Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Ribosomes are molecular machinery designed for the synthesis of polypeptide chains

Each E. coli Cell contains over 15,000 Ribosomes, which account for nearly a quarter of The Cell's dry weight. Prokaryotic ribosomes consist of approximately 65% rRNA and about 35% protein. The ribosomal particle has a Molecular Weight of ~2.8∙10⁶ daltons, a diameter of ~18 nm, and a sedimentation coefficient of 70S.

Prokaryotic ribosomes are composed of two subunits of unequal size (Fig. 29-11)—a large subunit with a sedimentation coefficient of 50S and a small subunit with a sedimentation coefficient of 30S. The weight of the large subunit is 1.8∙10⁶ daltons, and that of the small subunit is 1.0∙10⁶ daltons. The 50S subunit contains one molecule of 23S rRNA (~3200 NUCLEOTIDES), one molecule of 5S rRNA (~120 nucleotides), and 34 Proteins. The 30S subunit contains one molecule of 16S rRNA (1600 nucleotides) and 21 proteins. The subunit proteins are designated by numbers: from L1 to L34 in the large (50S) subunit (where L stands for Large) and from S1 to S21 in the small (30S) subunit (where S stands for Small). All E. coli ribosomal proteins have been isolated, and many of them have been sequenced; they differ noticeably from one another. Their molecular weights range from 6,000 to 75,000.

The nucleotide sequences of single-stranded E. coli rRNAs have also been determined. Each of the three rRNAs possesses a specific three-dimensional Structure dictated by the pattern of intramolecular base pairing. Fig. 29-12 illustrates the proposed conformation of 5S rRNA corresponding to the maximum number of paired bases. rRNAs apparently serve as scaffolds upon which the polypeptide components are assembled in a strictly defined order. If the 21 Polypeptides and 16S rRNA of the 30S subunit are isolated in pure form and then mixed in the appropriate sequence at the correct Temperature, the macromolecules spontaneously reconstitute into 30S subunits that are identical in structure and activity to native ones. Similarly, the 50S subunit can be spontaneously reconstructed from its 34 polypeptides and its 5S and 23S rRNAs, provided that the 30S subunit is also present in the mixture. Presumably, each of the 55 prokaryotic ribosomal proteins plays a specific role in polypeptide synthesis, functioning either as an enzyme or as an "assistant" in the overall process. However, these specific Functions have currently been established for only a few ribosomal proteins.

Although ribosomes are usually depicted as symmetrical structures—with the 30S subunit resting like a cap on the nearly spherical 50S subunit (as shown in Fig. 29-11)—these subunits are actually arranged asymmetrically and have quite irregular shapes. Fig. 29-13 shows the three-dimensional STRUCTURE OF THE E. coli 30S and 50S ribosomal subunits, constructed from X-ray crystallography and Electron Microscopy data. The two intricately shaped subunits fit together, leaving a cleft between them. The mRNA molecule passes through this cleft, along which the ribosome moves during Translation. The newly synthesized polypeptide chain emerges from this cleft.

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Fig. 29-11. Composition of prokaryotic (A) and eukaryotic (B) cytoplasmic ribosomes. Ribosomes from eukaryotic Mitochondria and METABOLISM/14.html">Chloroplasts resemble those of Prokaryotic Cells.

Fig. 29-12 Schematic representation of one of the possible Secondary structure models for prokaryotic 5S rRNA, corresponding to the maximum number of intramolecular Base Pairs (indicated by red dashes).