NADPH, as well as several key precursor and intermediate compounds. METABOLISM/18.html">The Influence of some of these signals on flux division at branch points is outlined in Fig. 5.24. Here, we examine some of the most illustrative Examples of Metabolic Regulation, along with the coordinated control of multiple metabolic pathways that sometimes occurs.

Obviously, the intracellular ATP concentration must be maintained at an optimal level. The adenylate energy charge [4] can serve as a measure of the cellular energy status, defined as The ratio of the ATP concentration to the sum of the concentrations of AMP, ADP, and ATP:

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In most Cells, The energy charge ranges from 0.87 to 0.94.

To understand how such a high degree of ATP concentration constancy is achieved, let us first consider the EMP pathway (Fig. 5.25). The primary regulatory enzyme here is Phosphofructokinase, which is activated by ADP and inhibited by ATP. Thus, this enzyme slows down the metabolite flux through the EMP pathway when the energy charge is high, and vice versa. Pyruvate kinase, activated by AMP, acts in a similar manner. Recall that the phosphorylation of glucose, as well as the fructose-6-phosphate and phosphoenolpyruvate reactions, represent the only three irreversible steps in the EMP pathway. We can now understand the reason for this phenomenon: The First stage produces an ionized derivative that is securely retained within The Cell membrane, while the other two irreversible steps serve as regulatory points.

The energy charge also affects the activity of the TCA cycle. Isocitrate dehydrogenase is activated by ADP and inhibited by ATP. Since one of the primary Functions of the EMP and TCA pathways is the phosphorylation of ADP—leading to an increase in the cellular energy charge—it is easy to see that the predominant mode of regulation in the scheme of Fig. 5.25 involves coordinated and concerted feedback inhibition.

FIG. 5.25. Regulation of the EMP metabolic pathway and the TCA cycle by adenylates. Dashed arrows indicate the allosteric effect on a given enzyme; the ⊕ sign denotes enzyme activation, and the ⊖ sign denotes its inhibition.

Control over the supply of precursor molecules and biosynthetic processes obviously requires other regulatory systems. Citrate, for example, serves as an allosteric modulator of several enzymes. It enhances the inhibitory effect of ATP on phosphofructokinase. This reflects the amphibolic Nature of the EMP and TCA pathways, whereby six-carbon compounds can be utilized in either pathway when the energy charge is high and nutrients are abundant. Citrate is also a potent activator of the first reaction in FATTY ACID Biosynthesis [Equation (5.34)].

The Regulation of Biosynthetic pathways is typically centered at the steps immediately following a branch point. An example of such regulation is found in the BIOSYNTHESIS OF AMINO Acids of the aspartate family, as illustrated in Fig. 5.12, where dashed lines indicate feedback inhibition pathways. The benefits of a branched reaction sequence with multiple regulated steps are obvious: a limited number of reactions yields several end products, with the synthesis pathway of each amino acid being independently regulated. At first glance, this system might appear to have a flaw: high concentrations of Lysine and Threonine should presumably slow down the synthesis of homoserine and Methionine. In reality, however, this does not happen because The conversion of aspartate to aspartyl-$eta$-phosphate is catalyzed by three different enzymes. The activity of one of these enzymes is unregulated, the second is inhibited by lysine, and the third is inhibited by threonine. Different enzymes catalyzing the same reaction are called isozymes or Isoenzymes. Similarly, the conversion of aspartate semialdehyde to homoserine is catalyzed by two isoenzymes, one of which is unregulated while the second is inhibited by threonine. These facts help to elucidate how efficient and effective control of the reaction network is achieved under any combination of elevated metabolite product concentrations.

The objective of a given biochemical engineering process may be The production of large quantities of specific metabolites. In such cases, the drive to maximize the yield of these compounds obviously conflicts with the life-support and Regulatory Systems of the native Organism. To compel the cell to produce the desired metabolites efficiently (from a human perspective, but not from the cell's "point of view"), attempts are frequently made to modify the metabolism-regulating enzymes so that the native regulatory systems fail to trigger, resulting in the overproduction of the target metabolites. We will return to this issue in the next chapter during our study of applied genetics. Here, we have only to consider the stoichiometric and energetic aspects of coupling Various metabolic pathways, as well as the overall stoichiometry of substrate utilization, cell growth, metabolite formation, and heat generation. As an Introduction to this topic, we will examine The Nature of the metabolic end products secreted by the cell into the medium.