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

Protein Biosynthesis
Translation and general requirements for protein synthesis in a cell-free system
Ribosomes

As is well known, Living organisms are divided into two major groups based on their cellular Structure: PROKARYOTES AND EUKARYOTES. The former lack a membrane-bound Nucleus, Cell/35.html">Mitochondria, or METABOLISM/14.html">Chloroplasts, and are represented primarily by microorganisms. In contrast, the Eukaryotic Cells of animals and plants, including Fungi, contain membrane-bound nuclei, as well as mitochondria (and in some cases chloroplasts) and other subcellular Organelles.

Both types of cells contain Ribosomes, though eukaryotic ribosomes (mol. mass 4.2 ∙ 10⁶) are significantly larger (23 nm in diameter) than prokaryotic ribosomes (mol. mass 2.5 ∙ 10⁶, 8 nm in diameter). Ribosomes are typically characterized by their sedimentation rate in a centrifugal field, which is quantified by the sedimentation coefficient s in Svedberg units S (see Chapter 1). The value of s depends not only on particle size but also on shape and density, meaning it is not strictly proportional to size. A microbial cell contains approximately 10⁴ ribosomes, whereas a Introduction/5.html">Eukaryotic Cell contains about 10⁵.

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Fig. 14.2. Components of prokaryotic and eukaryotic ribosomes (schematic diagram).

Chemically, ribosomes are Nucleoproteins composed of RNA and Proteins. Eukaryotic 80S ribosomes contain roughly equal proportions of both, whereas in prokaryotic 70S ribosomes the RNA-to-protein ratio is 65% and 35%, respectively (Fig. 14.2). Ribosomal RNA is commonly referred to as rRNA. Both 80S and 70S ribosomes consist of two subunits, which can be visualized using an Electron microscope or after treating ribosomes with solutions containing low concentrations of Mg²⁺ ions. Under these conditions, ribosomes dissociate into subunits, which can then be separated by ultracentrifugation. One subunit is twice the size of the other. For instance, the s values for the subunits of 70S ribosomes are 50S and 30S, while for 80S ribosomes they are 60S and 40S, respectively (see Fig. 14.2). It is also worth noting that in E. coli, the large and small subunits contain 34 and 21 proteins, respectively, along with two molecules of rRNA with sedimentation coefficients of 23S and 5S in the large subunit, and one molecule of rRNA (16S) in the small subunit. All ribosomal proteins have not only been isolated but also sequenced, displaying a wide molecular weight range (from 6,000 to 75,000). It is believed that all 55 bacterial ribosomal proteins participate in polypeptide synthesis as Enzymes or structural components, though—with a few exceptions—the detailed Functions of most remain unclear. The 23S and 5S RNAs contain 3,200 and 120 NUCLEOTIDES, respectively, while the 16S RNA contains 1,540 nucleotides. Eukaryotic ribosomal subunits have a more complex structure, comprising four different rRNAs and more than 70 distinct proteins distributed across both subunits. Specifically, the large subunit (60S) contains three rRNAs of varying sizes: 28S (4,700 nucleotides), 5.8S (160 nucleotides), and 5S (120 nucleotides), along with approximately 49 proteins. The small subunit (40S) contains a single 18S rRNA molecule and about 33 proteins. Furthermore, the BIOLOGICAL FUNCTIONS OF eukaryotic ribosomal components are also most likely associated with Polypeptide chain synthesis, though their specific roles are not yet fully elucidated.

Ribosomes function as a sophisticated molecular "machinery" (or "factory") for Protein Synthesis. Elucidating the subtle mechanisms of ribosomal protein synthesis requires more precise data on the Structure and function of all ribosomal components. Recently, data have emerged indicating the probable three-dimensional Spatial Structure of both intact ribosomes and their subunits. In particular, it has been shown that the shape and dimensions of the 30S and 40S ribosomal subunits are determined not by the protein molecules, but by the Tertiary Structure of their constituent 16S and 18S rRNAs. Furthermore, according to Academician A.S. Spirin, preserving the spatial morphological model of the entire 30S subunit requires only two proteins (out of 21) located in specific topographic Regions of the 16S rRNA molecule.

It is known that rRNA is generated from a common precursor of all cellular RNA types, which in turn is synthesized on a DNA template in The Nucleus (see Chapter 13). Ribosomal proteins are of cytoplasmic origin and are subsequently transported into the nucleolus, where ribosomal subunits spontaneously assemble by combining proteins with their respective rRNAs. The assembled subunits are then transported, either together or separately, through the nuclear membrane pores back into the Cytoplasm, where a group of ribosomes associates with mRNA to form Polysomes or polyribosomes, which directly participate in protein synthesis.