Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Protein Biosynthesis
Translation and General Requirements for Protein Synthesis in a Cell-Free System

Translation is directly related to the mechanisms of hereditary information transfer, or Gene Expression, bridging the "four-letter language of Nucleic Acids and the twenty-letter vocabulary of Proteins." In other words, translation boils down to Protein Synthesis in Ribosomes. During this process, it is exclusively The nucleotide sequence within the mRNA that determines the Introduction/19.html">Primary Structure of a protein—that is, the strictly ordered sequence of individual amino acid residues in the synthesized protein molecule.

Let us examine the conditions necessary for carrying out protein synthesis in a Cell-free system. Three experimental approaches developed in the early 1950s played a pivotal role in MODERN CONCEPTS OF protein synthesis. First, in classical studies by P. Zamecnik and colleagues utilizing labeled Amino Acids, the site of protein synthesis was identified for the first time as the ribosome. Upon injecting rats with ¹⁵К-Amino Acids and determining the radioactivity of proteins across various subcellular Liver fractions obtained via differential centrifugation at various time intervals, it was demonstrated that the radioactive label appeared first in the microsomal fraction and only subsequently in other subcellular structures. Second, The addition of ATP to the cytosolic protein-synthesizing system triggered amino acid "activation" and its binding to a heat-stable, soluble form of RNA later designated as Transfer RNA (tRNA), leading to The formation of a complex subsequently named aminoacyl-tRNA. The Enzymes catalyzing this process are now known as Aminoacyl-tRNA synthetases. Third, The Role of adapter RNAs themselves in the translation process was elucidated.

* Experimental evidence has also been obtained for the presence of DNA in Mitochondria (accounting for about 1–2% of total cellular DNA). It is non-homologous and non-complementary to nuclear DNA. It has been established that Mitochondrial DNA encodes the synthesis of certain structural Proteins of the mitochondria themselves as well as specific mitochondrial RNAs.

Class="center">Table 14.1. COMPOSITION OF THE protein-synthesizing system in PROKARYOTES AND EUKARYOTES at various Stages of Protein Synthesis

Further research was directed toward identifying Other components of the protein-synthesizing system.

The protein-synthesizing system comprises a set of all 20 amino acids making up protein molecules; a minimum of 20 different tRNAs specific for a given enzyme and a specific amino acid; a set of at least 20 different aminoacyl-tRNA synthetase enzymes, which also exhibit dual Specificity for a particular Amino Acid and a single tRNA; ribosomes (more precisely, Polysomes consisting of 4–12 monoribosomes with attached mRNA); ATP and an ATP-generating enzyme system; GTP, which plays a specific role in the initiation and elongation stages of ribosomal protein synthesis; Mg²⁺ ions at a concentration of 0.005–0.008 M; mRNA as the primary component carrying the structural information for the protein being synthesized in the ribosome; and, finally, protein factors involved in synthesis at various levels of translation. The Main Components of the prokaryotic and eukaryotic protein-synthesizing systems across various stages of protein synthesis are summarized in Table 14.1.

Let us consider the Structure and function of the main Components of the protein-synthesizing system in greater detail.