Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter IV. MOLECULAR MECHANISMS OF HEREDITY AND REALIZATION OF GENETIC INFORMATION

CHAPTER 21. PROTEIN BIOSYNTHESIS IN RIBOSOMES

21.4. REGULATION OF TRANSLATION. ANTIBIOTICS AS TRANSLATION INHIBITORS

Representing The final stage of the multi-step Gene Expression process, ribosomal Polypeptide chain synthesis is controlled by cellular regulatory systems and influenced by various physiologically active compounds.

Class="center">MOLECULAR MECHANISMS OF translational control

The mechanism controlling Translation in Eukaryotic Cells involves the Covalent Modification of the eukaryotic Translation initiation factor 2 (eIF-2), which exists in dephosphorylated (active) and phosphorylated (inactive) forms.

Through reversible phosphorylation and dephosphorylation of the initiation factor eIF-2 mediated by a cAMP-dependent regulatory cascade, reticulocyte Ribosomes control the Synthesis of the Hemoglobin protein moiety, globin, in response to an adequate supply of the prosthetic group, heme.

The regulation proceeds According to the following scheme (Fig. 21.6):

Fig. 21.6. cAMP-dependent cascade system regulating globin translation (a — active, n/a — inactive forms of respective Proteins).

(1) globin synthesis initiation involves the initiation factor eIF-2, which can be present in dephosphorylated (active) and phosphorylated (inactive) forms;

(2) phosphorylation of the α-subunit of the globin synthesis initiation factor eIF-2 by a specific eIF-2 protein kinase inactivates the factor, thereby blocking translation initiation;

(3) The activity of the eIF-2 kinase that phosphorylates eIF-2 is controlled (also via phosphorylation-dephosphorylation) by another protein kinase, which is cAMP-dependent;

(4) in turn, the catalytic activity of the cAMP-dependent protein kinase is negatively controlled by heme, which acts as an inhibitor of this kinase, blocking the release of its catalytic subunit.

This mechanism ensures the mutual coordination of heme and globin levels: under high heme concentrations, the protein kinase phosphorylating eIF-2 is inhibited, leading to the accumulation of the dephosphorylated (active) molecular form of the initiation factor and, consequently, stimulation of globin synthesis. Conversely, when intracellular heme reserves are depleted, the ribosomal globin-synthesizing system enters an inactive state, halting hemoglobin production.

Effects of physiologically active compounds on translation

Ribosomal Translation Processes, which constitute the final stage of multi-step genetic expression, serve as targets for numerous physiologically active compounds, including drugs and toxins.

1. In clinical practice, as well as in experimental biology and medicine, Antibiotics that inhibit METABOLISM/35.html">Protein Biosynthesis in prokaryotic and eukaryotic organisms at various stages of translation have found widespread application:

(1) initiation inhibitors: streptomycin, aurintricarboxylic acid;

(2) elongation inhibitors: amicetin, chloramphenicol, erythromycin, cycloheximide, puromycin, Tetracyclines;

(3) termination inhibitors: anisomycin, chloramphenicol, erythromycin, lincomycin, streptomycin.

The effects of some common antibiotics on translation are summarized in Table 21.2:

Antibiotics that inhibit translation processes in prokaryotes are used as antibacterial drugs in the Treatment of infectious diseases and other conditions caused by microbial factors.

Antibiotics that inhibit translation in eukaryotic cells of higher organisms, particularly mammals, are used as antitumor agents. By inhibiting protein biosynthesis in malignant tumor cells, these antibiotics induce tumor regression.

2. Protective effects of interferons—proteins synthesized in the Human and Animal body (in lymphoid and other Tissues) with The properties of antiviral antibiotics and natural antitumor factors—are mediated by influencing the translation initiation process in eukaryotic cells.

The antiviral mechanism of interferons is mediated through the phosphorylation of cellular initiation factors eIF-2, which, as noted above, can exist in dephosphorylated (active) and phosphorylated (inactive) forms.

Table 21.2. Properties of Antibiotics as Translation Inhibitors

The synthesis of interferons is stimulated by Viruses invading the cells of The Human Body. In turn, interferons induce the synthesis of protein Kinases that phosphorylate translation initiation factors eIF-2, which, according to the scheme discussed above, suppresses The biosynthesis of both viral proteins and all Proteins of the host Cell infected by the virus. As a result, this triggers the death of the host cell, thereby preventing the Replication of virion particles.

In addition to their antiviral activity, certain interferons inhibit the proliferation of malignant tumor cells, making these compounds promising as antitumor agents.

3. A potent translation inhibitor in eukaryotes is diphtheria toxin, produced by Corynebacterium diphtheriae cells, which causes a dangerous disease in children and adults that resulted in high mortality rates prior to the Introduction of specific immunization.

The toxin is a protein with a Molecular Weight of 61 kD that, upon entering the human body, is proteolytically cleaved into two polypeptide chains: an active A-fragment (mw 21 kD) and an inactive B-fragment (mw 40 kD).

The Molecular Mechanism of the toxic action of diphtheria toxin relies on the catalytic activity of the A-fragment, which is an ADP-ribosyltransferase enzyme that transfers ADP-ribose from an NAD molecule to the elongation factor eEF-2:

The ADP-ribose residue attaches to the diphthamide amino acid residue in the eEF-2 elongation factor molecule, causing the latter to lose its ability to carry out the translocation of the peptide residue from the A-site to the P-site of the ribosome (see section 21.3). Thus, The process of Translation elongation in The Cell is blocked, leading to a general cessation of Protein Synthesis.

The A-fragment of diphtheria toxin is a biological toxin with an extremely high lethal effect: a single molecule of this peptide is sufficient to kill the target cell it has entered.