Biochemistry and Molecular Biology - Belyasova N.A. 2002

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

The REGULATION OF ENZYMATIC Activity is a process just as vital to the successful functioning of a Cell as the Introduction/30.html">Regulation of Gene expression at the transcriptional level. These mechanisms allow Cells and the entire Organism to precisely coordinate numerous branching metabolic reactions, 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 acting over the course of 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 accomplished through several pathways, among which Allosteric Regulation and covalent modification are the most common.

Not all Enzymes are subject to allosteric regulation; rather, it occurs only in those whose molecular Structure includes an allosteric site (from the Greek allos meaning "other" and stereos meaning "solid" or "space")—a region distinct from the Active Site that is characterized by a high affinity for regulatory molecules. Such enzymes are called allosteric enzymes. Their activity is regulated through the participation of low-molecular-weight substances (effectors), the common property of which is The ability to interact with the allosteric site, leading to a distortion of the protein molecule's conformation. This distortion is transmitted to the active site, resulting in changes to the enzyme's activity and The rate of the corresponding reaction.

Effectors can act as both inhibitors of enzyme activity and 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 within 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 rate of synthesis 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; instead, 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 with an effector (activator) can be illustrated by aspartate transcarbamylase (ATCase), which catalyzes the first reaction in pyrimidine biosynthesis. This enzyme is activated by adenosine triphosphate (ATP), a purine nucleotide. It should be noted that ATCase is simultaneously 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 same allosteric site. Thus, by regulating the activity of a single enzyme, the coordinated Synthesis of purine and pyrimidine NUCLEOTIDES is ensured.

Mutational damage to the allosteric site can cause an enzyme to lose its ability to bind effector molecules and consequently alter its activity in response. This phenomenon is utilized in microorganism Selection to obtain mutants with desensitized enzymes. Such microorganisms are often producers of BIOLOGICALLY ACTIVE SUBSTANCES, and metabolite analogs are used for their selection. For instance, 5-methyltryptophan is capable of inhibiting the activity of anthranilate synthase just like tryptophan, 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, mutant strains of E. coli that can grow on media with 5-methyltryptophan are known. These bacteria harbor anthranilate synthase that is insensitive to feedback inhibition (desensitized) and synthesize tryptophan in excess quantities, secreting it into the external medium.

Another widespread mechanism for regulating enzyme activity is covalent modification, which involves the attachment or removal of a small chemical group from the enzyme. Through such modifications, a completely inactive form of an enzyme typically becomes active, or conversely, a fully active enzyme is inactivated. Phenomena classified under covalent modification include Limited proteolysis (shortening of polypeptide chains), phosphorylation-dephosphorylation, adenylylation-deadeenylylation, 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 a specific Serine residue and is reactivated upon the removal of the phosphate. 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 act as Enzyme Inhibitors. These interactions block the Active Site of the enzyme. Protein-mediated inhibition is of particular importance in 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 participate.