Protein biosynthesis occurs in all organs, tissues and cells. The largest amount of protein is synthesized in the liver. Ribosomes carry out protein biosynthesis. The chemical nature of ribosomes is nucleoproteins consisting of RNA (50-65%) and proteins (35-50%). Ribonucleic acid are the constituent parts of the granular endoplasmic reticulum, where biosynthesis and movement of the synthesized protein molecules occur.
Ribosomes in the cell are in the form of clusters from 3 to 100 units - polysomes (polyribosomes). Ribosomes are usually interconnected by a kind of filament visible under an electron microscope - i-RNA.
Each ribosome is capable of independently synthesizing one polypeptide chain, a group of several such chains and protein molecules.
Stages of protein biosynthesis
Amino Acid Activation. Amino acids enter the hyaloplasm from the intercellular fluid as a result of diffusion, osmosis, or active transfer. Each type of amino and imino acid interacts with an individual enzyme - aminoacyl synthetase. The reaction is activated by cations of magnesium, manganese, cobalt. An activated amino acid arises.
Protein biosynthesis (second stage) - the interaction and connection of the activated amino acid with t-RNA. Activated amino acids (aminoacyladenylate) are transferred via enzymes to cytoplasmic t-RNA. The process is catalyzed by aminoacyl RNA synthetases. The remainder of the amino acid is connected by a carboxyl group to the hydroxyl second atom of the Carbon ribose atom of the t-RNA nucleotide.
Protein biosynthesis (third stage) - transportation of the complex of activated amino acids with t-RNA to the ribosomes of the cell. The amino acid is bound to t-RNA and is transferred from hyaloplasm to the ribosome. The process is catalyzed by specific enzymes, of which there are at least 20 in the body. Some amino acids are transported by several t-RNAs (for example, valine and leucine, by three t-RNAs). The energy of GTF and ATP is used in this process. The fourth stage of biosynthesis is characterized by the binding of aminoacyl-t-RNA to the complex of i-RNA - ribosome. Aminoacyl-t-RNA, approaching the ribosome, interacts with i-RNA. Each t-RNA has a site consisting of three nucleotides - anticodon. In i-RNA, a region with three nucleotides corresponds to it - a codon. Each codon corresponds to an anticodon t-RNA and one amino acid. During biosynthesis, amino acids are attached to the ribosomes in the form of aminoacyl-tRNAs, which are subsequently formed in the polypeptide chain in the order determined by the placement of codons in i-RNA.
The next stage of protein biosynthesis is the initiation of the polypeptide chain. After two neighboring aminoacyl-t-RNAs with their anticodons have joined the codons of i-RNA, conditions are created for the synthesis of the polypeptide chain. A peptide bond is formed. These processes are catalyzed by peptide synthetases, activated by Mg cations and protein-triggering factors F1, F2, F3. The source of chemical energy is guanosine triphosphate.
Termination of the polypeptide chain. The ribosome, on the surface of which the polypeptide chain was synthesized, reaches the end of the i-RNA chain, then “jumps” out of it. A new ribosome joins the opposite end of i-RNA in its place, which synthesizes another polypeptide molecule. The polypeptide chain is disconnected from the ribosome and secreted into the hyaloplasm. This reaction is carried out using a specific release factor (factor R), which is connected to the ribosome and facilitates the hydrolysis of the ether linkage between the polypeptide and t-RNA.
In the hyaloplasm, simple and complex proteins are formed from polypeptide chains . Secondary, tertiary and, in many cases, quaternary structures of the protein molecule are formed. Thus, protein biosynthesis in the cell occurs.