Main components of the protein-synthesizing system
The key difference between translation and other template-based biosyntheses is that there is no complementary match between the nucleotide template and the synthesized polypeptide. To translate the four-letter nucleotide "language" into the twenty-letter amino acid language, a biological code and a complex apparatus are required. Its components include:
- mRNA — the informational template containing a sequence of codons.
- Amino acids — substrates for protein synthesis.
- tRNA — adapters featuring an acceptor (-CCA) end for binding an amino acid and an anticodon for recognizing mRNA.
- Aminoacyl-tRNA synthetases — specific enzymes that catalyze the attachment of amino acids to tRNA.
- Ribosomes — ribonucleoprotein structures where the physical assembly of the chain takes place.
- Protein factors — extraribosomal initiation (IF), elongation (EF), and termination (RF) proteins.
- Energy and cofactors — ATP and GTP molecules, as well as magnesium ions necessary for stabilizing the ribosome structure.
Amino acid activation
Before an amino acid enters the ribosome, it must be activated. This process is carried out by aminoacyl-tRNA synthetases (there are 20 types of such enzymes in the cell — one for each proteinogenic amino acid).
Interestingly, the enzyme exhibits absolute specificity for the amino acid, but only relative specificity for the tRNA. Due to the degeneracy of the genetic code, a single amino acid can be carried by several different isoacceptor tRNAs (they differ in anticodon structure but bind the same amino acid).
During the reaction, the enzyme attaches the alpha-carboxyl group of the amino acid to the 3'-OH end of the tRNA. The process proceeds with the consumption of ATP energy.
Example of aspartate activation: Asp + tRNA + ATP → Asp-tRNA + AMP + pyrophosphate.
The specificity of the resulting molecule is typically denoted by a superscript, for example: Met-tRNA^Met.
Translation initiation
Synthesis begins with the assembly of an active ribosome. More than 10 initiation factors (IF) participate in this process. Initiation steps:
- A preinitiation complex is formed consisting of the small ribosomal subunit (40S), initiator Met-tRNA^Met, IF factors, and GTP energy.
- The complex attaches to the 5'-end ("cap") of the mRNA and begins scanning — moving along the chain until it encounters the start codon AUG.
- The anticodon of Met-tRNA^Met complementary recognizes the start codon.
- The large subunit (60S) joins, GTP is hydrolyzed, and the IF protein factors are released.
The result of initiation is an assembled 80S ribosome with two formed active sites. The P-site (peptidyl site) is occupied by the initiator tRNA, while the A-site (aminoacyl site) is completely vacant and ready to accept the next molecule.
Elongation (chain extension)
The elongation cycle repeats multiple times and consists of three consecutive stages:
- Binding of aa-tRNA. A new transfer RNA enters the vacant A-site. A mandatory condition is that its anticodon must be complementary to the mRNA codon. The process proceeds with the participation of elongation factor EF1 and GTP energy.
- Peptide bond formation. Methionine (or the already synthesized peptide) is transferred from the P-site to the amino group of the new amino acid in the A-site. This reaction is catalyzed by peptidyl transferase. Notably, the active center of this enzyme is formed by the ribosomal RNA of the large subunit, meaning it possesses ribozyme activity. Dipeptidyl-tRNA is formed.
- Translocation. The ribosome shifts along the mRNA by exactly one codon (in the 5' to 3' direction). This requires the EF2 factor and GTP hydrolysis. As a result, the peptidyl-tRNA moves to the P-site, the empty tRNA leaves the ribosome, and the A-site becomes vacant.
Translation termination
The polypeptide assembly process is completed when one of the stop codons enters the A-site: UAG, UGA, or UAA.
Instead of new tRNAs, specific protein termination factors—RF1 and RF3—interact with the stop codon (the numbers indicate their structural similarity to homologous prokaryotic factors).
Under the action of peptidyl transferase, hydrolytic cleavage of the completed polypeptide chain from the final tRNA occurs. Following the release of the protein, the ribosomal subunits dissociate, which is also accompanied by the consumption of a GTP molecule.
