Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

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

CHAPTER 21. PROTEIN BIOSYNTHESIS IN RIBOSOMES

21.2. RIBOSOMAL PROTEIN-SYNTHESIZING SYSTEM

The modern era in understanding the cellular and MOLECULAR MECHANISMS OF Translation began with the studies of P. Zamecnik and his colleagues, who used C¹⁴-labeled Amino Acids to elucidate the Intracellular Localization of Protein Synthesis. It was demonstrated that the synthesis of radioactive (C¹⁴) Proteins in the Liver of experimental rats occurs in the microsomal fraction, specifically in Ribosomes. The Incorporation of Amino acids into polypeptide chains in vitro required the presence of messenger RNAs — mRNA — and Transfer RNAs (tRNAs) as carriers of amino acids to the sites of protein synthesis (P. Zamecnik, F. Lipmann, 1960).

Class="center">Components of the ribosomal protein-synthesizing system

The components of the protein-synthesizing system that carry out translation in prokaryotic and Eukaryotic Cells are:

ribosomes — ribonucleoprotein structures with sedimentation coefficients of 70s and 80s in PROKARYOTES AND EUKARYOTES, respectively, which interact during translation with Other components of the protein synthesis system; the three-dimensional Structure of ribosomes is shown in Fig. 21.2;

Fig. 21.2. Model of the 70s ribosome structure (four-side view).

The three-dimensional image of the small ribosomal subunit resembles a telephone handset, and the large one resembles a ladle.

mRNA, or another template polyribonucleotide that programs The sequence of amino acid incorporation into the polypeptide chain According to the information contained in genetic DNA;

α-L-amino acids in an amount corresponding to the Amino Acid Composition of fully functional proteins (typically, about 20 amino acids);

tRNA, which function as adapters in translation by interacting with mRNA codons and specific amino acids — about 20 Different types of tRNA, corresponding to the number of amino acids they accept (Fig. 21.3);

Fig. 21.3. Secondary (a) and tertiary (b) structure of Yeast Alanine tRNA (DHU - dihydrouridine; mG, ml — methylated guanosine and inosine).

Aminoacyl-tRNA synthetases (ARSases) — Enzymes that activate Amino Acids and attach amino acid residues to the 3'-ends of the acceptor stems of tRNA. ARSases are enzymes with high Specificity for both a specific Amino Acid and its corresponding tRNA;

regulatory proteins — protein factors of initiation (IF), elongation (EF), and termination, or releasing factors (RF); eukaryotic protein factors are designated as eIF, eEF, and eRf, respectively;

Coenzymes — GTP, ATP.

Eukaryotic ribosomes

In cells of nuclear organisms, ribosomes have a somewhat more complex biochemical composition and molecular Organization than in prokaryotes (Fig. 3.10).

Ribosomes are capable of reversible dissociation into two subunits:

Under in vitro conditions, ribosomal dissociation into subunits occurs when the concentration of Mg²⁺ ions decreases. In The Cell, a dynamic equilibrium exists between subunits and intact ribosomes; the latter are formed predominantly during translation. During translation, a certain number of ribosomes (from a few units to several dozen) can interact with a single mRNA molecule, forming polyribosomes or Polysomes.

In eukaryotic cells, ribosomes can function either as ribosomes bound to the membranes of The Endoplasmic reticulum ("rough endoplasmic reticulum" — RER) or as free ribosomes not bound to RER membranes. The ratio between free and membrane-bound ribosomes changes under various physiological states; under cellular pathology conditions, the number of non-membrane-bound ribosomes increases significantly (Yu.I. Gubsky, 1989).

Transfer RNAs and Amino Acid Activation

The General Structural Features of various tRNAs were examined in detail above (Chapter 3, Section 3.4). For each of the 20 a-L-amino acids, there is at least one specific type of tRNA. At the same time, different tRNA molecules share similarities in secondary and tertiary structure, which is due to the common nature of their biochemical function. An important structural feature of tRNA is the presence of a specific nucleotide triplet within the anticodon loop—the anticodon—which is complementary to the mRNA codon and ensures the pairing between tRNA and mRNA (codon-anticodon interaction) during The formation of the initiation complex. It is precisely these two biochemical properties of tRNA—The ability to interact with a specific amino acid, on the one hand, and the ability to interact with a specific mRNA codon, on the other—that form the Molecular Basis of the adaptor function of tRNA, i.e., the capacity to bridge two information flows, «nucleotide» and «amino acid», during the phenotypic expression of Genetic information.

The interaction between a tRNA and its corresponding amino acid requires mutual recognition (recognition) of these molecules, which takes place only in the presence of specialized proteins possessing specific recognition sites for both the tRNA and the α-L-amino acid. This process of recognition and subsequent joining of two Biomolecules of different classes occurs in two stages and is catalyzed by aminoacyl-tRNA synthetases.

Diagram of tRNA-amino acid interaction

1. Activation of The amino acid involving ATP to form aminoacyl adenylate:

2. Interaction of aminoacyl adenylate with the 3'-OH group of the terminal adenylyl residue on the acceptor arm of the tRNA, resulting in the formation of aminoacyl-tRNA: