Let us know what is Protein Synthesis Process. In simple words, protein synthesis means the production of protein molecules either from scratch or by breaking down or converting other biomolecules.
Protein synthesis involves several small steps—synthesis of amino acids, transcription of mRNA, translation of mRNA into protein, and post-transcriptional processing of that protein. Proteins are nothing but long chains of amino acids linked together in an orderly manner.
The process of making new proteins is known as protein synthesis. In biological systems, this occurs within the cell. In prokaryotes it occurs in their cytoplasm. It begins in the nucleus with the production of a transcript (mRNA) of the coding sequence of DNA in eukaryotes. This mRNA transcript then leaves the cell nucleus. It makes its way to ribosomes attached to Golgi bodies in the cytoplasm, where it is translated into a protein molecule with a codon – specific amino acid sequence.
Stages of Protein Synthesis Process:
There are 5 major steps in the protein synthesis process:
- activation of amino acids
- Transfer of amino acids in tRNA
- beginning of the polypeptide chain
- chain termination and
- translocation of protein molecule
This is the basic rule of thumb for prokaryotes, whereas eukaryotes have some additional steps due to their cell complexity. After that we will discuss the above mentioned steps in detail.
Activation of Amino Acids:
The reaction occurs when amino acids come to interact with ATP molecules catalyzed by aminoacyl RNA synthetase. The aminoacyl-AMP-enzyme complex is produced as a result of the reaction between amino acids (AA) and adenosine triphosphate (ATP), which is mediated by the above enzymes. The premises are as follows:
AA + ATP Enzyme -AA – AMP – Enzyme Complex + PP
It is worth noting that different aminoacyl RNA synthetases are required for different amino acids.
Transfer of Amino Acids to tRNA:
The generated AA-AMP-enzyme complex reacts with a particular tRNA. As a result, amino acids are carried over to the tRNA. As a result, enzymes, as well as AMPs, are released.
So the complex becomes:
AA – AMP – Enzyme Complex + tRNA – AA – tRNA + AMP Enzyme
Beginning of the Polypeptide Chain:
The ribosome accepts the charged tRNA. In all organisms, protein synthesis occurs in ribosomes that are normally associated with Golgi bodies in the cytoplasm. The SOS subunit of the 70S type interacts with the ribosome mRNA. Ribosomes are small complex molecules, which are responsible for protein synthesis and are made up of 2 components- rRNA (ribosomal RNA) and protein ribosomes also catalyze the formation of peptide bonds (enzyme-ribozyme-in bacteria). Ribosomes are classified into two types: large and small.
Scientists represent each amino acid by three nucleic acid sequences known as codons. Based on the arrangement of the nitrogenous bases, this information is expressed in mRNA. is present in . The amino acid methionine is transcribed by the codon AUG as an initiation codon, but rarely by GUG (for valine), which is always responsible for starting the polypeptide chain in prokaryotes. In prokaryotes, the formation of the starting amino acid methionine is essential.
There are two binding sites for amino-acyl-tRNA in the ribosome.
- A site or amino-acyl (acceptor site).
- The peptidyl site, often referred to as the “P” site, is a type of peptide (donor site). Each site is composed of different parts of the SOS and 30S subunits. Only the P site can bind to the initial formyl methionine tRNA (aa,fMet tRNA).
In the first step, the amino-acyl-tRNA complex is attached to an elongation factor called the “two complex”. This complex contains one molecule of bound GTP. The amino-acyl-tRNA-Tu-GTP complex is then attached to the 70S initiation complex. When the GTP molecule hydrolyzes, the TU-GDP complex is released from the 70S ribosome. The new aminoacyl tRNA now arrives and attaches itself to the aminoacyl or A site of the ribosome.
The tRNAs at the A site and P site of the ribosome are linked using peptide bonds. We consider this to be the beginning of the second phase of elongation. In the next step, the formyl methionine acyl group that was initially created is transferred from the tRNA that was attached to the amino group of the new amino acid that arrived at the A site. Peptide synthesis is catalyzed by the ribosomal enzyme peptidyl transferase found in the 50S subunit. A dipeptidyl-tRNA molecule is generated at the A site, while an empty tRNA remains attached to the P site of the mRNA.
