Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Proteins. Organizational Features and Functions of Enzymes
Regulation of Enzyme Activity

The REGULATION OF ENZYMATIC Activity is a process just as vital for the successful functioning of a Cell as the Introduction/30.html">Regulation of Gene expression at the METABOLISM/31.html">Transcription level. The existence of these mechanisms allows Cells and the entire Organism to precisely coordinate numerous branching metabolic pathways, ensuring the highest and most efficient level of metabolism, as well as rapid adaptability to changing environmental conditions. At the same time, the Regulation of enzyme synthesis is a slower mechanism operating over many minutes or even hours, whereas changes in enzymatic activity occur instantaneously and take effect within minutes or seconds. The Regulation of Enzyme Activity can be described as the "fine-tuning" of cellular metabolism.

The regulation of enzymatic activity can be achieved through several pathways, among which Allosteric Regulation and covalent modification are the most common.

Allosteric regulation does not affect all Enzymes, but only those whose molecular Structure includes an allosteric center (from the Greek allos meaning "other" and stereos meaning "solid" or "space")—a site distinct from the active center, characterized by high affinity for regulatory molecules. Such enzymes are called allosteric. Their activity is regulated with the participation of low-molecular-weight substances (effectors), whose common property is The ability to interact with the allosteric center, leading to a distortion of the protein molecule's conformation. This conformational change is transmitted to the active center, thereby altering the enzyme's activity and The rate of the corresponding reaction.

Effectors can act as both inhibitors of enzyme activity and their activators. An example of the Inhibition of enzymatic activity is the reduction in The activity of anthranilate synthase—the first enzyme in the Tryptophan Biosynthesis pathway in E. coli—when there is an excess of tryptophan in The Cell. In this case, tryptophan, as the end product of the aforementioned biosynthetic pathway, acts as an inhibitor of the key enzyme, which coordinates the synthesis rate of this Amino Acid and allows the cell to conserve its resources. After all, when tryptophan is in excess, such as when it is present in the growth medium, the cell has no need to expend building blocks and energy on its synthesis, as it can utilize the exogenous amino acid. Indeed, it has been experimentally proven that during growth, Bacteria predominantly utilize Amino Acids, Purines, and Pyrimidines added to the growth medium, and that these compounds exert an inhibitory effect on their own synthesis from precursor molecules. Because in this case tryptophan is the end product of a biosynthetic pathway whose rate decreases upon inhibition of the key enzyme, this type of regulation is termed "feedback inhibition" or "retroinhibition".

The increase in allosteric enzyme activity upon binding to an effector (activator) can be illustrated by aspartate transcarbamoylase (ATCase), which catalyzes the first reaction of pyrimidine biosynthesis. This enzyme is activated by adenosine triphosphate (ATP), a purine nucleotide. It should be noted that simultaneously, ATCase is inhibited by cytidine triphosphate (CTP), one of the End products of the aforementioned biosynthetic pathway, with both the activator and the inhibitor binding to the exact same allosteric center. Thus, by regulating the activity of a single enzyme, the coordination of purine and pyrimidine nucleotide synthesis is ensured.

Mutational damage to the allosteric center can cause the enzyme to lose its ability to bind effector molecules and consequently change its activity in response to them. This phenomenon is utilized in microorganism Selection to obtain mutants with desensitized enzymes. Such microorganisms are frequently producers of BIOLOGICALLY ACTIVE SUBSTANCES, and metabolite analogs are used for their selection. For instance, 5-methyltryptophan, much like tryptophan, is capable of inhibiting anthranilate synthase activity, but it cannot replace tryptophan within a Protein Structure. Therefore, E. coli bacteria are unable to form colonies on a synthetic medium containing this substance. However, certain E. coli mutants are known to grow on media with 5-methyltryptophan. These bacteria harbor feedback-resistant (desensitized) anthranilate synthase in their cells and synthesize tryptophan in excess quantities, secreting it into the external environment.

Another widespread pathway for regulating enzyme activity is covalent modification—the attachment or removal of a small chemical group to or from the enzyme. Through such modifications, either a completely inactive form of the enzyme typically becomes active, or conversely, a fully active enzyme is inactivated. Phenomena of covalent modification include Limited proteolysis (shortening of polypeptide chains), phosphorylation–dephosphorylation, adenylylation–deadenylylation, Acetylation–deacetylation, and others. For example, mammalian Glycogen synthase, which catalyzes The conversion of glucose to glycogen, is inactivated following the covalent attachment of a phosphate group to the side chain of one of its Serine residues and is reactivated upon phosphate Cleavage. Other Examples of Covalent Modification of enzymes are described in Chapter 3.

A special case of ENZYME ACTIVITY REGULATION is represented by Protein-Protein Interactions, in which specific Proteins fulfill The Role of Enzyme Inhibitors. In such interactions, the active center of the enzyme is blocked. Protein-mediated inhibition is of particular importance for regulating the activity of proteinases involved in Post-translational protein modification. This facilitates Changes in the maturation rate of many proteins crucial to the cell, and consequently, in the intensity of the processes in which these proteins take part.