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

Stoichiometry and Energetics of Metabolic Conversions
Organization and Regulation of Metabolism

Like any chemical plant, a Cell must possess regulatory systems to manage the intricate web of Chemical Reactions taking place within it. These systems must ensure the supply of Materials, energy, and electrons According to the cell's needs, as well as a high efficiency in utilizing these resources. In addition, reaction sequences and cycles must be organized so that they are amenable to effective control and regulation. In this section, we will examine how the flow of substances through numerous branched and closed Metabolic pathways of The Cell is regulated via the modulation

of The activity of several Key Enzymes and, furthermore, consider the main Structural Features of the metabolic network. To appreciate the importance and efficiency of these structures and regulatory systems, one need only look at the METABOLIC ACTIVITY OF E. coli Bacteria. In a nutrient-rich medium, these Cells divide every 20 min, carrying out a vast number of chemical reactions with striking precision, productivity, and balance (Table 5.6).

Class="center">Table 5.6. Biosynthetic activity during a 20-minute Cell Division cycle of E. coli^a

^а Lehninger A. L., Bioenergetics, 2d ed., р. 123, W. A. Benjamin, Inc., Palo Alto, Ca., 1965.

Protein Synthesis serves as a prime example. The data in the table show that, on average, a cell synthesizes 1,400 protein molecules per second. Since proteins are relatively large Biopolymers, with the synthesis of each creating an average of about 300 covalent bonds, roughly 420,000 peptide bonds are formed per second in a single cell. Furthermore, because proteins are informational polymers, their constituent monomer units must be linked in a strictly defined sequence.

The living cell pays for this enormous Rate of protein synthesis with a substantial fraction of the chemical energy generated during METABOLISM. The bacterium invests almost all its energy in Biosynthesis, with about 88% of it consumed in protein synthesis. The data in Table 5.6 also indicate that approximately 2.5 million ATP molecules are expended per second for biosynthetic purposes. Since an E. coli cell contains a total of only about five million ATP molecules, this reserve would sustain the cell for merely two seconds of life activity. This fact highlights the remarkable rates of ATP regeneration required for normal cell function, as well as the critical importance of regulating The rate of ATP production to match the cell's demands. Similar Conclusions can be drawn regarding other key participants in primary metabolic processes, such as NADH and NADPH.