The ribosome travels along the mRNA towards its 3′ terminal codon in the third stage of elongation (i.e., from the first to the second codon and then from the second to the third on the mRNA). Because the dipeptidyl tRNA is still attached to the second codon, ribosome movement causes the dipeptidyl tRNA to move from the A-site to the P-site. , The empty tRNA is released as a result of this translocation.
The third mRNA codon is now at the A-site, while the second codon is at the P-site. The translocation step is the movement of the ribosome along the mRNA. The elongation factor G is necessary for this step (also known as translocase). In addition, another molecule of GTP is hydrolyzed at the same time. The transfer requires energy, which is provided by the hydrolysis of GTP.
Termination also requires the actions of three termination or releasing factors known as R1, R, and S. The transfer requires energy, which is provided by the hydrolysis of GTP. This recurrent process for chain elongation results in the lengthening of the polypeptide chain. Since the ribosome carries each codon towards the 3′ end of the mRNA, it is at this time that the polypeptide chain with the last amino acid to bind to it comes.
Chain End:
One of the 3 terminal codons of mRNA marks the end of the polypeptide. UAG (amber), UAA (ocher) and UGA (opal) are called stop codons. They can also be considered stop signals.
The terminal codon is immediately followed by the last and last amino acid codon. The polypeptide chain, tRNA, and mRNA are then released. The ribosome subunits separate. Termination also requires the actions of three termination or releasing factors known as R1, R, and S.
Transfer of Protein Molecule:
Two types of polyribosome sheaves have been discovered to be involved in this process:
- free polyribosomes
- Membrane-bound polyribosomes.
Upon completion of protein synthesis in the free ribosome, the prepared ribosome releases the protein into the cytoplasm. Special processes are used to transport some of these specialized proteins to the mitochondria and nucleus.
On the other hand, in membrane-bound polyribosomes, a polypeptide chain that develops on mRNA is introduced into the lumen of the ER membrane. Some proteins also form parts of the membrane structure.
Nevertheless, only a few proteins are released into the lumen and integrate into Golgi body vesicles. They can also transform proteins through glycosylation, which is the addition of sugar residues. As a result, the vesicles form a bond with the plasma
Membrane and proteins are released sooner or later.
Protein Synthesis Process in Prokaryotes:
Prokaryotes are simple organisms and only these 5 steps are involved in protein synthesis.
- activation of amino acids
- Transfer of amino acids in tRNA
- beginning of the polypeptide chain
- chain termination and
- translocation of protein molecule
Prokaryotes have a single DNA molecule that is used for protein synthesis by transcription and translation. The DNA molecule is so simple that its protein synthesis process does not even require post-transcriptional and post-translational modifications.
Process of Protein Synthesis in Eukaryotes:
Protein synthesis in eukaryotes is a bit more complex due to the presence of introns in the RNA that is transcribed. Therefore only introns must be removed by the transcriptional process of splicing to ligature exons and produce mRNA that can be translated into proteins.
Eukaryotic hnRNA has a total of 4 steps in post-transcriptional processing:
- Introns are removed from mRNA by splicing . Introns are regions of the genome that do not code for proteins. The remainder of the mRNA is composed entirely of protein-coding parts called exons. In the diagram, ribonucleoproteins are small nucleoproteins that contain RNA and are essential for the splicing process.
- Some nucleotides in mRNA are changed during editing. Because of the editing, a human protein called apoB, which aids in the transport of lipids in the blood, has two different versions. Because editing inserts an earlier stop signal into the mRNA, one variant is shorter than the other.
- The ” head of the mRNA” is given a methylation cap via a 5′-capping. This cap helps the ribosome to recognize where to bind the mRNA and prevents it from being broken down.
- Polyadenylation gives the mRNA a “tail”. A chain of As forms the tail (adenine base). This means that the mRNA has now lost its functional requirement and is no longer of any use and can be discarded. It also aids in the export of mRNA from the nucleus and protects the mRNA from enzymes that can degrade it.
Protein Synthesis Example:
One type of protein synthesis that occurs in neurons is called renewed protein synthesis.
In neurons, “de novo protein synthesis” refers to protein synthesis that occurs outside the boundary of the soma or cell body. Both compartments of neurons i.e. the dendritic compartment (long cavity) and the axonal compartment (star or spider-shaped cavity) can produce such “extrasomal” proteins.
This means that the protein is synthesized without any prior knowledge of its codon structure, so the proteins produced are also an enigma regarding the origin and their function for scientists.
