Synonym: Protein synthesis
German: Proteinbiosynthese, Proteinsynthese
During the first step of proteinsynthesis, the 20 different [[proteinogenic] amino acids are activated. They are all esterified with a tRNA specific to them. This takes place in the cytoplasm by aminoacyl-tRNA synthetases, a group of enzymes, which are specific only to a certain amino acid respectively. The activation energy is supplied by ATP consumption.
By binding the mRNA and the first aminoacyl-tRNA to a free 30S ribosome subunit, an initiation complex is formed, to which the 50S subunit is attached in the second step. The first aminoacyl-tRNS begins the process as an N-Acyl derivative to make sure that the synthesis of the polypeptide chain always starts at the beginning of the genetic message.
The peptide chain is elongated by the continuous attachment of amino acyl residues. These residues are translated by aminoacyl-tRNA esters, each determined by a codon in the mRNA. After each new peptide is attached, both mRNA and the peptidyl-tRNA chain are moved a little along the ribosome, in order to properly position the next codon. The necessary reactive power is supplied by GTP consumption.
In the last step of the protein synthesis, the polypeptide chain is terminated by adequate termination signals (three special stop codons) in the mRNA. Eventually, the completed chain separates from the ribosome. The release of the polypeptide-tRNA from the ribosome is initiated, when stop codon is reached, by a specific protein factor (release factor) which is attached to the ribosome and hydrolytically cleaves the ester bond between polypeptide and tRNS. Thereupon, the 70S-ribosome leaves the mRNA in free form. It can enter a new cycle if it first dissociates in its 50S and 30S subunits, for which one of the specific initiation factors is required.
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