Briefly: answers to topical questions
Which antibiotics inhibit the translation process in bacteria?
Antibiotics that inhibit translation in bacteria suppress protein synthesis on prokaryotic ribosomes and block various stages of translation.
- Tetracyclines — block the A-site of the ribosome.
- Levomycetin (chloramphenicol) — binds to the 50S ribosomal subunit, inhibits peptidyl transferase, and blocks peptide bond formation.
- Streptomycin / aminoglycosides — disrupt initiation and code reading.
- Erythromycin / macrolides — block translocation.
- Doxycycline and clindamycin — suppress protein synthesis.
Antibacterial drugs that suppress translation are specific to the prokaryotic protein-synthesizing system.
What is a polysome and what is its function?
A polysome is a complex consisting of a single mRNA molecule, multiple ribosomes, and peptide chains of varying lengths being synthesized by them. The function of polysomes is the simultaneous active synthesis of proteins.
Two main types are distinguished:
- Membrane-bound polysomes — synthesize export proteins, membrane proteins, and lysosomal and peroxisomal enzymes.
- Free polysomes — remain in the hyaloplasm and synthesize proteins for the cell's own needs (cytosol, nucleus, mitochondria, cytoskeleton).
What are the differences in translation initiation between prokaryotes and eukaryotes?
The main differences in translation initiation lie in the mechanism of start codon recognition and the structure of mRNA signaling sequences.
| Sign | Prokaryotes | Eukaryotes |
|---|---|---|
| Initiation signal | Shine-Dalgarno sequence | Cap structure at the 5' end of mRNA |
| Start search mechanism | Interaction of the mRNA region with the 3' end of rRNA | Terminal initiation (scanning the chain to the AUG codon) |
In eukaryotes, the small subunit binds to the cap structure with the participation of a protein complex of initiation factors and then moves along the chain.
What is post-translational protein modification?
Post-translational modifications are chemical alterations and structural changes of polypeptide chains occurring after the completion of translation for the protein to acquire functional activity.
Key processes:
- Partial proteolysis — removal of a part of the polypeptide chain.
- Folding — formation of the spatial structure with the participation of chaperone proteins.
- Amino acid modifications — phosphorylation, carboxylation, hydroxylation, glycosylation, iodination, acylation.
- Disulfide bond formation — occurs between cysteine residues.
- Addition of prosthetic groups — addition of non-protein components.
- Formation of oligomeric structures — formation of the quaternary structure.
To memorize the names of the ribosome active sites: A-site (aminoacyl) — always accepts a new Amino acid. P-site (peptidyl) — firmly holds the growing Peptide.
Class and exam questions
What is the main coding problem in translation?
The fact that the template (mRNA) is built from only 4 types of nucleotides, while the final product (protein) consists of 20 types of amino acids. There is no chemical complementarity between them, so the biological code and tRNA adaptors are required for decoding.
Which enzyme catalyzes peptide bond formation and where is it located?
The reaction is catalyzed by peptidyl transferase. The uniqueness of this enzyme is that its active site is formed by the ribosomal RNA (rRNA) of the large ribosomal subunit. This is a classic example of a ribozyme.
Why are there only 20 aminoacyl-tRNA synthetases, but significantly more types of tRNA?
Due to the degeneracy of the genetic code, the same amino acid can be encoded by several different codons. Accordingly, isoacceptor tRNAs with different anticodons exist, but the enzyme recognizes all of them, possessing only relative tRNA specificity.
At which stages of translation is GTP energy expended?
GTP is used at the stage of preinitiation complex formation (initiation), binding of a new aa-tRNA in the A-site, translocation of the ribosome along the mRNA (elongation), and during the dissociation of ribosomal subunits (termination).
What else to study on this topic
- Structure and properties of the genetic code
- Structure of transfer RNAs: acceptor end and anticodon loop
- Mechanism of mRNA scanning and the role of the 5'-cap in initiation
- Ribozyme activity of rRNA: catalysis mechanisms
- Features of isoacceptor tRNAs
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