General Microbiology - Schlegel, H. 1987

Regulation of Metabolism

In preceding chapters dedicated to microbial METABOLISM, we repeatedly addressed the REGULATION OF METABOLISM and growth by environmental factors. Pasteur's Discovery of the inhibition of Yeast Fermentation by atmospheric oxygen is a prime example of such regulation, studied in great detail. It has also long been known that certain Enzymes involved in The breakdown of a specific substrate are synthesized only in its presence. In denitrifying Bacteria, nitrate Respiration can commence only in the absence of O₂: oxygen suppresses both The formation of the nitrate-reducing enzyme System and Its function. Changes in pH in cultures of Enterobacter or Clostridium can alter the course of fermentation and influence The Nature of the products formed. In phototrophic bacteria, oxygen and light affect pigment synthesis. These and many other environment-induced changes are driven by specialized regulatory mechanisms.

The multitude of metabolic processes required for the synthesis of various substances and Cell growth demands a high degree of coordination. Each metabolic pathway comprises multiple enzymatic reactions. Metabolic processes provide energy in a biologically available form, facilitate the synthesis of simple building blocks and complex macromolecules, and ensure cell Replication. The necessity to compete with other living organisms has driven the evolution of mechanisms that, on the one hand, enable adaptation to changing environmental conditions and, on the other hand, optimally coordinate diverse metabolic processes. The targets of such optimization include enzyme Proteins along with their synthesis and functioning. The Regulation of cellular metabolism occurs at two levels: the level of enzyme synthesis and the level of activity modification.

The first type of regulation is characteristic of many metabolic pathways. As a rule, the synthesis of multiple enzymes belonging to the same pathway is regulated simultaneously. The purpose of this regulation is to ensure an appropriate ratio between The rate of Synthesis of specific enzymes and the rate of synthesis of total cellular protein. This rate is determined by the frequency of structural Gene Transcription.

Many enzymes are produced continuously regardless of environmental conditions; such enzymes are termed constitutive (correspondingly, one also speaks of constitutive genes and constitutive enzyme synthesis).

The formation of catabolic enzymes is regulated via induction. From the standpoint of cellular metabolic economy, it is advantageous for enzymes involved in utilizing a substrate and incorporating its breakdown products into Intermediary Metabolism to be synthesized only when that substrate is present in the nutrient medium. All other catabolic enzymes that The Cell is capable of synthesizing should not be produced unless they are needed.

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The formation of anabolic enzymes is regulated via repression. From the perspective of this same metabolic economy, it is advantageous for enzymes of a specific biosynthetic pathway not to be synthesized if the end product is already present in the medium. Therefore, in the presence or accumulation of such an end product, the rate of synthesis of all enzymes specific to that biosynthetic pathway decreases.

Enzymes required for the synthesis of essential Cytology/cytology/6.html">Cellular Structural Components are usually produced continuously, but their formation is suppressed (repressed) if the end product is present in excess. In such cases, one speaks of end-product repression.

Regulation at the level of enzyme activity is typically characteristic only of Key Enzymes in cellular metabolism. The catalytic activity of enzymes participating in a given biosynthetic pathway can undergo changes; it can increase (under the action of a positive effector) or decrease (under the action of a negative effector). In end-product inhibition (feedback inhibition), the end product inhibits The activity of the first enzyme involved in that reaction chain.

Both types of regulation—Induction and Repression on the one hand, and modification of enzyme activity on the other—lead to an almost identical outcome: they affect the throughput capacity of a given metabolic pathway. Induction and repression act slowly and can be regarded as mechanisms of coarse regulation. Changes in the activity of a key enzyme manifest instantaneously, representing fine regulation.