Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Amino Acids and Nucleotides
The biosynthesis of amino acids is also regulated by changes in enzyme concentration

Another mechanism regulating Amino acid Biosynthesis relies on the controlled adjustment of the concentration of Enzymes involved in the pathway. If Cells are well supplied with a particular Amino Acid and have no further need for it, they contain very little of the enzymes required for its synthesis. Conversely, if the concentration of this amino acid drops and becomes insufficient, the cells begin to produce more of these enzymes. This type of regulation is achieved through shifts in The activity of the genes encoding the corresponding enzymes. Whenever the end product of a biosynthetic pathway is present in adequate concentration, the genes encoding the enzymes of that pathway are inactivated, or repressed. When the concentration of the pathway product falls, these genes are derepressed, and the enzymes are synthesized in larger quantities. We will later examine how enzyme synthesis is regulated through genetic repression mechanisms (Chapter 29).

Fig. 22-10 illustrates three Isoenzymes (designated as A, B, and C) that lack allosteric modulators. The activity of these isoenzymes is regulated by altering their rate of synthesis within The Cell; they are referred to as repressible enzymes. The synthesis of isoenzymes A and B is repressed in E. coli when sufficient Methionine is present. Similarly, the synthesis of isoenzyme C is repressed if isoleucine is available in adequate amounts in the medium. The mechanism regulating amino acid biosynthesis via repression and dereplication (Chapter 29) typically responds more slowly than Allosteric Regulation.

Thus, we see that a complex network of diverse regulatory mechanisms operates in The biosynthesis of this group of related Amino Acids. Rapidly growing bacterial cells coordinate the synthesis of all amino acids with exquisite efficiency, rivaling a computer in this regard.