9.4Protein Synthesis

Fig. 9-4 Schematic diagram of protein synthesis

During protein synthesis, three bases of mRNA (a codon) continuously form pairs with three bases of aminoacyl-tRNA (an anticodon). In this reaction, amino acids are arranged through
tRNA in the order of the mRNA codes (Fig. 9-4); amino acids and tRNA are dissociated; and amino acids are linked to each other following the order of the mRNA codes. This series of reactions occurring in the ribosome is termed translation.

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Translation Initiation

Initiation of translation is a complex reaction (Fig. 9-5). First, the initiation factors (IFs) dissociate the large and small subunits of the ribosome, and mRNA and Met-tRNA (fMet-tRNA in eubacteria) bind to the small subunit. The large subunit then binds to the small subunit, thereby forming a complex consisting of a ribosome, mRNA, and Met-tRNA. This is called the initiation complex. More details about the process, in which many IFs are involved, can be found in Fig. 9-5.

Fig. 9-5 Formation of the translation initiation complex in eukaryotes

“e” in front of the initiation factor indicates eukaryote

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Peptide Chain Elongation

Fig. 9-6 Elongation reactions of peptide chains

aa: amino acid

Peptide chain elongation is also a complex reaction (Fig. 9-6). At the onset of this reaction, Met-tRNA is situated at the P site (1), and aminoacyl-tRNA, which is bound to elongation factors (EFs), comes and binds to the A site. Next, the ester bond between the first amino acid in the P site (Met) and tRNA is cleaved (2), and a peptide bond is formed between methionine and the amino acid in the A site (3). Subsequently, the empty tRNA is moved to the E site, and peptidyl tRNA*2 is moved to the P site by the activity of other EFs (4). At the same time, the ribosome moves over mRNA by three bases. When the empty tRNA is released from the E site, the process restarts (5). In this manner, elongation reactions proceed successively. As each amino acid is added in this process, two GTP molecules are hydrolyzed. Synthesis proceeds from the side of the free amino group (N terminus) to that of the carboxyl group (C terminus).

*2 A tRNA that is bound to a peptide is formed on ribosomes during protein synthesis.

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Termination of Translation

Fig. 9-7 Termination of translation reaction

aa: amino acid

The termination reaction occurs when the codon is a termination codon (Fig. 9-7). The release factor (RF) involved in the reaction enters the A site corresponding to the termination codon of mRNA. A peptidyl tRNA moves to the P site, and the link between the polypeptide chain and tRNA is hydrolyzed by the enzymatic activity of rRNA, which releases the completed protein and subsequently tRNA and mRNA, thereby terminating translation.

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Multiple ribosomes are concurrently attached to each mRNA molecule engaged in protein synthesis, with the number of attached ribosomes corresponding to mRNA length. Multiple ribosomes that are bound together via mRNA are called a polysome (or polyribosome). Cells with active protein synthesis contain a large number of polysomes. The rate at which amino acids are linked is considered to be approximately 20 amino acids per second in prokaryotes. Assuming that the average molecular weight of an amino acid is 114, it can be considered that a protein with a molecular weight of 135,000 will be synthesized in 1 min. The molecular weights of many proteins are below 50,000, therefore these proteins are synthesized within 30 s. The rate is much slower in eukaryotes, at approximately two amino acids per second.

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