Protein synthesis represents the final stage in the translation of genetic information from DNA, via messenger RNA (mRNA), to protein. It can be viewed as a four-stage process, consisting of amino acid activation, translation initiation, chain elongation and termination. The events are similar in both prokaryotes, such as bacteria, and higher eukaryotic organisms, although in the latter there are more factors involved in the process.
To begin with, each of the 20 cellular amino acids are combined chemically with a transfer RNA (tRNA) molecule to create a specific aminoacyl-tRNA for each amino acid. The process is catalyzed by a group of enzymes called aminoacyl-tRNA synthetases, which are highly specific with respect to the amino acid that they activate. The initiation of translation starts with the binding of the small subunit of a ribosome, (30S in prokaryotes, 40S in eukaryotes) to the initiation codon with the nucleotide sequence AUG, on the mRNA transcript. In prokaryotes, a sequence to the left of the AUG codon is recognized. This is the Shine-Delgrano sequence and is complementary to part of the small ribosome subunit. Eukaryotic ribosomes start with the AUG nearest the 5'-end of the mRNA, and recognize it by means of a "cap" of 7-methylguanosine triphosphate. After locating the cap, the small ribosome subunit moves along the mRNA until it meets the first AUG codon, where it combines with the large ribosomal subunit.
In both prokaryotes and eukaryotes, the initiation complex is prepared for the addition of the large ribosomal subunit at the AUG site, by the release of initiation factor (IF) 3. In bacteria, the large 50S ribosomal subunit appears simply to replace IF-3, with IF-1 and IF-2. In eukaryotes, another factor eIF-5 (eukaryotic initiation factor 5), catalyses the departure of the previous initiation factors and the joining of the large 60S ribosomal subunit. In both cases the release of initiation factor 2 involves the hydrolysis of the GTP bound to it. At this stage, the first aminoacyl-tRNA, Met-tRNA, is bound to the ribosome. The ribosome can accommodate two tRNA molecules at once. One of these carries the Met-tRNA at initiation, or the peptide-tRNA complex during elongation and is thus called the P (peptide) site, while the other accepts incoming aminoacyl-tRNA and is therefore called the A (acceptor) site. What binds to the A site is usually a complex of GTP, elongation factor EF-TU and aminoacyl-tRNA. The tRNA is aligned with the next codon on the mRNA, which is to be read and the elongation factor guides it to the correct nucleotide triplet. The energy providing GTP is then hydrolysed to GDP and the complex of EF-TU:GDP leaves the ribosome. The GDP is released from the complex when the EF-TU complexes with EF-TS, which is then replaced by GTP. The recycled EF-TU: GTP is then ready to pick up another aminoacyl-tRNA for addition to the growing polypeptide chain. On the ribosome, a reaction is catalysed between the carboxyl of the P site occupant and the free amino group of the A site occupant, linking the two together and promoting the growth of the polypeptide chain. The peptidyl transferase activity which catalyses this transfer is intrinsic to the ribosome. The final step of elongation is the movement of the ribosome relative to the mRNA accompanied by the translocation of the peptidyl-tRNA from the A to the P. Elongation factor EF-G is involved in this step and a complex of EF-G and GTP binds to the ribosome, GTP being hydrolysed in the course of the reaction. The de-acylated tRNA is also released at this time.
The end of polypeptide synthesis is signalled by a termination codon contacting the A site. Three prokaryotic release factors (RF) are known: RF-1 is specific for termination codons UAA and UAG, while RF-2 is specific for UAA and UGA. RF-3 stimulates RF-1 and RF-2, but does not in itself recognize the termination codons. RF-3 also has GTPase activity and appears to accelerate the termination at the expense of GTP. Only one eukaryotic release factor is known and it has GTPase activity.
At any one time, there can be several ribosomes positioned along the mRNA and thus initiation, elongation and termination proceed simultaneously on the same length of mRNA. The three dimensional structure of the final protein begins to appear during protein synthesis before translation is completed. In many cases, after the synthesis of the amino acid chain, proteins are subjected to further reactions which convert them to their biologically active forms, e.g., by the attachment of chemical groups or by removal of certain amino acids--a processes known as post-translational modification.
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