Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Regulation of Biosynthetic Processes
An Overview of Cellular Metabolism

Before moving on to the mechanisms that regulate Biosynthesis, let us briefly summarize our current understanding of METABOLISM.

At one time, The Cell was widely viewed merely as a "bag of Enzymes." Indeed, the complex process of metabolism can be explained by the action of several thousand enzymes that accelerate specific reactions of their substrates. Enzymatic reactions can, in principle, take place even without enzymes, since they are fundamentally driven by the inherent reactivity of the substrates. The Role of enzymes is simply to channel these reactions along specific metabolic pathways, which are most often organized into cycles.

Some of these pathways involve energy-yielding reactions that store energy as ATP, the bulk of which is subsequently used to power the reductive processes of biosynthesis. These reductive pathways yield less reactive, hydrophobic lipid groups and amino acid side chains that are essential for the assembly of insoluble intracellular structures. The Structural Organization of natural Oligomeric Proteins, membranes, microtubules, and fibers results from aggregation driven by a combination of hydrophobic interactions, electrostatic forces, and Hydrogen Bonds. The ultimate outcome of metabolism is the synthesis of complex molecules that spontaneously interact with one another in highly specific ways to form the structures required by the Organism—such as lipid-rich Cytoplasmic membranes that, together with embedded proteins, regulate the influx of substances into Cells.

Nothing in the cell is static. Structures are constantly being built and broken down. Everything undergoes interconversion at varying rates. Hydrolytic enzymes attack all the polymers that make up cells, and active catabolic reactions break down the monomers generated by these attacks. Membrane structures also undergo modifications through hydroxylation and glycosylation. These reactions provide the driving force that transports the debris resulting from membrane breakdown to the cell surface. At the same time, other processes, including degradation by lysosomal enzymes, allow membrane-building Materials to re-enter the cell. Oxidative processes lead to The breakdown of hydrophobic substances, such as sterols and Fatty acids of Membrane Lipids, converting them into more soluble compounds that are subsequently degraded and fully oxidized.

Any factor that affects The rate of a reaction involved in the biosynthesis or breakdown of any cell component must directly or indirectly influence the overall metabolic picture. Thus, it is safe to say that any chemical reaction contributing even a minor part to metabolism can act as a regulator. Because molecules can interact in a myriad of ways, the number of reactions exerting regulatory control over metabolism is vast. Small molecules act on macromolecules as effectors, altering the conformation and reactivity of Biopolymers. Enzymes interact with one another, which can result in Cleavage, oxidation, or The formation of cross-linked aggregates. Transferases attach phosphate, glycosyl, methyl, and other groups to various active sites on proteins, and the catalytic activity of such modified proteins can change significantly. The number of such interactions capable of exerting a noticeable effect on Metabolic Regulation in an organism can reach millions. It is hardly surprising, therefore, that modern biochemical journals are filled with papers examining a vast array of diverse regulatory mechanisms.

Despite this complexity, however, the existence of certain regulatory mechanisms has been conclusively demonstrated. Two Types of regulation based on the feedback principle have already been discussed above. One operates in enzyme synthesis and involves the repression of this synthesis by an excess of the enzyme (Chapter 6, Section E,2), while the other provides rapid control of enzyme activity through inhibition (Chapter 6, Section E,4). Under conditions of a constant Cell Growth Rate, feedback regulation may be sufficient to ensure a balanced and proportional increase in the concentration of all cell constituents. This situation is observed, for example, in the logarithmic phase of bacterial growth (Chapter 6, Section B) or in rapidly growing animal embryos, where all required nutrients are supplied by a relatively constant maternal Blood supply.

A completely different situation is seen in the adult human, whose body is practically no longer growing. The metabolism of many parts of such an organism can vary greatly over time and depending on its physiological state. For instance, the organism can abruptly shift from normal Nutrition to starvation, or from a state of rest to heavy exertion. Metabolism under intense physical stress differs from that during routine activity. A high-fat diet requires a completely different metabolic profile than a high-carbohydrate diet. The requisite regulatory mechanisms must in these cases respond quickly and flexibly to such changes. In the following sections, we will examine some of the ways in which the breakdown and biosynthesis of Introduction/36.html">CARBOHYDRATES and lipids are regulated in the animal body.