GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

20. METABOLIC REGULATION

20.1. REGULATION OF ENZYME SYNTHESIS

20.1.1. Induction of Enzyme Synthesis

Many Enzymes are synthesized by The Cell regardless of environmental conditions. Such enzymes are referred to as constitutive (hence the terms constitutive genes and constitutive enzyme synthesis). However, A number of enzymes are inducible—they are produced by the cell only in the presence of a specific substrate. The synthesis of most enzymes involved in substrate Catabolism requires induction. Thus, The formation of catabolic enzymes is regulated via induction.

Enzyme synthesis can be induced by a substrate or an intermediate product of a reaction.

Substrate induction. A classic example of such induction is the induction of β-galactosidase, an enzyme required for the utilization of lactose by E. coli Cells. This enzyme splits lactose into glucose and galactose. Cells growing on glucose contain only trace amounts of this enzyme. When grown on lactose (the substrate), The activity of β-galactosidase increases a thousandfold.

If The breakdown of a substrate yields a series of intermediates (A, B, C, D) involving enzymes a, b, c, d (along with enzyme в), substrate induction can be either coordinated (where the substrate induces the simultaneous formation of all enzymes from a to d) or sequential (where enzyme synthesis occurs step by step: first в, then a, and so on) (Fig. 20.1).

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Fig. 20.1. Coordinated and sequential induction of enzyme synthesis by a substrate (schematic representation)

Coordinated enzyme synthesis provides the cell with the advantage of being able to rapidly respond to the appearance of a substrate. In sequential induction, The rate of substrate conversion and, consequently, Cell Growth Rate increase slowly, because the concentration of the first reaction product must reach a certain threshold level before it can stimulate the Synthesis of the second enzyme.

Induction by reaction products. In this type of induction, the synthesis of catabolic enzymes is induced by the product of the first or a subsequent reaction in a given catabolic pathway. This occurs, for example, during the breakdown of Tryptophan (Fig. 20.2). Tryptophan catabolism proceeds via formylkynurenine, kynurenine, and anthranilate to pyrocatechol. Kynurenine serves as the inducer for the corresponding group of enzymes.

Fig. 20.2. Induction of enzyme synthesis by reaction products (schematic representation). The intermediate product kynurenine acts as the inducer for the group of enzymes involved in tryptophan degradation