When a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the “chain initiating” codon. Then, as shown in Figure 1, while the mRNA travels through the ribosome, a protein molecule is formed—a process called translation. Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is “read” as it passes through the playback head of a tape recorder. Then, when a “stop” (or “chain terminating”) codon slips past the ribosome, the end of a protein molecule is signaled and the protein molecule is freed into the cytoplasm.
Fig1. A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein.
Polyribosomes. A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in Figures 1 and 2. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called polyribosomes.
Fig2. The physical structure of the ribosomes, as well as their functional relation to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules.
It is especially important to note that an mRNA can cause the formation of a protein molecule in any ribosome; that is, there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place.
Many Ribosomes Attach to the Endoplasmic Reticulum. In Chapter 2, it was noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the reticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum.
Figure 2 shows the functional relation of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum mem brane into the endoplasmic matrix.
It should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell.
Chemical Steps in Protein Synthesis. Some of the chemical events that occur in the synthesis of a protein molecule are shown in Figure 3. This figure shows representative reactions for three separate amino acids: AA1, AA2, and AA20. The stages of the reactions are as follows:
1. Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.
2. The activated amino acid, having an excess of energy, then combines with its specific tRNA to form an amino acid–tRNA complex and, at the same time, releases the adenosine monophosphate.
3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in appropriate sequence to form a protein molecule.
Fig3. Chemical events in the formation of a protein molecule. AMP, adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA.
Then, under the influence of the enzyme peptidyl transferase (one of the proteins in the ribosome), peptide bonds are formed between the successive amino acids, thus adding progressively to the protein chain. These chemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell.
Peptide Linkage. The successive amino acids in the protein chain combine with one another according to the typical reaction:
In this chemical reaction, a hydroxyl radical (OH−) is removed from the COOH portion of the first amino acid and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called peptide linkage. As each additional amino acid is added, an additional peptide linkage is formed.
