GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

20. METABOLIC REGULATION

20.1. REGULATION OF ENZYME SYNTHESIS

20.1.2. Repression of enzyme synthesis

There are two types of enzyme synthesis repression: 1) end-product repression; 2) catabolite repression.

End-product repression. This type of repression regulates the synthesis of anabolic Enzymes (i.e., enzymes involved in constructive METABOLISM—biosynthetic processes). From an economic standpoint, it is advantageous for The Cell not to synthesize enzymes of a particular biosynthetic pathway if the end product is already present in the environment. Therefore, in the presence or accumulation of such an end product, the synthesis rate of all enzymes specific to that biosynthetic pathway decreases (Fig. 20.3).

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Fig. 20.3. End-product repression of enzyme synthesis (schematic diagram)

In branched biosynthetic pathways, regulation is more complex. Examples include the systems for Amino acid synthesis (the "aromatic Amino acid family", the "aspartic acid family") (see Fig. 13.2). Enzymes located prior to the branch point are subject to repression by all end products acting simultaneously (multivalent repression). The synthesis of these enzymes is inhibited only when all end products are present in the medium; if they are added individually, no effect is observed.

Catabolite repression. While end-product repression acts on biosynthetic pathways, catabolite repression regulates catabolic reactions. When a growth medium contains two different substrates, Bacteria generally prefer the substrate that provides faster growth. This preferred substrate represses the synthesis of enzymes required for the utilization of the second substrate. Catabolite repression underlies The phenomenon of diauxie (the sequential utilization of multiple substrates) (see Fig. 6.8). Diauxie was discovered in 1942 by the French scientist J. Monod. He established that a Bacillus subtilis culture growing on a medium containing both glucose and arabinose initially utilizes only glucose; then, once the glucose is depleted, arabinose assimilation begins. This phenomenon was termed "catabolite repression" by B. Magasanik.