Ribosomes Site and Function of Protein Synthesis

At the conclusion of this chapter, we describe the source of proteins ribosomes Ribosomes in the cell are the site and function of protein synthesis. Ribosomes (molecular weight 2 x 10 -6 x 10 ) are ribonuclear protein complexes responsible for protein synthesis in aU cells. A single cell contains 20,000 ribosomes. From studies using x-ray crystallography, we know that each ribosome consists of two major subunits, called the 30s and 50s units (s, sedimentation constant see Chapter 11). The whole ribosome is 70s, not 80s, because there are intermolecular and intramolecular force involved. The two subunits, when put together, result in either an expansion or shrinkage in size. 

The large (50s) subunit contains 23s rRNA, 5s rRNA, and over 30 proteins, and the small (30s) subunit contains 16s rRNA and about 20 proteins. The complete 70s ribosome is formed by association of the 30s and 50s subunits through a networks of bridges. The 30s subunit mediates the interaction between mRNA codons and tRNA anticodons on which the fidelity of protein translation depends. The 50s subimit includes the activity that catalyzes peptide bond formation—peptide transferase and the binding site for GTP— which are the binding protein factors that assist in the initiative elongation and termination phases of protein synthesis. 

Bridges are found not only between the two subunits but also between the subimits and tRNAs. The tRNAs function is to orient the subunits and facilitate movement. They also may function in signaling between the subunits and coordinating the multiple steps in the cycle of peptide elongation. 

FIGURE 18.14 See color insert. Protein synthesis on the prokaryotic ribosome. [Source Borman, S., C EN (ACS) 54, October 2 (2000). Original in color.] 

50s subunit above, the 30s subunit below, the E- (red), P- (orange), and A- (gold) tRNAs are visible in the interface cavity with their anticodon arms pointed downward into the 30s subunit. 

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