Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Bacteria contain constitutive and inducible enzymes

Constitutive Enzymes are those that are present in bacterial Cells in constant amounts, regardless of the Organism's metabolic state. Examples include enzymes involved in major Catabolic pathways, such as Glycolysis. In contrast, the concentration of inducible enzymes varies within The Cell. Typically, an inducible enzyme is present in a bacterial cell only in trace amounts; however, if its substrate is added to the medium—especially if this substrate serves as the cell's sole carbon source—the concentration of such an enzyme can rapidly increase a thousandfold or even more. Under these conditions, inducible enzymes may be required to transport the substrate into the cell and convert it into a metabolite that the cell can utilize. One such inducible enzyme is ß-galactosidase, which catalyzes the first step in lactose utilization, namely its hydrolytic Cleavage into D-glucose and D-galactose (Sec. 15.10). Under normal conditions, if glucose is abundant in the medium, E. coli does not metabolize lactose because each cell contains only about five molecules of ß-galactosidase. However, if E. coli cells are placed in a medium with lactose as the sole energy and carbon source, within just 1–2 minutes the cells begin synthesizing ß-galactosidase in large quantities, reaching levels exceeding 1,000 molecules per cell. The induced ß-galactosidase hydrolyzes lactose into D-glucose and D-galactose, which can then be used as sources of energy and carbon. If these lactose-induced E. coli cells are subsequently transferred to a fresh medium containing glucose instead of lactose, further synthesis of ß-galactosidase ceases immediately. Thus, enzyme induction is an economical process; inducible enzymes are synthesized only when needed. A substance capable of inducing the synthesis of an enzyme or a group of enzymes is called an inducer.

If a ß-galactoside such as lactose is added to E. coli cells in the absence of glucose, they begin synthesizing large quantities not only of ß-galactosidase, but also of two other functionally related Proteins: ß-galactoside permease and protein A. Permease is a membrane protein that facilitates The transport of ß-galactosides from the external environment into the cell. The function of protein A is not entirely clear, although it may play an important role in the metabolic utilization of galactosides. When a single inducer triggers the synthesis of a group of interrelated enzymes or proteins, as in this case, the process is termed coordinated induction. Today, we know that E. coli and other Bacteria are capable of synthesizing A wide variety of interrelated enzymes or groups of enzymes in response to various specific Inducers. This capability enables bacteria to rapidly adapt to new environments and efficiently utilize a diverse range of nutrients available in their surroundings.