Protein synthesis 😆 Flashcards
Each codons consists of 3 bases (triplet)
There are 64 codons
They are all written in the 5’ to 3’ directions
61 codons code for amino acids
The other 3 (UAA, UGA, UAG) are stop codons (nonsense codons) that terminate translation
There is one start codon (initiation codon), AUG coding for methionine
Protein synthesis begins with methionine in eukaryotes and formylmethionine in prokaryotes
The Genetic Code
Proteins translated on ribosomes associated with the rough endoplasmic reticulum:
Secreted proteins
Proteins inserted into the cell membrane
Lysosomal enzymes
Proteins translated on free cytoplasmic ribosomes:
Cytoplasmic proteins
Mitochondrial proteins
Translation
Characteristics of the Genetic Code
Specific
Universal
Redundant/Degenerate
Nonoveerlapping/Commaless
A specific codon always codes from the same amino acid
Specific
It has been conversed from very early stages of evolution with only slight differences in the manner in which the code translated
Universal
A given amino acid may have more than one triplet coding for it
Redundant/Degenerate
Code is read from a fixed starting point as a continuous sequence of bases, taken 3 at a time
Nonoverlapping/Commaless
Requirement of Translation
- All the amino acids that eventually appear in the protein
- At least one specific type of RNA for each amino acid
- One aminoacyl-tRNA synthetase for each amino acid
- mRNA coding for the protein to be synthesized
- Fully competent ribosomes
- Protein factors
- ATP and GTP as energy sources
Has an attachment site for a specific amino acid at its 3’end
Also has an anticodon region that can recognize the codon specifying the amino acid the tRNA is carrying
tRNA
Accurate base pairing is required only in the first 2 nucleoside positions of an mRNA codon, so codon differing in the 3rd wobble position may code for the same tRNA/amino acid
tRNA Wobble
Large complexes of protein and rRNA
Consists of 2 subunits
Eukaryotes 60s and 40s = 80s
Prokaryotes 50s and 30s = 70s
Ribosomes
Each ribosomes has 3 binding sites
A site codon
P site codon
E site codon
Binds an incoming aminoacyl-tRNA
A site codon
Occupied by peptidyl-tRNA
P site codon
Occupied by the empty tRNA as it is about to exit the ribosomes
E site codon
Amino-acetyl-tRNA synthetase (1 per AA) uses an ATP scrutinizes an AA before and after it binds to tRNA
If incorrect, bond is hydrolyzed by synthetase. The AA-tRNA bond has energy for formation of peptide bond
A mischarged tRNA reads usual codon, but inserts wrong AA
Charging
Steps in Translation
Step 1 : initiation
Step 2 : elongation
Step 3 : termination
Activated by GTP hydrolysis, initiation factors (elFs) help assemble the 40s ribosomal subunit with the initiator tRNA and are released when the mRNA and the ribosomal unit assemble with the complex
In prokaryotes, a purine-rich region (the SHINE-DALGARNO SEQUENCE) of the mRNA base pairs with a complementary sequence
The 5’-cap in eukaryotic mRNA is used to position that structure on the ribosome
INITIATION
- Aminoacyl-tRNA binds to A site (except for initiator methionine). Elongation factors direct the binding of the appropriate tRNA to the codon in the empty A site
- The enzyme peptidyltransferase catalyzed peptide bond formation, transfers growing polypeptide to amino acid in A site
- Ribosome advance 3 nucleosides toward 3’ end of RNA, moving peptidyl tRNA to P site (translocation)
ELONGATION
Releasing factors are proteins that hydrolyze the peptidyl-tRNA bond when a stop codon (UAA, UAG, UGA) occupies the A site
Completed protein is released from ribosome through simple hydrolysis and dissociates
TERMINATION
Energy requirement of Translation
- tRNA aminoacylation (ATP –>AMP)
- Loading tRNA onto ribosome (GTP –> GDP)
- Translocation (GTP –> GDP)
Post-translational modification
Trimming excess amino acids Phosphorylation Glycosylation Hydroxylation Proteins that are defective or destined for rapid turnover are marked for destruction by ubiquitin and are degraded by proteasomes
An exotoxin of CORYNEBACTERIUM DIPHTHERIAE inactivates eEF-2 and thereby specifically inhibits mammalian protein synthesis
Diphtheria toxin
